-
HYDROCARBONS
Tekst voor de cursus Grondstoffen en het Systeem Aarde (HD
698)H.E.Rondeel, december 2001
Teksten gebaseerd op:Blackbourn, G.A. (1990) Cores and core
logging for geologists. Whittles Publ.,Caithness. 113 pp.Shauer
Langstaff, C. & D. Morrill (1981) Geologic cross sections.
IHRDC, Boston. 108 pp.Stoneley, R. (1995) An introduction to
petroleum exploration for non-geologists. Oxford University
Press,Oxford. 119 pp.Waples, D. (1981) Organic geochemistry for
exploration geologists. Burgess Publ. Co., Mineapolis.
151pp.Waples, D.W. (1985) Geochemistry in petroleum exploration.
Reidel Publ. Co, Dordrecht & IHRDC,Boston. 232 pp.
MuzakiSticky NoteBab 9 neun
Bab 'Petroleum Traps' di-skip dulu
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HYDROCARBONS
CONTENTS
1 -
INTRODUCTION.............................................................................................................................
5
FORMATI0N OF 0IL AND
GAS.........................................................................................................
5
2 - ORGANIC
FACIES..........................................................................................................................
6
THE CARBON CYCLE
.......................................................................................................................
6FACTORS INFLUENCING ORGANIC
RICHNESS............................................................................
7
PRODUCTIVITY
..............................................................................................................................
7PRESERVATION..............................................................................................................................
8DILUTION
.....................................................................................................................................
11
SUMMARY
.......................................................................................................................................
12
3 - ORGANIC CHEMISTRY
..............................................................................................................
13
INTRODUCTION..............................................................................................................................
13NAMES AND
STRUCTURES...........................................................................................................
13
HYDROCARBONS
.........................................................................................................................
13NONHYDROCARBONS
.................................................................................................................
15
4 -
KEROGEN......................................................................................................................................
17
INTRODUCTION..............................................................................................................................
17KEROGEN
FORMATION.................................................................................................................
17KEROGEN COMPOSITION
.............................................................................................................
18KEROGEN
MATURATION..............................................................................................................
20
INTRODUCTION
...........................................................................................................................
20EFFECTS OF MATURATION ON KEROGENS
.............................................................................
21HYDROCARBON
GENERATION...................................................................................................
22
SUMMARY
.......................................................................................................................................
23
5 - BITUMEN, PETROLEUM, AND NATURAL
GAS......................................................................
24
INTRODUCTION..............................................................................................................................
24COMPOUNDS PRESENT IN BITUMEN AND PETROLEUM
......................................................... 24
GENERAL CLASSES OF COMPOUNDS
.......................................................................................
24SPECIFIC
COMPOUNDS..............................................................................................................
25
FACTORS AFFECTING COMPOSITION OF BITUMEN AND
PETROLEUM................................ 25SOURCE AND DIAGENESIS
.........................................................................................................
25RESERVOIR
TRANSFORMATIONS...............................................................................................
26COMPARISON OF BITUMEN AND PETROLEUM
.......................................................................
27NATURAL GAS
..............................................................................................................................
28
SUMMARY
.......................................................................................................................................
28
6 -
MIGRATION..................................................................................................................................
29
DEFINITIONS...................................................................................................................................
29PRIMARY
MIGRATION...................................................................................................................
29
MECHANISMS...............................................................................................................................
29DISTANCE AND DIRECTION
.......................................................................................................
30
SECONDARY
MIGRATION.............................................................................................................
31MECHANISM.................................................................................................................................
31
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Contents
DISTANCE AND DIRECTION
.......................................................................................................
31ACCUMULATION............................................................................................................................
32
INTRODUCTION
...........................................................................................................................
32CLASSICAL
TRAPS........................................................................................................................
33KINETIC TRAPS
............................................................................................................................
33TAR-MAT TRAPS
...........................................................................................................................
34GAS HYDRATES
............................................................................................................................
34
EFFECTS ON OIL AND GAS COMPOSITION
................................................................................
34SIGNIFICANCE FOR EXPLORATION
............................................................................................
35
7 - PETROLEUM TRAPS
...................................................................................................................
36
THE REPRESENTATION OF TRAPS
..............................................................................................
36STRUCTURAL TRAPS
.....................................................................................................................
37STRATIGRAPHIC
TRAPS................................................................................................................
41COMBINATION
TRAPS...................................................................................................................
42HYDRODYNAMIC TRAPS
..............................................................................................................
43THE RELATIVE IMPORTANCE OF TRAPS
...................................................................................
43EXERCISES
......................................................................................................................................
45
8 - SOURCE-ROCK
EVALUATION..................................................................................................
49
DEFINITION OF SOURCE
ROCK....................................................................................................
49PRINCIPLES OF SOURCE-ROCK EVALUATION
..........................................................................
49
QUANTITY OF ORGANIC MATERIAL
..........................................................................................
49MATURITY OF ORGANIC
MATERIAL..........................................................................................
49CONTAMINATION AND
WEATHERING.......................................................................................
52ESTIMATION OF ORIGINAL SOURCE CAPACITY
......................................................................
52
INTERPRETATION OF SOURCE-ROCK DATA
.............................................................................
53QUANTITY OF ORGANIC MATERIAL
..........................................................................................
53TYPE OF ORGANIC
MATTER.......................................................................................................
53MATURITY.....................................................................................................................................
54COALS AS SOURCE ROCKS
.........................................................................................................
54
SUMMARY
.......................................................................................................................................
55EXERCISES
......................................................................................................................................
56
9 - PREDICTING THERMAL MATURITY
......................................................................................
60
INTRODUCTION..............................................................................................................................
60CONSTRUCTION OF THE GEOLOGICAL MODEL
.......................................................................
60
BURIAL-HISTORY
CURVES..........................................................................................................
61TEMPERATURE
HISTORY............................................................................................................
61SPECIAL CONSIDERATIONS ABOUT BURIAL-HISTORY CURVES
............................................ 62
CALCULATION OF
MATURITY.....................................................................................................
63FACTORS AFFECTING THERMAL
MATURITY............................................................................
64POTENTIAL PROBLEMS WITH MATURITY
CALCULATIONS.....................................................
65
EXERCISES
......................................................................................................................................
66
10 - QUANTITATIVE ASSESSMENT
...............................................................................................
69
OIL IN PLACE
..................................................................................................................................
69RESERVES........................................................................................................................................
69
DISCOVERED
RESERVES.............................................................................................................
70UNDISCOVERED RESERVES
.......................................................................................................
72ULTIMATE
RESERVES..................................................................................................................
73
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Organic Facies - 5
1 - Introduction
FORMATI0N OF 0IL AND GASProponents of the organic origin of oil
and gas have given us a general picture of how organic
matterderived from dead plants is converted to hydrocarbons.
Although the transformation process is verycomplex, with many
details still poorly understood, it is known that organic debris
derived fromplants and algae is best preserved in fine-grained
sediments deposited in the absence of oxygen.Low-temperature
chemical and biological reactions (called diagenesis) that occur
during transportto and early burial in the depositional environment
modify this organic matter. Many of the chemicalcompounds present
in sediments are in fact derived from bacteria, and were formed as
dead organicmatter was converted to microbial tissues.Most of this
organic matter is transformed during diagenesis info very large
molecules, the largest ofwhich are called kerogen. These play a key
role as the precursors for oil and much natural gas.The earliest
stage of hydrocarbon generation occurs during diagenesis. Certain
microorganisms,called methanogens, convert some of the organic
debris to biogenic methane. Formation of biogenicmethane has been
recognized for a long time, but only within the last few years have
we realized thatin many areas a large portion of the natura!-gas
reserves are biogenic.As burial depth increases, porosity and
permeability decrease, and temperature increases. Thesechanges lead
to a gradual cessation of microbial activity, and thus eventually
bring organicdiagenesis to a halt. As temperature rises, however,
thermal reactions become increasinglyimportant. During this second
transformation phase, called catagenesis, kerogen begins
todecompose into smaller, more mobile molecules. In the early
stages of catagenesis most of themolecules produced from kerogen
are still relatively large; these are the precursors for
petroleum,and are called bitumen . In the late stages of
catagenesis and in the final transformation stage,
calledmetagenesis, the principal products consist of smaller gas
molecules.In recent years this relatively simple picture of
hydrocarbon generation has been complicated slightlyby our growing
awareness that kerogens formed from different kinds of organic
matter, or underdifferent diagenetic conditions, are chemically
distinct from each other. These differences can have asignificant
effect on hydrocarbon generation.Once formed, oil and gas molecules
can be expelled from the source rock into more permeablecarrier
beds or conduits. Migration through these conduits often leads to
traps, where hydrocarbonmovement ceases and accumulation
occurs.
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Organic Facies - 6
2 - Organic Facies
THE CARBON CYCLEBecause oil and gas are generated from organic
matter in sedimentary rocks, we need tounderstand how this organic
matter came to be preserved in the rocks. Preservation of
organicmaterial is actually a rare event. Most organic carbon is
returned to the atmosphere through thecarbon cycle; less than 1% of
the annual photosynthetic production escapes from the carboncycle
and is preserved in sediments. Oxidative decay of dead organic
matter is a highly efficientprocess mediated largely by
microorganisms.Preservation of organic matter begins with
photosynthesis. Some of the organic material insediments consists
of fragments of plants or algae that derived their energy from the
sun. A largefraction, however, comprises microbial tissue formed
within the sediments by the bacterialtransformation of plant and
algal debris. Zooplankton and higher animals contribute
relativelylittle organic matter to sediments. The recently
discovered deep-sea ecosystems in the PacificOcean that derive
their energy from oxidation of sulfides in hydrothermal vents are
interesting
but volumetrically unimportant.Despite the great imbalance in
biomass between terrestrial plants (450 billion metric tons [t])
andaquatic phytoplankton (5 billion t), the yearly productivity of
both groups is about equal, as aconsequence of the much more rapid
reproduction of simple aquatic organisms. Because of
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Organic Facies - 7
extensive oxidation of land-plant debris in soils, however, much
of the terrestrial organic materialis already highly oxidized when
it arrives in the sediments.Although some destruction of organic
material occurs during transport to the depositionalenvironment, a
great deal of the oxidation of organic matter occurs within the
sedimentsthemselves. Total Organic Carbon (TOC) values decrease
monotonically through the first 300meters of burial before
levelling out at about 0.1%, suggesting that either depth or
organic-carbon content eventually limits diagenesis. Depth could
interfere with microbial diagenesis whencompaction reduces pore
sizes and nutrient fluxes in interstitial waters. On the other
hand, thelow TOC values could indicate that the remaining organic
matter has no more nutritional value,and that the microbes have
given up trying to digest it. Each factor may be dominant
underdifferent conditions.Although oxidative decay destroys most of
the yearly production, over vast amounts of geologictime the small
fraction that escaped the carbon cycle has built up extremely large
quantities oforganic matter (20,000,000 billion t) dispersed in
fine-grained sedimentary rocks. Only a smallfraction of this
(10,000 billion t, or about 0.05%) occurs in economic deposits of
fossil fuels.When we consider inefficiencies in discovery and
recovery, only one molecule out of about everyone million
successfully negotiates the journey from living organism to the
gasoline pump.
FACTORS INFLUENCING ORGANIC RICHNESSIn order for organic-rich
rocks to be formed, significant amounts of organic matter must
bedeposited and protected from diagenetic destruction. The three
primary factors influencing theamount of organic matter in a
sedimentary rock are productivity, preservation, and
dilution.Productivity is the logical place to begin our analysis,
because without adequate productivity,accumulation of organic-rich
sediments cannot occur.
PRODUCTIVITYA partial listing of the many factors influencing
productivity would include nutrient availability,light intensity,
temperature, carbonate supply, predators, and general water
chemistry. Each ofthese categories could in turn be further
subdivided. For example, nutrient availability woulddepend on such
factors as water circulation patterns, orogeny and erosion,
volcanism,paleoclimate, and recycling by organic decay.Nutrient
availability is, in fact, one of the critical parameters governing
productivity. Shallow-marine environments, where there is local
recycling of nutrients from decaying organisms andinflux of fresh
nutrients from terrestrial sources, are therefore much more
productive than theopen ocean.In relatively unrestricted marine
environments, watercirculation patterns are particularlyimportant
for supplying nutrients and thus controlling productivity. Bodies
of water naturallydevelop density stratification, with a preference
for horizontal water movement within eachdensity layer. Nutrients
dissolved in waters below the photic zone therefore go
unutilized,because under normal circumstances they cannot move
upward into the zone of photosynthesis.Only where there is
upwelling of subsurface waters can these nutrients return to the
photic zone.Upwelling occurs where bulk movement of surface water
away from a particular area allowsdeeper water to ascend to replace
it. If this deeper water is enriched in nutrients,
highphotosynthetic productivity will occur at the site of
upwelling. In the modern world there arezones of intense seasonal
upwelling off the west coasts of California, Peru, Namibia,
andNorthwest Africa that result from the movement of surface waters
away from these coasts. Thereis another zone of seasonal upwelling
off the Horn of Africa in the Indian Ocean as a result of
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Organic Facies - 8
monsoonal winds that drive surface waters away from the coast.
All these areas exhibit highproductivity when upwelling
occurs.Theoretical models have been developed to predict upwelling
(and consequent productivity) inancient seas from input data on
continental configurations, landmasses, wind and watercirculation
patterns, and paleoclimates.Such models are interesting, and may in
fact prove useful in future exploration efforts. There are,however,
some problems associated with their application. First,
productivity is probably not asimportant a factor as preservation.
There are many more organic-rich facies resulting fromexcellent
preservation than from extremely high productivity. After all, if
on the average only 1%of organic matter is preserved, increasing
preservation rates is a very efficient way to increaseorganic
richness. Secondly, the accuracy with which we can reconstruct
continental positions,paleoclimatic conditions, and all the other
factors that influence upwelling loci is severelylimited,
especially in the Palaeozoic.
PRESERVATIONThe principal control on organic richness is the
efficiency of preservation of organic matter insedimentary
environments. Three factors affect the preservation (or
destruction) of organicmatter: the concentration and nature of
oxidizing agents, the type of organic matter deposited,and the
sediment-accumulation rate. Of these, oxidizing agents are probably
the most crucialfactor.
ANOXIA. Because most of the oxidation occurring in the water
column, soils, and sediments isbiological, and because most
biological oxidation processes require molecular oxygen,
thesimplest way to limit oxidation is to limit the supply of
oxygen. All large organisms requireoxygen in order to live,
although some species can tolerate extremely low oxygen levels
(0.5milliliters (mL) per liter (L)). At lower levels of dissolved
oxygen, many species disappear; theremaining individuals often
become dwarfed in an effort to survive in a hostile environment.
Atdissolved oxygen levels below about 0.2 mL/L, essentially the
only viable organisms are thosethat we call anaerobes,
microorganisms that utilize materials like sulfate or nitrate ions
insteadof molecular oxygen as electron acceptors in their metabolic
processes.We call the zone in which oxygen contents are high the
oxic zone; the zone where oxygen fallsbelow 0.2 mL/L is called the
anoxic zone. Processes that occur in these two zones are
calledaerobic and anaerobic, respectively. The term dysaerobic has
been used to describe processesoccurring in the transitional zone
(0.2-0.5 mL/L), and we could coin the term dysoxic to describethe
zone itself. The term "anoxic" literally means "having no oxygen,"
hut because of the radicalchange in biota that occurs at about 0.2
mL/L, its use in practice has been expanded to includevery low
oxygen levels as well.Anoxia is of tremendous importance in the
preservation of organic matter in sediments, becausewhen the
availability of oxygen is limited, diagenesis is restricted to
anaerobic processes. Theseanaerobic processes are inefficient
compared with aerobic diagenesis, and are usually limited inscope
by the availability of sulfate or nitrate. Thus if anoxia can
develop, preservation of organicmatter will be much enhanced.Anoxic
sediments are not always easy to recognize, because some of the
commonly usedindicators of anoxia may be misleading. Anoxic
sediments always contain elevated TOC values(generally above 2% and
always above 1% ). However, much oxic sediment also contains
largeamounts of organic matter, especially of woody origin. TOC
values alone must therefore be usedwith caution. The presence of
undegraded marine organic material is a strong indication ofanoxia,
because marine organic matter is consumed preferentially by
organisms. Its presence in
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Organic Facies - 9
rocks therefore indicates that diagenesis was stopped
prematurely, most likely by absence ofoxygen.Color is not a
reliable indicator. All anoxic sediments will be very dark gray or
black whendeposited. Many black rocks, however, are not rich in
organic carbon; they often owe their darkcolor to finely divided
pyrite or to particular chert phases. Color should be used mainly
as anegative criterion: If a rock is not very, very dark, it cannot
represent an anoxic facies.The presence of pyrite itself can also
be deceptive. Although pyrite does indeed form underanoxic
conditions, and its presence indicates that the anaerobic reduction
of sulfate ion did occur,there is no guarantee that anoxia was
present at the sea floor; it may well have developed afterburial.
Furthermore, anoxia can be very local; intense pyritization of
benthic bivalves istestimony to the fact that pyrite is not a good
indicator of bottom-water anoxia at the time ofdeposition.Finally,
anoxic sediments show preserved depositional laminae on a
millimeter or submillimeterscale. The laminae prove that burrowing
fauna were absent, and therefore that dissolved-oxygenlevels were
below 0.2 mL/L. Conversely, the presence of bioturbation indicates
that the bottomwaters were not anoxic, although stunted burrows can
be used as evidence of dysoxia.The ultimate implications of anoxia
for petroleum exploration are great; it has been estimated, infact,
that most of the world's oil was generated from source beds
deposited under anoxicconditions. It therefore behoves us to
understand the conditions under which anoxia develops.
STAGNANT BASINS. Truly stagnant basins are actually quite rare;
slow circulation orturnover of the water column occurs almost
everywhere. Nevertheless, it is instructive toconsider complete
stagnation, particularly in understanding lacustrine beds. If an
isolated body ofwater is deep enough, and if the climate is
subtropical or tropical, then permanent densitystratification will
arise as a result of temperature differences within the water
column. Depths inexcess of 200 m are required to prevent mixing
during storms, and warm climates are necessaryto avoid overturn
caused by freeze-thaw cycles. The cooler, denser waters remain at
the bottom,leading to the eventual development of a pycnocline
(density interface) which preventsinterchange between the two
layers. Lack of communication between the layers
prohibitsreplenishment of oxygen in the bottom layer. Therefore,
once the original oxygen has beenconsumed in oxidizing organic
matter, no more oxygen can enter, and both the waters in thebottom
layer and the underlying sediments will become anoxic.Marine basins
are seldom isolated enough to fit well into the stagnant-basin
model, but limnicenvironments often are. Among the ancient lake
beds thought to have been deposited inpermanently stratified waters
are the well-known Green River Shale (middle Eocene, Wyoming),the
Elko Formation (Eocene/Oligocene, Nevada), and strata from several
basins in China. Lakedeposits associated with continental rifting,
especially during the Triassic along the margins ofthe developing
Atlantic Ocean, are anoxic in some of the places where they have
been penetrated.Lakes in failed rifts can also contain
organic-rich, anoxic sediments. Lakes of the Rift Valley ofEast
Africa are excellent modern analogs receiving much attention from
both researchers andexplorationists at the present time.
OXYGEN-MINIMUM LAYER (OML). The oxygen-minimum layer is a layer
of subsurfacewater that has a lower dissolved-oxygen content than
the water layers either above or below.This oxygen minimum develops
when the rate of consumption of oxygen within that layerexceeds the
rate of influx of oxygen to it. Consumption of oxygen results from
decay of deadorganisms that have sunk from the photic zone above.
The oxygen minimum layer usually beginsimmediately below the photic
zone, where photosynthesis and turbulence can no longercontribute
oxygen to the water. The supply of fresh oxygen is therefore
limited to horizontal
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Organic Facies - 10
movement of oxygen-bearing waters. However, because these
horizontally moving waters also liewithin the oxygen minimum layer,
the oxygen they can contribute is limited. Below the OMLoxygen
levels again increase, as a result of diminished oxygen demand,
since most organic matterwas destroyed within the overlying
OML.Although an oxygen-minimum layer exists virtually everywhere in
the ocean, its intensity variesgreatly. Intensely developed OMLs
occur in areas of high productivity and, to a lesser extent,
inareas of poor circulation. Wherever an intensely developed OML
intersects the sediment-waterinterface, sediments will be deposited
under low-oxygen conditions. Any organic matter arrivingin those
sediments will have an excellent chance to escape
oxidation.Bottomset beds associated with prograding delta systems
can be rich in organic matter if they arelaid down within a
well-developed oxygen-minimum layer. In contrast, foreset beds
within thesame system are leaner in organic matter because they are
deposited above the OML.There are other ancient and modern examples
of organic-rich rocks deposited under anoxic ornear-anoxic
conditions associated with OMLs. These include the modern
Peru-Chile shelf (highproductivity associated with upwelling) and
occurrences of black sediments of Aptian toTuronian age in the
North Atlantic.It has been proposed that at certain times in the
past (e.g., mid-Cretaceous, Late jurassic, LateDevonian) the world
oceans were severely depleted in dissolved oxygen. This depletion
wasprobably the result of the complex interplay of several factors,
including paleoclimate and watercirculation. During those times the
OML expanded both upward and downward because of poorsupply of
oxygen to subsurface waters. In times like the mid-Cretaceous, when
a majortransgression had greatly increased the continental shelf
area, an upward expansion of the OMLled to a tremendous increase in
the surface area covered by anoxic bottom waters. It is
notcoincidental that these were times of deposition of large
amounts of organic-rich rocks in manyparts of the world.
RESTRICTED CIRCULATION. Settings in which circulation is
restricted are much morecommon than stagnant basins. Furthermore,
because of their connection with the open-marinerealm, those
environments can also incorporate the features of an
oxygen-minimum-layer model.Shallow Silling. Circulation is often
restricted by the presence of a sill, the point of
connectionbetween the restricted area and the open-marine
environment. Where the sill is shallow, thewaters entering or
leaving the basin are near surface. In an evaporitic environment
(Karabogaz inthe Caspian Sea) there is a net flow of water into the
basin, whereas in a fluvially dominatedsystem (Black Sea) the net
flow of surface water is out over the sill. In either case, if the
basin isdeep enough, permanent density stratification will develop,
with the bottom layer almost isolatedfrom the open-marine waters.
In actuality there is a lazy turnover of the bottom waters, but it
istoo slow to disturb the anoxia which develops in the bottom
layer.Shallowly silled basins often yield evaporites, which could
be excellent hydrocarbon sourcerocks. Evaporitic environments
combine the opportunity for abundant growth of algae with
idealconditions for preservation. Nutrients are concentrated by
evaporation, and grazers andpredatory organism are eliminated by
the high salinities. High productivity reduces oxygenlevels, and
high hydrogen-sulfide concentrations create conditions poisonous to
predators. Theresult is often deposition of organic-rich laminae
within evaporites, or as lateral faciesequivalente thereof.Coal
Swamps. Large amounts of organic material are preserved in coal
swamps as a result ofthe combined effects of poor water
circulation, high influxes of organic matter, and
diminishedbacterial activity. Coal swamps can develop under a
variety of conditions in both marine andnon-marine environments.
Although circulation in coal swamps is generally sluggish,
theshallowness of the swamps prevents the waters themselves from
becoming anoxic. Anoxia
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Organic Facies - 11
develops within the sediments rather than in the water column.
Phenolic bactericides derivedfrom lignin hinder bacterial decay in
the water and throughout the sediment column. Lack ofsulfate in
non-marine swamps further prevents anaerobic microbial destruction
of the organicmatter.Coals are important source rocks for gas
accumulations, but their supposedly low potential forgenerating oil
is to be reconsidered.Oxic Settings. Most depositional settings not
specifically catalogued above will be more or lesswell oxygenated,
and therefore wi11 contain primarily oxidized organic matter.
Near-shoreoxidizing facies sometimes have high TOC values, but the
organic material is almost invariablywoody. Abyssal sediments are
notoriously low in organic carbon as the result of the
combinedeffects of high oxygen levels in abyssal waters, very slow
sedimentation rates, and lowproductivity in the overlying pelagic
realm. The hydrocarbon-source potential of all of theseoxidizing
facies is low, and more favorable for gas than for oil.
TYPE OF ORGANIC MATTER. Organic matter of algal
(phytoplanktonic) origin isconsumed more readily by organisms than
are other types of organic material, because itschemical components
are digestible and provide precisely the nutrients required by
scavengersand predators. Nitrogen and phosphorus are in particular
demand; their virtual absence in muchterrestrial organic material,
especially in structural (woody) material, renders it of
littlenutritional value. Furthermore, the phenolic components
present in lignin-derived terrestrialmaterial are toxic to many
micro-organism, thus preventing extensive diagenesis of
suchmaterial.Any extensive organic diagenesis is therefore likely
to eliminate algal organic matter first. Thatmaterial which remains
is dominantly of terrestrial origin, and may include woody,
cellulosic,lignitic, cuticular, or resinous material, all of which
are chemically quite distinct from each other.It may also contain
very resistent organic debris derived from erosion of ancient
rocks, forestfires, and other oxidative processes.
RAPID SEDIMENTATION AND BURIAL. Rapid sedimentation and burial
con also enhancepreservation. TOC values increase as
sediment-accumulation rates increase, as a result of morerapid
removal of organic material from the zone of microbial
diagenesis.Rapid burial is accomplished by a high influx of
inorganic detritus, biogenic inorganic sediment, ororganic
material. Rapid deposition of inorganic detritus is common in
turbidites and in prodeltashales. The extremely high accumulation
rates for biogenic carbonates and siliceous sediments inzones of
high productivity promote preservation of the associated algal
protoplasm. Coals alsoaccumulate very rapidly and, with their high
concentrations of organic matter, provide an idealmeans of
maintaining low-oxygen conditions.Rapid settling of organic debris
through the water column is also important, because
extensivedecomposition occurs during its fall to the ocean floor.
In fact, much of the organic material thatdoes reach the bottom in
deep waters arrives in relatively large fecal pellets, which settle
severalorders of magnitude faster than individual
phytoplankton.
DILUTIONAlthough high sediment-accumulation rates enhance
preservation of organic matter, at very highaccumulation rate
dilution may become a more important factor than increased
preservation.Dilution does not reduce the total amount of organic
matter preserved, but it does spread thatorganic material through a
larger volume of rock. The net result is a reduction in TOC
values.
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Organic Facies - 12
Dilution effects depend upon rock lithology. Biogenic sediments,
in which the organic andinorganic materials arrive together, are
not as strongly affected by dilution. Shales, in contrast,show
strong dilution effects when accumulation rates are very high.
Facies changes fromcarbonates to shales may create large dilution
effects that can be wrongly interpreted asindicating changes in
oxygen levels.
SUMMARYThere are three principal factors that affect the amount
of organic matter in sedimentary rocks:primary photosynthetic
productivity, effectiveness of preservation, and dilution by
inorganicmaterial. Of these, preservation is generally the most
important.Productivity can be predicted by locating ancient sites
of marine upwellings. Our ability to makeaccurate predictions is
limited, however, by uncertainties about exact continental
positions andconfigurations in the past, lack of knowledge of
seawater chemistry and nutrient availability atthose times, and a
very imperfect understanding of oceanic- and
atmospheric-circulation patterns.Consequently, such models are not
yet of much practical value for the distant past.Preservation is
best accomplished where oxygen is excluded from bottom waters.
There are anumber of mechanisms by which oxygen depletion may be
fostered and maintained, includingstagnancy or near-stagnancy, a
strongly developed oxygen-minimum layer, and rapid burial. It
isoften very difficult to separate the influences of these various
factors in a particular depositionalenvironment.Rapid accumulation
of sediment shortens the residence time of organic matter in the
zone ofdiagenesis and thus promotes preservation. If the rapidly
accumulating sediment is mainlyclastic, however, dilution effects
may lead to lower TOC values in spite of enhanced
preservationrates. In biogenic sediments or coals, in contrast,
where sediment-accumulation rates are directlyproportional to
organic-carbon-accumulation rates, dilution is far less
marked.Because of its role in creating rocks with excellent
hydrocarbon-source potential, anoxia inbottom waters is a
phenomenon whose effects we should learn to recognize in ancient
rocks.Some of the commonly applied criteria are apt to be
misleading, however. It is important to beable to distinguish local
anoxia or anoxia developed deep within sediments from anoxia
inducedby anoxic bottom waters. The most reliable criteria for
bottom-water anoxia are the preservationof fine depositional
laminae, and the presence of high TOC values coupled with the
occurrence ofundegraded marine organic matter.Anoxic events in the
past were probably not as large in scale or as long lasting as some
workershave suggested. Although certain periods undeniably contain
more than their share of anoxicrocks, anoxic sediments were
deposited discontinuously through time and space. Direct control
ofthe anoxia was thus probably local, as a result of high
productivity or sluggish circulation. As inthe modern oceans, such
events were often interrupted for long periods before anoxia
wasreinduced.Models that integrate the concepts of organic richness
with depositional cycles and faciesanalysis will be valuable tools
for understanding hydrocarbon systems in basins. To derivemaximum
value from our analyses, we should always strive to place the
organic rich rocks in thelarger context of basin evolution through
time and space.
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Organic Chemistry - 13
3 - Organic Chemistry
INTRODUCTIONAnyone who uses petroleum geochemistry must be
familiar with basic chemical terminology. Theobjective of this
chapter is to acquaint the reader with the names of common
compounds and withseveral different conventions for drawing their
structures. This objective is very different trom thatof a normal
course in organic chemistry, in which one must also learn all the
reactions of manyclasses of compounds. The chemical reactions of
interest to us are very few and are discussed onlybriefly. All
compounds containing carbon atoms, except carbon dioxide,
carbonates, and metalcarbides, are termed organic. This usage is
historical and does not imply that all such compoundsare
necessarily derived from living organisms. Organic chemistry is
thus the study of carbon-containing compounds, and organic
geochemistry the study of organic compounds present ingeological
environments.
NAMES AND STRUCTURES
HYDROCARBONSIn chemical terms a hydrocarbon is a compound
containing only the elements carbon and hydrogen.Petroleum and
natural gas are themselves often referred to as "hydrocarbons," but
that usage isincorrect trom the chemist's point of view because
those materials often contain substantial amountsof nitrogen,
sulfur, oxygen, trace metals, and other elements. In this chapter
we restrict the usage ofthe term hydrocarbon to the standard
chemical one; elsewhere in this text usage will vary, as it doesin
the real world.
Examples of hydrocarbons are methane, ethane, and cyclohexane,
whose structures are shownbelow.
In each of these compounds, and indeed in every carbon compound
(except a few highly unstableones created only in laboratories),
every carbon atom forms four bonds. Similarly, hydrogen alwaysforms
one bond; oxygen and sulfer, two bonds; and nitrogen, three bonds.
Carbon atoms like toform bonds with each other, creating long
chains and ring structures. This unique property ofcarbon is
responsible for the existence of literally millions of different
organic compounds.Writing the detailed structure of a simple
molecule like methane is no problem, especially if one hasto do it
only occasionally. If one wants to draw large molecules, however,
the explicit inclusion ofevery atom and every bond becomes
extremely tedious. Several different types of shorthand
havetherefore developed to facilitate drawing organic molecules.One
common convention is to represent all the hydrogen atoms attached
to a given carbon atom bya single H, using a subscript on the H to
denote the total number of hydrogens around that atom.The
structures of methane and ethane are thus represented by CH4 and
CH3CH3 respectively.We can make other logical simplifications for
longer carbon chains. The following representationsof n-pentane are
equivalent: CH3CH2CH2CH2CH3 or CH3(CH2)3CH3.
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Organic Chemistry - 14
An even quicker shorthand that uses no letters at all has
evolved. Each carbon atom is representedby a point, and
carbon-carbon bonds are shown as lines connecting those points.
Hydrogen atomsand bonds to hydrogen atoms are not shown at all.
Because we know that each carbon atom formsfour bonds and each
hydrogen atom forms one bond, simple inspection shows how mant'
hydrogenatoms each carbon atom must have. For example, n-pentane
and cyclohexane are represented by theline structures shown
below.
The zigzag configuration illustrated for n-pentane isadopted to
show clearly each carbon atom.The simplest series of hydrocarbons
has linear structures;these molecules are called n-alkanes or
nparains. Theletter n stands for normal, and indicates that there
is nobranching in the carbon chain. We have ahreadyencountered
n-pentane; the names of the other ninesimplest n-alkanes are given
in the following table. Note
that the name of each compound ends in -ane, as in "alkane." The
first four names are irregular, butthe prefixes denoting the number
of carbon atoms in the other alkanes are derived from
Greeknumbers.
Names and formulas of the ten smallest n-alkanesMethane CH4
CH4Ethane C2H6 CH3CH3Propane C3H8 CH3CH2CH3Butane C4H10 CH3 (CH2)2
CH3Pentane C5H12 CH3 (CH2)3 CH3Hexane C6H14 CH3 (CH2)4 CH3Heptane
C7H16 CH3 (CH2)5 CH3Octane C8H18 CH3 (CH2)6 CH3Nonane C9H20 CH3
(CH2)7 CH3Decane C10H22 CH3 (CH2)8 CH3
Carbon atoms need not always bond together in a linear
arrangement. Branching can occur, givingrise to a vast number of
possible structures.The term methyl, which we used earlier, is the
adjectival form of the word methane. In the case of 2-methylhexane
(C7H16) the basic structure is hexane; a CH3 (methyl) group is
attached to the secondcarbon atom. Other adjectival forms are made
by dropping the -ane ending and adding yl (forexample, ethyl and
propyl).Among the most important branched hydrocarbons in organic
geochemistry are the isoprenoids.Regular isoprenoids consist of a
straight chain of carbon atoms with a methyl branch on everyfourth
carbon. Isoprenoids ranging in length from six to forty carbon
atoms have been found inpetroleum and rocks.
We have also seen that carbon atoms can be arranged in rings.
These cyclic compounds (callednaphthenes) are named by counting the
number of carbon atoms in the ring and attaching the
prefixcyclo.All the compounds mentioned above are called saturated
hydrocarbons or saturates, because theyare saturated with respect
to hydrogen. That is, no more hydrogen can be incorporated into
themolecule without breaking it apart.Another important group of
hydrocarbons is the unsaturates, which, in contrast, are able
tocombine with additional hydrogen. Many unsaturated compounds have
carbon-carbon double
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Organic Chemistry - 15
bonds; these compounds are called alkenes. Examples are ethene
(C2H4) , propene (C3H6), andcyclohexene (C6H10), the structures of
which are shown below. They are named in a similar mannerto the
alkanes, except that the ending -ene indicates the presence of a
double bond.
Because alkenes are highly reactive, they do not long persist in
geologic environments. In thelaboratory they are readily converted
to alkanes by the addition of hydrogen in the presence of
acatalyst. By hydrogenation ethene thus reacts to form ethane.
A variety of reactions, including hydrogenafion, converts
alkenes to alkanes and cyclic compoundsduring diagenesis.Aromatics
form an extremely important class of unsaturated hydrocarbons. At
first glancearomatics appear to be nothing more than cyclic alkenes
containing several double bonds, but theyactually have completely
different chemical properties from alkenes and are unusually
stable.Although they are unsaturated, they do not add hydrogen
easily. Their stability permits aromatics tobe important
constituents of oils and sediments.Aromatics possess a system of
alternating single and double bonds within a cyclic structure.
Asimplified notation for drawing these molecules permits us to
represent the double-bond system by acircle within the ring. The
circle indicates that the electrons in the double bonds are
delocalized;that is, they are free to move throughout the cyclic
system instead of being held between twoparticular carbon atoms. It
is this delocalization of electrons which makes aromatic
compoundsvery stable.Some aromatic molecules are very large.
Polycyclic aromatic hydrocarbons having fused ringstructures are
quite common. The extreme case is graphite, which is an
almost-endless sheet ofaromatic rings.The hydrocarbons we discussed
so far are relatively simple molecules. Although they are
veryimportant constituents of petroleum, these compounds are quite
different trom the majority of theorganic molecules found in living
organisms. Most biological molecules are larger and morecomplex
than the simple hydrocarbons; the majority contain oxygen,
nitrogen, phosphorus, sulfur,or other elements. The hydrocarbons
present in petroleum are mostly the end products of
extensivedegradation of biogenic molecules. In fact, some complex
hydrocarbons that are found in fossilorganic material can be
related directly to individual biological precursors.
NONHYDROCARBONSAtoms other than hydrogen and carbon that occur
in petroleum, bitumen, and kerogen are calledheteroatoms; the
compounds in which they occur are called heterocompounds.
Heterocompoundsare also called NSO compounds, because the most
common heteroatoms are nitrogen, sulfur, andoxygen. Fossil organic
matter often contains a vide variety of heterocompounds, of which
some arebiogenic and others are formed during diagenesis. Many of
the heterocompounds present inorganisms are converted to
hydrocarbons during diagenesis and catagenesis.
Many common NSO compounds are not directly related to biogenic
precursors. Among the mostimportant NSO compounds are the
asphaltenes, which are large, highly aromatic materials of
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Organic Chemistry - 16
varying structure. They have many characteristics in common with
kerogen, but asphaltenemolecules are smaller and more aromatic than
most kerogens.
Many nonhydrocarbon molecules common to living organisms are
also present in sediments. Amongthese are lignin, carbohydrates,
and amino acids. Lignin is an important component of wood,providing
much of the structural support for large land plants. It is a
polymer consisting of manyrepetitions and combinations of three
basic aromatic subunits.Lignin monomers are linked topether to form
molecules having molecular weights from 3000 to10,000 atomic mass
units. Upon decomposition lignin forms phenolic compounds, which
arearomatics having a hydroxyl group (OH) attached. Because phenols
are potent bactericides, ligninis rather resistant to degradation,
and thus tends to become concentrated as other organic matter
isdecomposed.Carbohydrates include starch, sugars, and cellulose;
the latter is the most abundant organiccompound in the biosphere.
Like lignin, it is an important constituent of terrestrial organic
matter.Although cellulose is quite resistant to decomposition under
some conditions, most carbohydratesare attacked readily by
microorganisms. Lignin and cellulose are major constituents of
humic coals.Amino acids are the building blocks of proteins. They
are rapidly metabolized by virtually allorganisms, however, and
thus are seldom preserved in sediments (exceptions occur in shell
materialand in bones, where small amounts of preserved amino acids
can be used to date specimens)
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Kerogen - 17
4 - Kerogen
INTRODUCTIONKerogen is normally defined as that portion of the
organic matter present in sedimentary rocks thatis insoluble in
ordinary organic solvents. The soluble portion, called bitumen,
will be discussed in afollowing chapter. Lack of solubility is a
direct result of the large size of kerogen molecules, whichhave
molecular weights of several thousand or more. Each kerogen
molecule is unique, because ithas patchwork structures formed by
the random combination of many small molecular fragments.The
chemical and physical characteristics of a kerogen are strongly
influenced by the type ofbiogenic molecules from which the kerogen
is formed and by diagenetic transformafions of thoseorganic
molecules.Kerogen composition is also affected by thermal
maturation processes (catagenesis and metagenesis)that alter the
original kerogen. Subsurface heating causes chemical reactions that
break off smallfragments of the kerogen as oil or gas molecules.
The residual kerogens also undergo importantchanges, which are
reflected in their chemical and physical properties.Kerogen is of
great interest to us because it is the source of most of the oil
and some of the gas thatwe exploit as fossil fuels. Diagenetic and
catagenetic histories of a kerogen, as well as the nature ofthe
organic matter from which it was formed, strongly influence the
ability of the kerogen togenerate oil and gas. A basic
understanding of how kerogen is formed and transformed in
thesubsurface is therefore important in understanding how and where
hydrocarbons are generated,whether these hydrocarbons are mainly
oil or gas, and how much oil or gas can be expected.The term
kerogen was originally coined to describe the organic matter in oil
shales that yielded oilupon retorting. Today it is used to describe
the insoluble organic material in both coals and oilshales, as well
as dispersed organic matter in sedimentary rocks. The amount of
organic matter tiedup in the form of kerogen in sediment is far
greater than that in living organisms or in economicallyexploitable
accumulations of coal, oil, and natural gas.Coals are a subcategory
of kerogen. Humic coals are best thought of as kerogens formed
mainlyfrom landplant material without codeposition of much mineral
matter. Algal (boghead) coals areformed in environments where the
source phytoplankton lack both calcareous and siliceous
skeletalcomponents. Oil shales, in contrast, have more mineral
matter than algal coals, with some of theinorganic matrix often
being contributed by the algae themselves. Coals and oil shales
shouldtherefore be viewed merely as sedimentary rocks containing
special types of kerogens in very highconcentrations.
KEROGEN FORMATIONThe process of kerogen formation actually
begins during senescence of organisms, when thechemical and
biological destruction and transformation of organic tissues begin.
Large organicbiopolymers of highly regular structure (proteins and
carbohydrates, for example) are partially orcompletely dismantled,
and the individual component parts are either destroyed or used to
constructnew geopolymers, large molecules that have no regular or
biologically defined structure. Thesegeopolymers are the precursors
for kerogen but are not yet true kerogens. The smallest of
thesegeopolymers are usually called fulvic acids; slightly larger
ones, humic acids; and still larger ones,humins. During the course
of diagenesis in the water column, soils, and sediments, the
geopolymersbecome larger, more complex, and less regular in
structure. True kerogens, having very highmolecular weights,
develop after tens or hundreds of meters of burial.The detailed
chemistry of kerogen formation need not concern us greatly.
Diagenesis results mainlyin loss of water, carbon dioxide, and
ammonia from the original geopolymers. If anaerobic sulfate
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Kerogen - 18
reduction is occurring in the sediments, and if the sediments
are depleted in heavy-metal ions (whichis often the case in
nonclastic sediments but is seldom true in shales), large amounts
of sulfur maybecome incorporated into the kerogen structure. The
amount of sulfur contributed by the originalorganic matter itself
is very small. Carboncarbon double bonds, which are highly
reactive, areconverted into saturated or cyclic structures.Kerogen
formation competes with the destruction of organic matter by
oxidative processes. Mostorganic oxidation in sedimentary
environments is microbially mediated. Microorganisms prefer
toattack small molecules that are biogenic, or at least look very
much like biogenic molecules.Geopolymers are more or less immune to
bacterial degradation, because the bacterial enzymesystems do not
know how to attack them. In an oxidizing environment many of the
small biogenicmolecules will be attacked by bacteria before they
can form geopolymers. In a low-oxygen(reducing) environment, in
contrast, the subdued level of bacterial activity allows more time
for theformation of geopolymers and, therefore, better organic
preservation.Kerogens formed under reducing conditions will be
composed of fragments of many kinds ofbiogenic molecules. Those
kerogens formed under oxidizing conditions, in contrast, contain
mainlythe most resistant types of biogenic molecules that were
ignored by microorganisms duringdiagenesis.
KEROGEN COMPOSITIONBecause each kerogen molecule is unique, it
is somewhat fruitless to attempt a detailed discussionof the
chemical composition of kerogens. Even if such a description were
possible, it would not beof great and direct significance to
exploration geologists. What is within our reach, and ultimatelyof
much greater practical value, is developing a general method of
describing gross kerogencomposition and relating it to
hydrocarbon-generative capacity. One way that we can begin is
byclassifying kerogens into a few general types.About a decade ago
workers at the French Petroleum Institute developed a useful scheme
fordescribing kerogens that is still the standard today. They
identified three main types of kerogen(called Types I, II, and III)
and have studied the chemical characteristics and the nature of
theorganisms from which all types of kerogens were derived.
Subsequent investigations have identifiedType IV kerogen as
well.
The four types of kerogen, the macerals that they arecomposed
of, and their organic precursors
Transformation of organic material in sediments andsedimentary
rocks.
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Kerogen - 19
Type I kerogen is quite rare because it is derived principally
from lacustrine algae. The best-knownexample is the Green River
Shale, of middle Eocene age, from Wyoming, Utah, and
Colorado.Extensive interest in those oilshale deposits has led to
many investigations of the Green River Shalekerogens and has given
Type I kerogens much more publicity than their general
geologicalimportance warrants. Occurrences of Type I kerogens are
limited to anoxic lakes and to a fewunusual marine environments.
Type I kerogens have high generative capacities for
liquidhydrocarbons.Type II kerogens arise from several very
different sources, including marine algae, pollen andspores, leaf
waxes, and fossil resin. They also include contributions from
bacterial-cell lipids. Thevarious Type II kerogens are grouped
together, despite their very disparate origins, because they
allhave great capacities to generate liquid hydrocarbons. Most Type
II kerogens are found in marinesediments deposited under reducing
conditions.Type III kerogens are composed of terrestrial organic
material that is lacking in fatty or waxycomponents. Cellulose and
lignin are major contributors. Type III kerogens have much
lowerhydrocarbon-generative capacities than do Type II kerogens
and, unless they have small inclusionsof Type II material, are
normally considered to generate mainly gas.Type IV kerogens contain
mainly reworked organic debris and highly oxidized material of
variousorigins. They are generally considered to have essentially
no hydrocarbon-source potential.Hydrogen contents of immature
kerogens (expressed as atomic H/C ratios) correlate with
kerogentype. In the immature state, Type I (algal) kerogens have
the highest hydrogen contents becausethey have few rings or
aromatic structures. Type II (liptinitic) kerogens are also high in
hydrogen.Type III (humic) kerogens, in contrast, have lower
hydrogen contents because they containextensive aromatic systems.
Type IV kerogens, which mainly contain polycyclic aromatic
systems,have the lowest hydrogen contents.Heteroatom contents of
kerogens also vary with kerogen type. Type IV kerogens are highly
oxidizedand therefore contain large amounts of oxygen. Type III
kerogens have high oxygen contentsbecause they are formed from
lignin, cellulose, phenols, and carbohydrates. Type I and Type
IIkerogens, in contrast, contain far less oxygen because they were
formed from oxygen-poor lipidmaterials.
Van Krevelen diagram showing maturationpathways for Types 1 to
IV kerogens astraced by changes in atomic HIC and OICratios. The
shaded areas approximatelyrepresent diagenesis, catagenesis,
andmetagenesis, successively.
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Kerogen - 20
Sulfur and nitrogen contents of kerogens are also variable and,
in some cases, interrelated. Nitrogenis derived mainly from
proteinaceous material, which is destroyed rapidly during
diagenesis. Mosthigh-nitrogen kerogens were therefore deposited
under anoxic conditions where diagenesis wasseverely limited.
Because lignins and carbohydrates contain little nitrogen, most
terrestriallyinfluenced kerogens are low in nitrogen.Kerogen
sulfur, in contrast, is derived mainly from sulfate that was
reduced by anaerobic bacteria.High-sulfur kerogens (and coals) are
almost always associated with marine deposition, because
freshwaters are usually low in sulfate. Sulfur is only incorporated
into kerogens in large quantities wheresulfate reduction is
extensive and where Fe +2 ions are absent (organic-rich, anoxic,
marine,nonclastic sediments). Many high-sulfur kerogens are also
high in nitrogen.The division of kerogens into Types I-IV on the
basis of chemical and hydrocarbon-generativecharacteristics has
been supported by another independent scheme for classifying
kerogens usingtransmitted-light microscopy. Kerogen types are
defined by the morphologies of the kerogenparticles. In many cases
the original cellular structure is still recognizable, proving the
origin of theparticle. In others the original fabric has
disappeared completely, forcing us to make assumptionsabout the
source organisms. Microscopic organic analysis has reached a fairly
high level ofrefinement and is often capable of assessing kerogen
type with good accuracy.The different types of kerogen particles
are called macerals, a term taken trom coal petrology.Macerals are
essentially organic minerals; they are to kerogen what minerals are
to a rock. Thekerogen in a given sedimentary rock includes many
individual particles that are often derived from avariety of
sources. Thus few kerogens consist of a single maceral type.Maceral
names were developed by coal petrologists to describe, wherever
possible, the materialsfrom which a maceral was derived. A list of
the most common macerals and their precursors isgiven in the table
presented earlier in this chapter.It is possible to make a
reasonably good correlation between kerogen type based on
chemicalcharacteristics and kerogen type based on visual
appearance. The correspondence is not perfect,however, because
there is not a perfect biological separation of the various types
of living organicmatter. The biggest problem comes in identifying
Type III kerogen. What appears to be vitrinite(Type III kerogen) by
visual analysis may have chemical characteristics intermediate
between TypeII and Type III kerogens because of the presence of
small amounts of resin or wax.
KEROGEN MATURATION
INTRODUCTIONVery important changes, called maturation, occur
when a kerogen is subjected to high temperaturesover long periods
of time. Thermal decomposition reactions, called catagenesis and
metagenesis,break off small molecules and leave behind a more
resistant kerogen residue. The small moleculeseventually become
petroleum and natural gas.By convention the term catagenesis
usually refers to the stages of kerogen decomposition duringwhich
oil and wet gas are produced. Metagenesis, which occurs after
catagenesis, represents dry-gas generation. Despite its name,
metagenesis is not equivalent to "metamorphism." Metagenesisbegins
long before true rock metamorphism, but it also continues through
the metamorphic stage.Although the terms catagenesis and oil
generation are often used synonymously, they are notprecisely
equivalent. Catagenesis and hydrocarbon generation occur
concurrently, but they reallyrepresent different aspects of the
same process. Catagenesis refers to transformations of
kerogenmolecules, whereas hydrocarbon generation focuses on the
production of hydrocarbon molecules. Inthis text we shall use the
terms somewhat interchangeably, especially when we are discussing
bothaspects simultaneously. In principle, however, they represent
fundamentally different perspectives.
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Kerogen - 21
This chapter will focus on those changes in the residual kerogen
that accompany catagenesis. Thecomposition of the products
(bitumen, oil, and gas) will be discussed in a following
chapter.Kerogen maturation is not a reversible process-any more
than baking a cake is reversible.Furthermore, the chemical process
of maturation never stops completely, even if drastic decreasesin
temperature occur. Chemical reaction-rate theory requires that the
rates of reactions decrease astemperature decreases, but it also
states that at any temperature above absolute zero reactions willbe
occurring at some definable rate. For practical purposes, however,
the rates of catagenesis aregenerally not important at temperatures
below about 70 C. Furthermore, in most cases decreasesof
temperature in excess of about 20-30 C due to subsurface events or
erosional removal willcause the rates of catagenesis to decrease so
much that it becomes negligible for practical purposes.It is
impossible to set precise and universal temperature limits for
catagenesis, because time alsoplays a role. Old rocks will often
generate hydrocarbons at significantly lower temperatures thanyoung
rocks, simply because the longer time available compensates for
lower temperatures. Thiscomplex interplay between the effects of
time and temperature on maturity is discussed in a
laterchapter.
EFFECTS OF MATURATION ON KEROGENSKerogen undergoes important and
detectable changes during catagenesis and metagenesis. Some ofthese
changes can be measured quantitatively, thus allowing us to judge
the extent to which kerogenmaturation has proceeded. The real
reason for following kerogen catagenesis, of course, is tomonitor
hydrocarbon generation. Although it is obvious that many measurable
changes in kerogensare related to hydrocarbon generation, it is
also true that other changes in kerogen properties havelittle or
nothing to do with it, and thus are not necessarily valid
indicators of hydrocarbongeneration. We shall look now at the
various techniques for estimating the extent of
hydrocarbongeneration from kerogen properties and see how closely
each of them is related to hydrocarbongeneration.As we saw earlier,
the cracking of any organic molecule requires hydrogen. The more
hydrogen akerogen contains, the more hydrocarbons it can yield
during cracking. Because many of the lightproduct molecules are
rich in hydrogen, the residual kerogen gradually becomes more
aromatic andhydrogen poor as catagenesis proceeds. Thus the steady
decrease in hydrogen content of a kerogen(usually measured as the
atomic hydrogen/carbon ratio) during heating can be used as an
indicatorof both kerogen catagenesis and hydrocarbon generation,
provided that the hydrogen content of thekerogen was known prior to
the onset of catagenesis.Nitrogen and sulfur are also lost from
kerogens during catagenesis. Nitrogen loss occurs primarilyduring
late catagenesis or metagenesis, after hydrogen loss is well
advanced. In contrast, much ofthe sulfur is lost in the earliest
stages of catagenesis, as evidenced by low maturity, high-sulfur
oilsfound in a number of areas, including the Miocene Monterey
Formation of southern California.The most important implication of
these chemical changes is that the remaining hydrocarbon-generative
capacity of a kerogen decreases during catagenesis and metagenesis.
All kerogensbecome increasingly aromatic and depleted in hydrogen
and oxygen during thermal maturation. Inthe late stages of
maturity, Types I, II, and III kerogens will therefore be very
similar chemically,possessing essentially no remaining hydrocarbon
generative capacity.Kerogen particles become darker during
catagenesis and metagenesis, much as a cookie brownsduring baking.
There is a steady color progression yellow-goldenorange-light
brown-dark brown-black as a result of polymerization and
aromatization reactions. These reactions are intimatelyrelated to
important changes in the chemical structure of kerogen, but they
are not necessarilyidentical with hydrocarbon generation. There is
therefore no necessary cause-and-effect relationship
-
Kerogen - 22
between kerogen darkening and hydrocarbon generation, and no
guarantee that a particular kerogencolor always heralds the onset
of oil generation.As kerogen matures and becomes more aromatic, its
structure becomes more ordered, because theflat aromatic sheets can
stack neatly. These structural reorganizations bring about changes
inphysical properties of kerogens. One property that is strongly
affected, and which can be used togauge the extent of molecular
reorganization, is the ability of kerogen particles to reflect
incidentlight coherently. The more random a kerogen's structure,
the more an incident light beam will bescattered, and the less it
will be reflected.Half a century ago coal petrologists discovered
that the percentage of light reflected by vitriniteparticles could
be correlated with coal rank measured by other methods.Because coal
rank is merely a measure of coal maturity, and because vitrinite
particles also occur inkerogens, the technique, called vitrinite
reflectance, has been widely and successfully applied inassessing
kerogen maturity.Cracking often produces free radicals, which are
unpaired electrons not yet involved in chemicalhonds. Kerogens,
especially highly aromatic ones, contain large numbers of unpaired
electrons. Theconcentration of free radicals in a given kerogen has
been found to increase with increasingmaturity. Free-radical
concentrations can be measured by electron-spin resonance.Kerogens
often fluoresce when irradiated. The intensity and wavelength of
the fluorescente arefunctions of kerogen maturity.Some properties
of kerogen change very little during catagenesis. For example,
carbon-isotopiccompositions of kerogens are affected little by
maturation. Except for darkening, the visualappearance of kerogen
also does not change during catagenesis: kerogen types are
generallyrecognizable until the particles become black and opaque,
somewhat beyond the oil-generationwindow.
Plot of bitumen generation as afunction of maturity (dashed
fine)compared to bitumen remaining inrock (solid line). The
differencebetween the two curves representsbitumen expelled from
the rock orcracked to light hydrocarbons.
HYDROCARBON GENERATIONAs kerogen catagenesis occurs, small
molecules are broken off the kerogen matrix. Some of theseare
hydrocarbons, while others are small heterocompounds. These small
compounds are much moremobile than the kerogen molecules and are
the direct precursors of oil and gas. A general name torthese
molecules is bitumen.Bitumen generation occurs mainly during
catagenesis; during metagenesis the chief product ismethane. If
neither expulsion from the source rock nor cracking of bitumen
occurred, there wouldbe a large and continuous build-up of bitumen
in the rock as a result of catagenetic decompositionof kerogen.
What actually occurs, however, is that some of the bitumen is
expelled from the sourcerock or cracked to gas, resulting in lower
bitumen contents in the source. Both curves are highly
-
Kerogen - 23
idealized, however, because natural variations among samples
cause much scatter in experimentaldata.It has become apparent in
recent years that not all kerogens generate hydrocarbons at the
samecatagenetic levels, as measured by parameters such as vitrinite
reflectance. Given the significantchemical differences among the
various types of kerogens, this result is hardly
surprising.Resinite and sulfur-rich kerogens are able to generate
liquid hydrocarbons earlier than otherkerogens because of the
particular chemical reactions occurring in those two materials.
Resiniteconsists of polymerized terpanes (ten-carbon isoprenoids)
that can decompose easily by reversingthe polymerization process.
Sulfur-rich kerogens decompose easily because carbon-sulfur
hbondsare weaker than any bonds in sulfur-poor kerogens.Effective
generation of hydrocarbons requires that the generated products be
expelled from thesource-rock matrix and migrated to a trap. Timing
and efficiency of expulsion depend on a numberof factors, including
rock physics and organic-geochemical considerations. We shall
consider thelatter briefly here.Many workers now believe that
microfracturing of source rocks is very important tor
hydrocarbonexpulsion. Microfracturing is related to overpressuring,
which in turn is partly attributed tohydrocarbon generation itself.
Rich rocks will become overpressured earlier than lean ones and
thuswill also expel hydrocarbons earlier. In very lean rocks
expulsion may occur so late that cracking ofthe generated bitumen
is competitive with expulsion. In such cases the expelled products
will bemainly gas.
SUMMARYKerogen begins to form during early diagenesis, when
large geopolymers are created frombiological molecules. The
chemical composition and morphology of kerogen macerals depend
bothon the type of original organic matter and on diagenetic
transformations. Numerous methods existfor tracing the history of a
kerogen and determining its original chemical and
physicalcharacteristics.Catagenesis of kerogen produces a more
aromatic, hydrogen-poor, residual kerogen as well assmall molecules
that are the direct precursors for petroleum and natural gas.
Several methods existfor estimating the extent to which hydrocarbon
generation has occurred in a given kerogen, butnone of these
measurements is closely linked to the actual process of hydrocarbon
generation.Thus, although we know that oil generation does occur
during the phase we call catagenesis, wecannot always define the
limits of hydrocarbon generation with great confidence.The chemical
composition of a kerogen controls the timing of hydrocarbon
generation and the typeof products obtained. Kerogens formed from
lipid-rich organic material are likely to generate
liquidhydrocarbons, whereas those kerogens that contain few lipids
will generate mainly gas. Kerogensformed from resinite will
generate condensates or light oils quite early. High-sulfur
kerogensgenerate heavy, high-sulfur oils at low levels of maturity.
Other kerogens usually follow a moretraditional model.Source rocks
that generate large amounts of hydrocarbons early are likely to
expel thosehydrocarbons early. Candidates for early expulsion would
be very organic rich rocks and thosecontaining resinite or
high-sulfur kerogens. Conversely, those rocks that generate few
hydrocarbonsmay not expel them until they have been cracked to
gas.
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Bitumen, Petroleum, and Natural Gas -24
5 - Bitumen, Petroleum, and Natural Gas
INTRODUCTIONPetroleum obtained from reservoir rocks and bitumen
extracted from fine-grained rocks have manysimilarities, but they
also exhibit many important differences. There is no doubt that
they arerelated; indeed, bitumen is almost universally accepted as
the direct precursor for petroleum.However, many unanswered
questions remain about the processes that transform bitumen
intopetroleum. Major compositional changes occur in going from
bitumen to petroleum, but we are notcertain whether they occur
mainly within the source rock or during migration through the
reservoirrock. We also do not know how much of the change involves
chemical reactions, and how much isdue to physical separation of
chemical compounds having very different properties. The influence
ofthe lithologies of source and reservoir rocks on these
compositional changes is poorly understood.Both bitumens and
petroleums exhibit a wide range of compositions. Much of this
variety is relatedto source-rock facies and the composition of the
kerogens that generated the bitumens. Maturityalso exerts control
over bitumen and petroleum composition. Reservoir transformations
in somecases greatly affect oil composition and properties.Bitumen
and petroleum compositions can also be used as tools in correlating
samples with eachother. Such correlations can be particularly
useful in establishing genetic relationships amongsamples. In order
to understand bitumen and petroleum compositions and to use them
forexploration, however, we must separate the characteristics
related to kerogen composition fromthose related to the
transformation of bitumen to petroleum and from those related to
changesoccurring in reservoirs. This chapter will compare and
contrast bitumen and petroleumcompositions and examine the factors
responsible for the observed differences.
COMPOUNDS PRESENT IN BITUMEN AND PETROLEUM
GENERAL CLASSES OF COMPOUNDSBoth bitumen and petroleum contain a
very large number of different chemical compounds. Some ofthese are
present in relatively large quantities, while others are only trace
contributors. In order toinvestigate the individual compounds
present, we first separate a crude oil or a bitumen into
severalfractions having distinct properties.Each of the fractions
contains certain types of chemical compounds. One fraction consists
mainly ofsaturated hydrocarbons; n-alkanes, branched hydrocarbons
(including isoprenoids), and cyclics.Saturated hydrocarbons are the
most thoroughly studied of the components of petroleum andbitumen
because they are the easiest to work with analytically.A second
fraction consists of aromatic hydrocarbons and some light
sulfur-containing compounds.Light aromatic hydrocarbons, like
benzene and toluene, have been studied in petroleums, but
thesecompounds are lost from bitumens during evaporation of the
solvent used in extracting the bitumenfrom the rock. Heavier
aromatic and naphthenoaromatic hydrocarbons, particularly those
derivedfrom diterpanes, triterpanes, and steranes, are more
commonly studied.Most of the NSO compounds appear in the remaining
two fractions. The lighter of these fractions,variously called
polars, NSOs, and resins, contains a wide variety of small and
medium-sizedmolecules with one or more heteroatoms. Few of these
heterocompounds have been studiedcarefully.The final fraction
contains very large, highly aromatic asphaltene molecules that are
often rich inheteroatoms. Asphaltenes tend to aggregate into stacks
because of their planarity, and formcomplexes with molecular
weights of perhaps 50,000. The large sizes of asphaltene units
render
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Bitumen, Petroleum, and Natural Gas - 25
them insoluble in light solvents. Asphaltenes can thus be
removed from oils or bitumens in thelaboratory or refinery by
adding a light hydrocarbon, such as pentane or propane. Because of
theirmolecular complexity and heterogeneity, asphaltene molecules
have not been studied in detail.
SPECIFIC COMPOUNDSBiomarkers. Many of the compounds and classes
of compounds that we find in crude oils andbitumens are called
biomarkers, an abbreviation for biological markers. These
compounds, whichare derived from biogenic precursor molecules, are
essentially molecular fossils. The most usefulbiomarkers serve as
indicators of the organisms from which the bitumen or petroleum was
derived,or of the diagenetic conditions under which the organic
matter was buried. In a few cases specificprecursor organisms or
molecules can be identified, whereas in other instances we may be
able tolimit the possible precursors to only a few species. In most
cases, however, although we know forcertain that the biomarker
molecule is biogenic, we are unable to use it as an "index fossil"
forspecific organisms.Other compounds. Many other types of organic
compounds in crude oils and bitumens are notconsidered to be
biomarkers because they cannot be related directly to biogenic
precursors. Theyare, however, of biological origin, but their
sources are simply no longer recognizable due todiagenetic and
catagenetic transformations.
FACTORS AFFECTING COMPOSITION OF BITUMEN AND PETROLEUM
SOURCE AND DIAGENESISBiomarkersn-Alkanes were among the first
biomarkers to be studied extensively. Their high concentration
inbitumens and oils is best explained by their existence in plant
and algal lipids, and by theircatagenetic formation from long-chain
compounds such as fatty acids and alcohols.Another important
indication of the origin of n-alkanes is the distribution of
individual homologs, ormembers of the n-alkane series. For the most
part n-alkanes present in terrestrial plants have oddnumbers of
carbon atoms, especially 23, 25, 27, 29, and 31 atoms.In contrast,
marine algae produce n-alkanes that have a maximum in their
distribution at C-17 or C-22, depending upon the species present.
The distributions are quite sharp, and no preference foreither odd-
or even-carbon homologs is evident.Many sediments, of course,
receive contributions of n-alkanes from both terrestrial and
marinesources. Their n-alkane distributions reflect this
mix.Sediments are also known that exhibit a strong preference for
n-alkanes having an even number ofcarbon atoms. These n-alkanes are
believed to be formed by hydrogenation (reduction) of long-chain
fatty acids and alcohols having even numbers of carbon atoms.
(Among the acids andalcohols present in living organisms,
even-carbon homologs predominate as strongly as do the odd-carbon
homologs among the n-alkanes.) Even-carbon preferences occur
principally in evaporiticand carbonate sediments, where input of
terrestrial n-alkanes is minimal and diagenetic conditionsare
highly reducing.Carbon Preference Index, or CPI, was developed as a
measure of the strength of the odd-carbonpredominance in n-alkanes
over the even alkanes (in the series from 23 upwards).The average
of two ranges is taken to minimize bias produced by the generally
decreasing n-alkaneconcentrations with increasing number of carbon
atoms. If the number of odd- and even-carbonmembers is equal, the
CPI is 1.0. If odd-carbon homologs predominate, the CPI is greater
than 1.0.However, because the concentration of n-alkanes often
decreases with increasing carbon number,the lower-carbon homologs
are given more weight in the calculation. CPI values can
therefore
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Bitumen, Petroleum, and Natural Gas -26
deviate from 1.0 even when no preference is distinguishable by
visual inspection of the distributioncurve.n-Alkane distributions
are greatly modified by thermal maturity. Chain lengths gradually
becomeshorter, and the original n-alkanes present in the immature
sample are diluted with new n-alkanesgenerated during catagenesis.
Because the newly generated n-alkanes show little or no
preferencefor either odd- or even-carbon homologs, CPI values
approach 1.0 as maturity increases.n-Alkane distributions in
bitumens and oils derived from algae do not show the influences
ofmaturity as clearly because the original CPI values are already
very close to 1.0. It is thereforeoften difficult to estimate
maturity levels in pelagic rocks on the basis of n-alkane
data.Parameters other than Biomarkers. Sulfur contents are also
strongly influenced by diageneticconditions. For economic and
environmental reasons, oils having more than about 0.5% sulfur
aredesignated as high-sulfur. Many high-sulfur oils contain 1%
sulfur or less, but in some areas sulfurcontents can reach 7%
(Monterey oils from the onshore Santa Maria area, southern
California, forexample). A few oils contain more than 10%.These
high-sulfur bitumens and crude oils are derived from high-sulfur
kerogens. As we sawearlier, sulfur is incorporated into kerogens
formed in nonclastic sediments that accumulate whereanaerobic
sulfate reduction is important. Most oils and bitumens derived from
lacustrine orordinary clastic marine source rocks will be low in
sulfur content, whereas those from euxinic oranoxic marine source
rocks will be high-sulfur.Sulfur occurs predominantly in the heavy
fractions of oils and bitumens, particularly in theasphaltenes.
High-sulfur oils therefore have elevated asphaltene contents.
RESERVOIR TRANSFORMATIONSIntroduction. There are two main types
of reservoir transformations that can affect crude oils(reservoir
transformations are not applicable to bitumen because, by
definition, the material in areservoir is petroleum). Thermal
processes occurring in reservoirs include cracking anddeasphalting.
Nonthermal processes are water washing and biodegradation. Of
these, cracking andbiodegradation are by far the most
important.Cracking and Deasphalting. Cracking, which breaks large
molecules down into smaller ones, canconvert a heavy,
heteroatom-rich off into a lighter, sweeter one. Waxy oils become
less waxy. APIgravities increase, and pour points and viscosities
decrease. When cracking is extreme, the productsbecome condensate,
wet gas, or dry gas.Cracking is a function of both time and
temperature, as well as of the composition of the oil and
thecatalytic potential of the reservoir rock. It is therefore
impossible to state that cracking alwaysoccurs at a certain depth
or reservoir temperature. Most oils, however, will be reasonably
stable atreservoir temperatures below about 90 C, regardless of the
length of time they spend there. On theother hand, a reservoir
above 120 C will contain normal oil only if the oil is a recent
arrival.Although the role of catalysis in hydrocarbon cracking in
reservoirs has not been proven, manyworkers suspect that clay
minerals are important facilitators of hydrocarbon breakdown.
Catalyticeffectiveness varies greatly from one clay mineral to
another, however, and our partialunderstanding of this difficult
subject is not of much practical use at the present time.Cracking
also brings about deasphalting, because asphaltene molecules become
less soluble as theoil becomes lighter. Precipitation of
asphaltenes in the reservoir will lower sulfur content andincrease
API gravity appreciably.Biodegradation and water washing. Water
washing involves selective dissolution of the mostsoluble
components of crude oils in waters that come in contact with the
oils. The smallesthydrocarbon molecules and the light aromatics,
such as benzene, are the most soluble. The effectsof water washing
are rather difficult to determine because they do not affect the
oil fractions that
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Bitumen, Petroleum, and Natural Gas - 27
are most frequently studied. Furthermore, in most cases the
effects are quite small because of thelow solubilities of all
hydrocarbons in water. Finally, water washing and biodegradation
often occurtogether, with the more dramatic effects of
biodegradation obscuring those of water washing.Biodegradation is a
transformation process of major importance. Under certain
conditions somespecies of bacteria are able to destroy some of the
compounds present in crude oil, using them as asource of energy.
The bacteria responsible for biodegradation are probably a mixture
of aerobic andanaerobic strains. Only aerobic bacteria are believed
to actually attack hydrocarbons, but anaerobesmay consume some of
the partially oxidized byproducts of initial aerobic attack.Because
biodegradation changes the physical properties of oils, it can have
serious negativefinancial implications. Heavily biodegraded oils
are often impossible to produce (Athabasca TarSands of Alberta,
Canada, and the Orinoco heavy oils of Venezuela, for example). If
production isphysically possible, it may be expensive or
uneconomic. It is therefore important to understandwhere and why
biodegradation occurs, and what its effects are on oil
composition.Biodegradation may actually start during oil migration
(provided required temperature and oxygenconditions are met),
because oil-water interactions are maximized then. Most
biodegradationprobably occurs within reservoirs, however, since the
length of time an oil spends in a reservoir isusually much longer
than its transit time during migration.Biodegradation can vary in
intensity from very light to extremely heavy. Because the chemical
andphysical properties of an oil change dramatically in several
predictable ways during biodegradation,biodegraded oils are easily
recognized. Many basins have at least a few biodegraded oils, and
insome areas they are epidemic.Bacteria that consume petroleum
hydrocarbons have strong preferences. Hydrocarbons are not
theirvery favorite foods, and they eat them only because there is
nothing else available. The preferredhydrocarbons are n-alkanes,
presumably because their straight-chain configurations allow
thebacterial enzymes to work on them most efficiently. Also
attractive to the "bugs" are long, alkylside-chains attached to
cyclic structures.After the n-alkanes and alkyl groups are
consumed, the bacteria begin to destroy compounds havingonly a
single methyl branch or those having widely spaced branches. Then
they move on to more-highly branched compounds, such as the
isoprenoids.In the last stages of biodegradation, polycyclic
alkanes are attacked.Because the hierarchy of bacterial attack on
crude oils is well known, it is possible to assess thedegree of
biodegradation by observing which compounds have been
destroyed.Sulfur contents of crude oils also increase as a result
of biodegradation. In a heavily biodegraded oilthe sulfur content
may increase by a factor of two or three. Sulfur is undoubtedly
concentrated inthe oil by selective removal of hydrocarbons, and
may also be added by bacterially mediated sulfatereduction.
COMPARISON OF BITUMEN AND PETROLEUMAlthough bitumens and crude
oils contain the same compounds, the relative amounts are
quitedifferent. In the process of converting bitumen to petroleum,
either the NSO compounds are lost inlarge quantities, or they are
converted to hydrocarbons. In actuality, both processes probably
occur,although selective loss of nonhydrocarbons during expulsion
is probably most effective inconcentrating the hydrocarbons.Bitumen
composition depends strongly on the lithology of the host rock.
Carbonates containbitumens that are much richer in heterocompounds
than are shales, and their hydrocarbon fractionsare more aromatic.
These differences are the result of the higher sulfur contents of
kerogens incarbonates. Oils derived from carbonate sources are also
richer in heterocompounds than oilssourced from shales.
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Bitumen, Petroleum, and Natural Gas -28
NATURAL GASNatural gas contains many different compounds,
although most of them are present only in tracequantities. The
principal components with which we shall be concerned are