Structural geology: Structural geology is the study of the architecture of the rocks(geometry of the rocks) with respect to their deformational histories. Use and importance: The study of geologic structures has been of prime importance in economic geology, both petroleum geology and mining geology. Folded and faulted rock strata commonly form traps for the accumulation and concentration of fluids such as petroleum and natural gas. Faulted and structurally complex areas are notable as permeable zones for hydrothermal fluids and the resulting concentration areas for base and precious metal ore deposits. Veins of minerals containing various metals commonly occupy faults and fractures in structurally complex areas. These structurally fractured and faulted zones often occur in association with intrusive igneous rocks. They often also occur around geologic reef complexes and collapse features such as ancient sinkholes. Deposits of gold, silver, copper, lead, zinc, and other metals, are commonly located in structurally complex areas. Structural geology is a critical part of engineering geology, which is concerned with the physical and mechanical properties of natural rocks. Structural fabrics and defects such as faults, folds, foliations and joints are internal weaknesses of rocks which may affect the stability of human engineered structures such as dams, road cuts, open pit mines and underground mines or road tunnels. Geotechnical risk, including earthquake risk can only be investigated by inspecting a combination of structural geology and geomorphology. In addition areas of karst landscapes which are underlain by underground caverns and potential sinkholes or collapse features are of importance for these scientists. In addition, areas of steep slopes are potential collapse or landslide hazards. Environmental geologists and hydrogeologists or hydrologists need to understand structural geology because structures are sites of groundwater flow and penetration, which may affect, for instance, seepage of toxic substances from waste dumps, or seepage of salty water into aquifers. Plate tectonics is a theory developed during the 1960s which describes the movement of continents by way of the separation and collision of crustal plates. It is in a sense structural geology on a planet scale, and is used throughout structural geology as a framework to analyze and understand global, regional, and local scale features. Measurement conventions(Strike&dip):The inclination of a planar structure in geology is measured by strike and dip. The strike is the line of intersection between the surface of dipping bed and a horizontal plane, , and the dip is the magnitude of inclination between bedding plane and horizontal plane.. For example; striking 25 degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.
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Structural & Petroleum Geology - KOYAPETEThe study of geologic structures has been of prime importance in economic geology, both petroleum geology and mining geology. Folded and faulted
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Transcript
Structural geology:
Structural geology is the study of the architecture of the rocks(geometry of the
rocks) with respect to their deformational histories.
Use and importance:
The study of geologic structures has been of prime importance in economic geology,
both petroleum geology and mining geology. Folded and faulted rock strata
commonly form traps for the accumulation and concentration of fluids such as
petroleum and natural gas. Faulted and structurally complex areas are notable as
permeable zones for hydrothermal fluids and the resulting concentration areas for
base and precious metal ore deposits. Veins of minerals containing various metals
commonly occupy faults and fractures in structurally complex areas. These
structurally fractured and faulted zones often occur in association with intrusive
igneous rocks. They often also occur around geologic reef complexes and collapse
features such as ancient sinkholes. Deposits of gold, silver, copper, lead, zinc, and
other metals, are commonly located in structurally complex areas.
Structural geology is a critical part of engineering geology, which is concerned with
the physical and mechanical properties of natural rocks. Structural fabrics and defects
such as faults, folds, foliations and joints are internal weaknesses of rocks which may
affect the stability of human engineered structures such as dams, road cuts, open pit
mines and underground mines or road tunnels.
Geotechnical risk, including earthquake risk can only be investigated by inspecting a
combination of structural geology and geomorphology. In addition areas of karst
landscapes which are underlain by underground caverns and potential sinkholes or
collapse features are of importance for these scientists. In addition, areas of steep
slopes are potential collapse or landslide hazards.
Environmental geologists and hydrogeologists or hydrologists need to understand
structural geology because structures are sites of groundwater flow and penetration,
which may affect, for instance, seepage of toxic substances from waste dumps, or
seepage of salty water into aquifers.
Plate tectonics is a theory developed during the 1960s which describes the movement
of continents by way of the separation and collision of crustal plates. It is in a sense
structural geology on a planet scale, and is used throughout structural geology as a
framework to analyze and understand global, regional, and local scale features.
Measurement conventions(Strike&dip):The inclination of a planar structure
in geology is measured by strike and dip. The strike is the line of intersection between
the surface of dipping bed and a horizontal plane, , and the dip is the magnitude of
inclination between bedding plane and horizontal plane.. For example; striking 25
degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.
Stress and Strain: The concepts of stress, strain and material behavior are fundamental to the understanding of geological structures including faults and folds.
Stress: Stress is the force applied to each unit area in particular direction.Measured in pascals, N/m2
Type of Stresses: 1-Normal stress:Perpendicular to plane a-Extensional stress b-Compressional stress 2-Shear stress:Parallel to plane
Strain: When rocks deform they are said to strain.A strain is a change in size, shape, or volumeof a material in response to applied stress. Strain is given infraction, no unit. Linear strain = ΔL / L.
Principal Stress Directions: The stress state at any given point can be described by a system of three principal stresses,normal to each other and along which there are noshear stress components. σ1 maximum principal stress. σ2 intermediate principal stress. σ3 minimum principal stress.
By understanding the constitutive relationships between stress and strain in rocks,
geologists can translate the observed patterns of rock deformation into a stress field
during the geologic past.
[
Why change in stress: Tectonic processes are responsible for the change in stress.
Three stages of deformation (strain) affected on the rocks:
1-Elastic deformation: the deformation of a body in proportion to the applied stress and its recovery once the stress is removed.
2-Elastic limit: it is limiting stress,if this exceeded, the body does not return to its original shape.
3-Plastic deformation:after the stress is released,the body not return to its original shape & size.
Two types of substances (rocks) affected by deformation:
Ductile rocks: The permanent deformation, without fracture in the shape of a solid.
Brittle rocks: The fracturing of a rock in response to stress with little or no permanent deformation prior to its rupture.
Geological Structures:
Common structures: 1. Faults 2. Folds 3. Joints 4. Unconformities
Implications: 1. Tectonic history 2. Mineral exploration 3. Gas and oil exploration 4. Geotechnical engineering
Fold :
The term fold is bent or curved of strata as a result of pressure and high temperature.
The basic cause is likely to be some aspect of plate tectonics.
Structure of a fold:
The upfold is called an anticline. The downfold is called a syncline.
The imaginary line joining the highest points along the upfold is called the crest line.
The flanks of a fold are known as the limbs.
The central line from which the rock strata dip away in opposing directions is called
Fold terminology. For more general fold shapes, a hinge curve replaces the hinge
line, and a non-planar axial surface replaces the axial plane.
Folds are classified by their size, fold shape, tightness, dip of the axial plane.
Strike and Dip Diagram
STRIKE: The direction of the line formed by the intersection of a horizontal plane with a bedding or fault plane. The trend of the rock/fault outcrop.
DIP: The angle formed by the intersection of a bedding or fault plane and the horizontal plane; measured in a vertical plane perpendicular to the strike.
Anticline: linear, strata normally dip away from axial center, oldest strata in center.
Syncline: linear, strata normally dip toward axial center, youngest strata in center.
Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.
Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.
Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.
symmetrical fold: two limbs are of equal steepness Assymmetrical fold: one limb is steeper than the other Recumbent fold: two limbs are nearly parallel
disconformities are parallel to bedding planes, they are difficult to see in nature(i.e.between sedimentary rocks showing visible indication of
depositional hiatus).
2) Angular Unconformity – A contact in which younger strata overlie an erosional surface on tilted or folded rock layers. This type of unconformity is easy to identify in nature(i.e. between tilted and
undeformed sediments).
Image provided by FCIT. Original image from Textbook of Geology by Sir Archibald
Geikie (1893).
3) Nonconformity – A contact in which an erosion surface on plutonic or metamorphic rock has been covered by younger sedimentary or volcanic rock.
4) Paraconformity- A contact between parallel layers formed by extended
periods of non-deposition (as opposed to being formed by erosion). These
are sometimes called "pseudounconformities") (i.e. a cryptic disconformity
for which there is not immediate evidence of missing sediment, but abrupt
changes in fossil fauna indicate adjacent beds are of significantly different
ages
.
What does the Grand Canyon tell us about unconformities and the base level
of erosion?
The time missing in an unconformity is known as a hiatus or lacuna. The
origin and history of an unconformity may be revealed by the study of cross
sections from the edge of sedimentary basins.
Since no single section likely records continuous sedimentation it is
important to demonstrate equivalence of widely separated sections through
the process of correlation to piece together a complete history of the planet.
Evidence of unconformities:
1- Sedimentary criteria in the continental environments:
a-Basal conglomerate.
b- Residual or weathered chert nodulus (chert in chalky Lst.)
c-Burried soil profile.
2- Sedimentary criteria in the non continental(marine) environments:
a-Glauconite:Green colour mineral(Fe.KSio2).
b-Phosphatized pebbles:Shell&bones of animals &fishes.
There are three hypothesis or theory for tectonic movements of the earth:
1-Seafloor spreading(1935-1940):
Age of oceanic crust; youngest (red) is along spreading centers.
Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic
crust is formed through volcanic activity and then gradually moves away from the
ridge. Seafloor spreading helps explain continental drift in the theory of plate
tectonics. When oceanic plates diverge, tensional stress causes fractures to occur in
the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to
form new sea floor. Older rocks will be found further away from the spreading zone
while younger rocks will be found nearer to the spreading zone
2-Continental drift(Wegener 1912):
Continental drift is the movement of the Earth's continents relative to each other by
appearing to drift across the ocean bed. The concept was independently (and more
fully) developed by Alfred Wegener in 1912. The theory of continental drift was
superseded by the theory of plate tectonics, which builds upon and better explains
why the continents move.
Alfred Wegener first proposes Continental Drift in his book published in 1915. Suggests that 200 million years ago there existed one large supercontinent
which he called Pangaea (All Land)(Figure). This was not really a new idea, but Wegener offered several lines of evidence in support of his proposal.
1. Fit of the Continents - Noted the similarity in the coastlines of North and South America and Europe and Africa. Today the fit is done at the continental shelf and it is nearly a perfect match.
2. Fossil Similarities - Mesosaurus, (Figure) reptile similar to modern alligator which lived in shallow waters of South America and Africa.
3. Rock Similarities a. Rocks of same age juxtaposed across ocean basins. (Figure)
b. Termination of mountain chains. (Figure)
4. Paleoclimatic Evidence a. Glacial deposits at equator b. Coral reefs in Antarctica
Idea was rejected by North American geologists because Wegener couldn't come up with a mechanism for continental drift. Suggested tidal forces, but physicists showed this to be impossible. Wegener dies in 1930 and his idea dies with him.
Pangea:
Pangea was a supercontinent which existed during the Permian Period about 225 million years ago.
Diagram of five maps of the Earth showing Pangea and the positions of the continents as they split apart over time, from the U.S.
Geological Survey. According to the continental drift theory, the supercontinent Pangaea began to break up about 225-200 million
years ago, eventually fragmenting into the continents as we know them today. Continental drift was the forerunner of the theory of
plate tectonics.
3- Plate Tectonics theory(1960-1970):
Map of the Earth's tectonic plates from the US Geological Survey.
The lithosphere consists of the Earth's crust and part of the uppermost mantle. The Earth's surface or lithosphere is divided into
about 7 large plates and 20 smaller ones.
Seven Major Plates (See Figure)
1. Pacific 2. North American 3. South American 4. African 5. Eurasian 6. Antarctic 7. Indo-Australian 8. There are dozens of smaller plates, 9. the seven largest of which are:
What is the origin of plate tectonics? The continents drift slowly (the timescale
for substantial change is 10-100 million years), but that they drift at all is
remarkable. The following figure illustrates the structure of the first 100-200
kilometers of the Earth's interior, and provides an answer to this question.
The lithosphere and the aesthenosphere
The crust is thin, varying from a few tens of kilometers thick beneath the
continents to less than 10 km thick beneath the many of the oceans. The crust
and upper mantle together constitute the lithosphere, which is typically 50-100
km thick and is broken into large plates (not illustrated). These plates sit on the
aesthenosphere.
The aesthenosphere is kept plastic (deformable) largely through heat generated
by radioactive decay. The material that is decaying is primarily radioactive
isotopes of light elements like aluminum and magnesium. This heat source is
small on an absolute scale (the corresponding heat flow at the surface out of the
Earth is only about 1/6000 of the Solar energy falling on the surface).
What forces drive plate tectonics?
Basically, the driving forces that are advocated at the moment, can be divided in three
categories:1- mantle dynamics related, 2- gravity related (mostly secondary forces),
3- Earth rotation related (secondary forces).
1-Mantle dynamics related driving forces:
Convection Currents The mantle is made of much denser, thicker material, because of this the plates
"float" on it like oil floats on water.
Many geologists believe that the mantle "flows" because of convection
currents. Convection currents are caused by the very hot material at the
deepest part of the mantle rising, then cooling, sinking again and then heating,
rising and repeating the cycle over and over. The next time you heat anything
like soup or pudding in a pan you can watch the convection currents move in
the liquid. When the convection currents flow in the mantle they also move the
crust. The crust gets a free ride with these currents. A conveyor belt in a
factory moves boxess like the convection currents in the mantle moves the
plates of the Earth.
Convection cells. Roughly circular.Mantle heat probably due to radioactive decay .
Plate tectonics is driven by the convection in the asthenosphere (part of the Earth's mantle).
Conceptual drawing of assumed convection cells in the mantle. Below a depth of about 700 km, the des cending slab begins to soften and flow, losing its form
Types of plate tectonic boundaries
There are three major types of plate tectonic boundaries. These include:
1. Divergent plate boundaries where plates move apart from one another.
2. Convergent plate boundaries where plates move toward one another.
3. Transform plate boundaries whe re plates slide past one another.
Artist's cross section illustrating the main types of plate boundaries. Cross section by José F. Vigil from This Dynamic Planet -- a wall
map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory.
Image courtesy of U. S. Geological Survey.
1. Divergent - where the plates are moving apart.
Animation of divergent plate motion at a mid-ocean ridge..
Examples: mid-ocean ridges such as the Mid-Atlantic Ridge (the site of sea-floor spreading), and continental rifts such as
the east African Rift system.
Animation of divergent plate motion. (Constructive margin).
2. Convergent - where the plates are moving toward one
Seismograph:Instrument which is recorded the waves of an earthquake.
Causes of earthquakes:
1-Superficial movements.(eg:dashing waves cause vibrations along the sea-
shores,such as Tsunamis travel rapidly & are very destructive when they reach
land with 750 Km|hour.
2-Volcanic eruptions.
3-Folding &faulting.
Classification of earthquakes:
The earthquakes have been classified on the basis of their:
1-Intensity,2-Causes of origin,3-Depth of the shock originated.
But the classification based on the depth of the shock originated is widely used
into the following three categories:
1-Shallow earthquake less than 50Km depth.
2-Intermediate earthquake 50-300 Km depth.
3-Deep earthquake :shock is originated from more than 300 Km depth.
(eg:earthquake in indian,1905 Penjab,1934Bihar).
Engineering consideration of earthquake:
He only solution that can be done at the best is to provide additional factors in
the design of structure to minimize the losses due to shock of an earthquake, this
can be done in the following way:
1-To collect sufficient data,regarding the previous seismic activity in the area.
2-To provide factors of safety, to stop or minimize the loss due to severe
earthquake.
(eg:the foundation of building,dam should rest on asolid rock bed,best materials
should be used in the petroleum engineering projects.)
Petroleum geology:
Petroleum geology is the study of origin, occurrence, movement, accumulation, and
exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines
that are applied to the search for hydrocarbons (oil exploration).
Introduction and Summary
PETROLEUM (rock-oil, from the Latin petra, rock or stone, and oleum, oil) occurs widely in the earth as gas, liquid, semisolid, or solid, or in more than one of these states at a single place. Chemically any petroleum is an extremely complex mixture of hydrocarbon (hydrogen and carbon) compounds, with minor amounts of nitrogen, oxygen, and sulfur as impurities. Liquid petroleum, which is called crude oil to distinguish it from refined oil, is the most important commercially. It consists chiefly of the liquid hydrocarbons, with varying amounts of dissolved gases, bitumens, and impurities. It has an oily appearance and feel; in fact, it resembles the ordinary lubricating oil sold at filling stations, is immiscible with water and floats on it, but is soluble in naphtha, carbon disulfide, ether, and benzene. Petroleum gas, commonly called natural gas to distinguish it from manufactured gas, consists of the lighter paraffin hydrocarbons, of which the most abundant is methane gas (CH4). The semisolid and solid forms of petroleum consist of the heavy hydrocarbons and bitumens. They are called asphalt, tar, pitch, albertite, gilsonite, or grahamite, or by any one of many other terms, depending on their individual characteristics and local usage. The general term "bitumen" has long been used interchangeably with "petroleum" for both the liquid and the solid forms. Hydrocarbon is a term often used interchangeably with "petroleum" for any of its forms.
A porous and permeable body of rock, called the reservoir rock, which is overlain by an impervious rock, called the roof rock, contains oil or gas or both, and is deformed or obstructed in such a manner that the oil and gas are trapped.
Commercial deposits of crude oil and natural gas are always found underground, where they nearly always occur in the water-coated pore spaces of sedimentary rocks. Being lighter than water, the gas and oil rise and are concentrated in the highest part of the container; in order to prevent their escape, the upper contact of the porous rock with an impervious cover must be concave, as viewed from below. Such a container is called a trap, and the portion of the trap that holds the pool of oil or gas is called the reservoir. The significant thing is that reservoirs can be of various shapes, sizes, origins, and rock compositions.
Any rock that is porous and permeable may become a reservoir, but those properties are most commonly found in sedimentary rocks, especially sandstones and carbonates. A trap may be formed, either ' wholly or partly, by the deformation of the reservoir rock, which may be accomplished by folding, faulting, or both, and in either a single episode or in several episodes.
Or a trap may be formed, either wholly or partly, by stratigraphic variations in the reservoir rock. These may be primary, such as original facies changes,
Petroleum-Bearing Rocks: Sedimentary rocks are the most important and interesting type of rock to the
petroleum industry because most oil and gas accumulations occur in them; igneous
and metamorphic rocks rarely contain oil and gas.
All petroleum source rocks are sedimentary.
Furthermore, most of the world’s oil lies in sedimentary rock formed from marine
sediments deposited on the edges of continents. For example, there are many large
deposits that lie along the Gulf of Mexico and the Persian Gulf.
Origin of the petroleum:
Abiogenic petroleum origin
Abiogenic petroleum origin is a hypothesis that was proposed as an alternative
mechanism of petroleum origin. Geologists now consider the abiogenic formation of
petroleum scientifically unsupported.
According to the abiogenic hypothesis, petroleum was formed from deep carbon
deposits, perhaps dating to the formation of the Earth. Supporters of the abiogenic
hypothesis suggest that a great deal more petroleum exists on Earth than commonly
thought, and that petroleum may originate from carbon-bearing fluids that migrate
upward from the mantle. The presence (oceans) of methane on Saturn's moon Titan
and in the atmospheres of Jupiter, Saturn, Uranus and Neptune is cited as evidence of
the formation of hydrocarbons without biology.
These abiogenic origin theories are:
1-Cosmic theory:Suppose that most of planet of solar system have saturated
hydrocarbons gases as semiliquid state.When the earth is cold the hydrocarbon
materials accumulated on the earth surface rocks &form accumulation of
hydrocarbon deposits.
2-Volcanic theory:Eruption of hydrocarbon gases from volcanic activity.
3-Magmatic theory:
4-Chemical theory:Ethane from polymerization.
Biogenic petroleum origin:
Oil forms from the decay and transformation of dead organisms buried in
sedimentary rocks .Petroleum geologists agree that oil originates from to vast
quantities of dead marine plankton or plant material that sank into the mud of shallow
seas. Under the resulting anaerobic conditions, organic compounds remained in a
reduced state where anaerobic bacteria converted to hydrocarbon materials,
Analysis of maturation involves assessing the thermal history of the source rock in
order to make predictions of the amount and timing of hydrocarbon generation and
expulsion.
Finally, careful studies of migration reveal information on how hydrocarbons move
from source to reservoir and help quantify the source (or kitchen) of hydrocarbons in a
particular area.
Factors required to make an Oil deposit :
• Source rock- rich in organic matter
• Burial heating- > maturation
• Reservoir rock- porous and permeable
• Trap-
• structural trap
• stratigraphic trap.
Source rock:
Characters of Source rock
� Black organic-rich marine shales
� Organic matter is preserved low-oxygen water
� Restricted marine basins and zones were water rises from the deep
In petroleum geology, source rock refers to rocks from which hydrocarbons have
been generated or are capable of being generated. They are organic-rich sediments
that may have been deposited in a variety of environments including deep water
marine, lacustrine and deltaic. Oil shale can be regarded as an organic-rich but
immature source rock from which little or no oil has been generated and expelled.
Types of source rock
Source rocks are classified from the types of kerogen that they contain, which in turn
governs the type of hydrocarbons that will be generated.
Type 1 source rocks are formed from algal remains deposited under anoxic conditions in deep lakes: they tend to generate waxy crude oils when submitted to thermal stress during deep burial.
Type 2 source rocks are formed from marine planktonic and bacterial remains preserved under anoxic conditions in marine environments: they produce both oil and gas when thermally cracked during deep burial.
Type 3 source rocks are formed from terrestrial plant material that has been decomposed by bacteria and fungi under oxic or sub-oxic conditions: they tend to generate mostly gas with associated light oils when thermally cracked during deep burial. Most coals and coaly shales are generally Type 3 source rocks.
Maturation and expulsion
With increasing burial by later sediments and increase in temperature, the kerogen
within the rock begins to break down. This thermal degradation or cracking releases
shorter chain hydrocarbons from the original large and complex molecules found in
the kerogen.
The hydrocarbons generated from thermally mature source rock are first expelled,
along with other pore fluids, due to the effects of internal source rock over pressuring
caused by hydrocarbon generation as well as by compaction. Once released into
porous and permeable carrier beds or into faults planes, oil and gas then move
upwards towards the surface, an overall buoyancy driven process known as secondary
migration.
Petroleum reservoir:
A petroleum reservoir, or oil and gas reservoir, is a subsurface pool of
hydrocarbons contained in porous or fractured rock formations. The naturally
occurring hydrocarbons, such as crude oil or natural gas, are trapped by overlying
rock formations with lower permeability.
Formation:
Crude oil found in all oil reservoirs formed in the Earth's crust from the remains of
once-living things. Crude oil is properly known as petroleum, and is used as fossil
fuel. Evidence indicates that millions of years of heat and pressure changed the
remains of microscopic plant and animal into oil and natural gas.
Although the process is generally the same, various environmental factors lead to the
creation of a wide variety of reservoirs. Reservoirs exist anywhere from the land
surface to 30,000 ft (9,000 m) below the surface and are a variety of shapes, sizes and
ages.
Classifiation of reservoir rocks: A-Primary classification( simple& broad):
The above series of diagrams is an attempt to illustrate a type of stratigraphic trap. In
the diagram at the upper left, we see a river that is meandering. As it does so, it
deposits sand along its bank. Further away from the river is the floodplain, where
broad layers of mud are deposited during a flood. Though they seem fairly constant,
rivers actually change course frequently, eventually moving to new locations.
Sometimes these new locations are miles away from their former path. In the diagram
at the upper right, we show what happens when a river changes its course. The sand
bars that were deposited earlier are now covered by the mud of the new floodplain.
These lenses of sand, when looked at from the side many years later (the bottom
diagram), become cut off from each other, and are surrounded by the mud of the
river's floodplain - which will eventually turn to shale. This makes for a perfect
stratigraphic trap.
Types of Hydrocarbon Trap
Diagrams of structural and stratigraphic traps
A trap is a geologic structure or a stratigraphic feature capable of retaining hydrocarbons. Hydrocarbon traps that result from changes in rock type or pinch-outs, unconformities, or other sedimentary features such as reefs or buildups are called stratigraphic traps. Hydrocarbon traps that form in geologic structures such as folds and faults are called structural traps. Any mixture of structural and stratigraphic elements is called a combination trap.
Seals
The seal is a fundamental part of the trap that prevents hydrocarbons from further
upward migration(anhydrite, gypsum)
A capillary seal is formed when the capillary pressure across the pore throats is
greater than or equal to the buoyancy pressure of the migrating hydrocarbons. They
do not allow fluids to migrate across them until their integrity is disrupted, causing
them to leak.
Oil and Gas Traps
All oil and gas deposits are found in structural or stratigraphic traps. You may have
heard that oil is found underground in “pools,” “lakes,” or “rivers.” Maybe someone
told you there was a “sea” or “ocean” of oil underground. This is all completely
Maturation of kerogen is a function of increased burial and temperature and is
accompanied by chemical changes.
As kerogen thermally matures and increases in carbon content, it changes form an
immature light greenish-yellow color to an overmature black, which is representative
of a progressively higher coal rank. Different types of kerogen can be identified, each
with different concentrations of the five primary elements, carbon, hydrogen, oxygen,
nitrogen, and sulphur, and each with a different potential for generating petroleum.
The organic content of a rock that is extractable with organic solvents is known as
bitumen. It normally forms a small proportion of the total organic carbon in a rock.
Bitumen forms largely as a result of the breaking of chemical bonds in kerogen as
temperature rises. Petroleum is the organic substance recovered from wells and found
in natural seepages. Bitumen becomes petroleum at some point during migration. Important chemical differences often exist between source rock extracts (bitumen) and
crude oils (petroleum).
Kerogen is of no commercial significance except where it is so abundant (greater than
10%) as to occur in oil shales. It is, however, of great geological importance because it is
the substance that generates hydrocarbon oil and gas. A source rock must contain
significant amounts of kerogen.
Crude Oil
Crude oil is a mixture of many hydrocarbons that are liquid at surface temperatures
and pressures, and are soluble in normal petroleum solvents. It can vary in type and
amount of hydrocarbons as well as which impurities it may contain.
Crude oil may be classified chemically (e.g. paraffinic, naphthenic) or by its density.
This is expressed as specific gravity or as API (American Petroleum Institute) gravity
according to the formula:
sp.grav.@60 F141.5
API o o - 131.5
Specific gravity is the ratio of the density of a substance to the density of water.
API gravity is a standard adopted by the American Petroleum Institute for expressing
the specific weight of oils.
The lower the specific gravity, the higher the API gravity, for example, a fluid
with a specific gravity of 1.0 g cm –3 has an API value of 10 degrees. Heavy oils are
those with API gravities of less than 20 (sp. gr. >0.93). These oils have frequently
suffered chemical alteration as a result of microbial attack (biodegradation) and other
effects. Not only are heavy oils less valuable commercially, but they are considerably
more difficult to extract. API gravities of 20 to 40 degrees (sp. gr. 0.83 to 0.93)
indicate normal oils.
Oils of API gravity greater than 40 degrees (sp. gr. < 0.83) are light.
Asphalt
Asphalt is a dark colored solid to semi-solid form of petroleum (at surface
temperatures and pressures) that consists of heavy hydrocarbons and bitumens. It can
occur naturally or as a residue in the refining of some petroleums. It generally
contains appreciable amounts of sulphur, oxygen, and nitrogen and unlike kerogen,
asphalt is soluble in normal petroleum solvents. It is produced by the partial
maturation of kerogen or by the degradation of mature crude oil. Asphalt is
particularly suitable for making high-quality gasoline and roofing and paving
materials.
Natural Gas
There are two basic types of natural gas, biogenic gas and thermogenic gas. The
difference between the two is contingent upon conditions of origin. Biogenic gas is a
natural gas formed solely as a result of bacterial activity in the early stages of
diagenesis, meaning it forms at low temperatures, at overburden depths of less than
3000 feet, and under anaerobic conditions often associated with high rates of marine
sediment accumulation. Because of these factors, biogenic gas occurs in a variety of
environments, including contemporary deltas of the Nile, Mississippi and Amazon
rivers. Currently it is estimated that approximately 20% of the worlds known natural
gas is biogenic.
Thermogenic gas is a natural gas resulting from the thermal alteration of kerogen due
to an increase in overburden pressure and temperature.
The major hydocarbon gases are: methane (CH4 ), ethane (C2H6), propane (C3H8),
and butane (C4H10).
The terms sweet and sour gas are used in the field to designate gases that are low or
high, respectively, in hydrogen sulfide.
Natural gas, as it comes from the well, is also classified as dry gas or wet gas,
according to the amount of natural gas liquid vapors it contains. A dry gas contains
less than 0.1 gallon natural gas liquid vapors per 1,000 cubic feet, and a wet gas 0.3 or
more liquid vapors per 1,000 cubic feet.
Condensates
Condensates are hydrocarbons transitional between gas and crude oil (gaseous in the
subsurface but condensing to liquid at surface temperatures and pressures).
Chemically, condensates consist largely of paraffins, such as pentane, octane, and
hexane.
There are five types of sedimentary rocks that are important in the production of hydrocarbons: Sandstones
Sandstones are clastic sedimentary rocks composed of mainly sand size particles or
grains set in a matrix of silt or clay and more or less firmly united by a cementing
material (commonly silica, iron oxide, or calcium carbonate). The sand particles
usually consist of quartz, and the term “sandstone”, when used without qualification,
indicates a rock containing about 85-90% quartz.
Carbonates, broken into two categories, limestones and dolomites.
Carbonates are sediments formed by a mineral compound characterized by a
fundamental anionic structure of CO3-2. Calcite and aragonite CaCO3, are examples
of carbonates. Limestones are sedimentary rocks consisting chiefly of the mineral
calcite (calcium carbonate, CaCO3), with or without magnesium carbonate.
Limestones are the most important and widely distributed of the carbonate rocks.
Dolomite is a common rock forming mineral with the formula CaMg(CO3)2. A
sedimentary rock will be named dolomite if that rock is composed of more than 90%
mineral dolomite and less than 10% mineral calcite.
Shales
Shale is a type of detrital sedimentary rock formed by the consolidation of fine-
grained material including clay, mud, and silt and have a layered or stratified structure
parallel to bedding. Shales are typically porous and contain hydrocarbons but
generally exhibit no permeability. Therefore, they typically do not form reservoirs but
do make excellent cap rocks. If a shale is fractured, it would have the potential to be a
reservoir.
Evaporites
Evaporites do not form reservoirs like limestone and sandstone, but are very
important to petroleum exploration because they make excellent cap rocks and
generate traps. The term “evaporite” is used for all deposits, such as salt deposits, that
are composed of minerals that precipitated from saline solutions concentrated by
evaporation. On evaporation the general sequence of precipitation is: calcite, gypsum
or anhydrite, halite, and finally bittern salts.
Evaporites make excellent cap rocks because they are impermeable and, unlike
lithified shales, they deform plastically, not by fracturing.
The formation of salt structures can produce several different types of traps. One type
is created by the folding and faulting associated with the lateral and upward
movement of salt through overlying sediments. Salt overhangs create another type of
trapping mechanism.
Exploration and Mapping Techniques
Exploration for oil and gas has long been considered an art as well as a science. It
encompasses a number of older methods in addition to new techniques. The
exploration is must combine scientific analysis and an imagination to successfully solve
the problem of finding and recovering hydrocarbons.
Subsurface Mapping Geologic maps are a representation of the distribution of rocks and other geologic
materials of different lithologies and ages over the Earth’s surface or below it. The
geologist measures and describes the rock sections and plots the different formations on a
map, which shows their distribution. Just as a surface relief map shows the presence of
mountains and valleys, subsurface mapping is a valuable tool for locating underground
features that may form traps or outline the boundaries of a possible reservoir.
Subsurface mapping is used to work out the geology of petroleum deposits. Threedimensional
subsurface mapping is made possible by the use of well data and helps to
decipher the underground geology of a large area where there are no outcrops at the
surface.
Some of the commonly prepared subsurface geological maps used for exploration and
production include; (1) geophysical surveys, (2) structural maps and sections, (3)
isopach maps, and (4) lithofacies maps.
1-Geophysical Surveys Geophysics is the study of the earth by quantitative physical methods. Geophysical
techniques such as seismic surveys, gravity surveys, and magnetic surveys provide a way
of measuring the physical properties of a subsurface formation. These measurements are
translated into geologic data such as structure, stratigraphy, depth, and position. The
practical value in geophysical surveys is in their ability to measure the physical properties
of rocks that are related to potential traps in reservoir rocks as well as documenting
regional structural trends and overall basin geometry.
a-Seismic Surveys The geophysical method that provides the most detailed picture of subsurface geology is
the seismic survey. This involves the natural or artificial generation and propagation of
seismic (elastic) waves down into Earth until they encounter a discontinuity (any
interruption in sedimentation) and are reflected back to the surface. On-land, seismic
“shooting” produces acoustic waves at or near the surface by energy sources such as
dynamite, a “Thumper” (a weight dropped on ground surface), a “Dinoseis” (a gas gun),
or a “Vibroseis” (which literally vibrates the earth’s surface).
Seismic waves travel at known but varying velocities depending upon the kinds of rocks
through which they pass and their depth below Earth’s surface. The speed of sound
waves through the earth’s crust varies directly with density and inversely with porosity.
Through soil, the pulses travel as slowly as 1,000 feet per second, which is comparable to
the speed of sound through air at sea level. On the other hand, some metamorphic rocks
transmit seismic waves at 20,000 feet (approximately 6 km) per second, or slightly less
than 4 miles per second. Some typical average velocities are: shale = 3.6 km/s;
sandstone = 4.2 km/s; limestone = 5.0 km/s.
b-Magnetic Surveys Magnetic surveys are methods that provide the quickest and least expensive way to study
gross subsurface geology over a broad area. A magnetometer is used to measure local
variations in the strength of the earth’s magnetic field and, indirectly, the thickness of
sedimentary rock layers where oil and gas might be found. Igneous and metamorphic
rocks usually contain some amount of magnetically susceptible iron-bearing minerals and
are frequently found as basement rock that lies beneath sedimentary rock layers.
Basement rock seldom contains hydrocarbons, but it sometimes intrudes into the
overlying sedimentary rock, creating structures such as folds and arches or anticlines that
could serve as hydrocarbon traps. Geophysicists can get a fairly good picture of the
configuration of the geological formations by studying the anomalies, or irregularities, in
the structures.
The earth’s magnetic field, although more complex, can be thought of as a bar magnet,
around which the lines of magnetic force form smooth, evenly spaced curves. If a small
piece of iron or titanium is placed within the bar magnet’s field it becomes weakly
magnetized, creating an anomaly or distortion of the field. The degree to which igneous
rocks concentrate this field is not only dependent upon the amount of iron or titanium
present but also upon the depth of the rock. An igneous rock formation 1,000 feet below
the surface will affect a magnetometer more strongly than a similar mass 10,000 feet
down.
.
c-Gravity Surveys The gravity survey method makes use of the earth’s gravitational field to determine the
presence of gravity anomalies (abnormally high or low gravity values) which can be
related to the presence of dense igneous or metamorphic rock or light sedimentary rock in
the subsurface. Dense igneous or metamorphic basement rocks close to the surface will
read much higher on a gravimeter because the gravitational force they exert is more
powerful than the lighter sedimentary rocks. The difference in mass for equal volumes of
rock is due to variations in specific gravity.
Some types of maps: 1-Structural Contour Maps
Contour maps show a series of lines drawn at regular intervals. The points on each line
represent equal values, such as depth or thickness. One type of contour map is the
structural map, which depicts the depth of a specific formation from the surface. The
principle is the same as that used in a topographic map, but instead shows the highs and
lows of the buried layers.
Contour maps for exploration may depict geologic structure as well as thickness of
formations. They can show the angle of a fault and where it intersects with formations
and other faults, as well as where formations taper off or stop abruptly. The subsurface
structural contour map is almost or fully dependent on well data for basic control
Cross-Sections Structural, stratigraphic, and topographic information can be portrayed on cross-sections
that reproduce horizontally represented map information in vertical section. Maps
represent information in the plan view and provide a graphic view of distribution. Cross
sections present the same information in the vertical view and illustrate vertical
relationships such as depth, thickness, superpostion, and lateral and vertical changes of
geologic features.
Raw data for cross-sections come from stratigraphic sections, structural data, well sample
logs, cores, wireline logs, and structural, stratigraphic, and topographic maps.
2-Isopach Maps Isopach maps are similar in appearance to contour maps but show variations in the
thickness of the bed. These maps may be either surface or subsurface depending on data
used during construction. Isopach maps are frequently color coded to assist visualization
and are very useful in following pinch outs or the courses of ancient stream beds.
Porosity or permeability variations may also be followed by such means. Geologists use
isopach maps to aid in exploration work, to calculate how much petroleum remains in a
formation, and to plan ways to recover it.
3-Lithofacies Maps Lithofacies maps show, by one means or another, changes in lithologic character and how
it varies horizontally within the formation. This type of map has contours representing
the variations in the proportion of sandstone, shale, and other kinds of rocks in the
formation.
Identification of source and reservoir rocks, their distribution, and their thickness’ are
essential in an exploration program, therefore, exploration, particularly over large areas,
requires correlation of geologic sections. Correlations produce cross-sections that give
visual information about structure, stratigraphy, porosity, lithology and thickness of
important formations. This is one of the fundamental uses of well logs for geologists.
Wells that have information collected by driller’s logs, sample logs, and wireline logs
enable the geologists to predict more precisely where similar rock formations will occur
in other subsurface locations.
Surface Geology There are several areas to look for oil. The first is the obvious, on the surface of the
ground. Oil and gas seeps are where the petroleum has migrated from its’ source
through either porous beds, faults or springs and appears at the surface. Locating seeps at
the surface was the primary method of exploration in the late 1800’s and before.
Seeps are abundant and well documented worldwide. Oil or gas on the surface, however,
does not give an indication of what lies in the subsurface. It is the combination of data
that gives the indication of what lies below the surface. Geologic mapping, geophysics,
geochemistry and aerial photography are all crucial aspects in the exploration for oil and
gas.
Subsurface Geology and Formation Evaluation Subsurface geology and formation evaluation covers a large range of measurement and
analytic techniques. To complete the task of defining a reservoir’s limits, storage
capacity, hydrocarbon content, produce ability, and economic value, all measurements
must be taken into account and analyzed.
First, a potential reservoir must be discovered before it can be evaluated. The initial
discovery of a reservoir lies squarely in the hands of the exploration is it using seismic
records, gravity, and magnetics.
There are a number of parameters that are needed by the exploration and evaluation team
to determine the economic value and production possibilities of a formation. These
parameters are provided from a number of different sources including, seismic records,
coring, mud logging, and wireline logging.
Log measurements, when properly calibrated, can give the majority of the parameters
required. Specifically, logs can provide a direct measurement or give a good indication
of:
1-Porosity, both primary and secondary.2-Permeability.3-Water saturation and hydrocarbon
movability.4-Hydrocarbon type (oil, gas, or condensate).5-Lithology.6-Formation dip and
structure.7-Sedimentary environment.Travel times of elastic waves in a formation
These parameters can provide good estimates of the reservoir size and the hydrocarbons
in place.
Logging techniques in cased holes can provide much of the data needed to monitor
primary production and also to gauge the applicability of waterflooding and monitor its
progress when installed. In producing wells, logging can provide measurements of :
Hole diameter (dh)— The size of the borehole determined by the diameter of the
drill bit.
References:
Raymond Siever, Sand, Scientific American Library, New York (1988), ISBN 0-7167-5021-X.
1. P.E. Potter, J.B. Maynard, and P.J. Depetris, Mud and Mudstones: Introduction and Overview Springer, Berlin (2005) ISBN 3-540-22157-3.
2. Georges Millot, translated [from the French] by W.R. Farrand, Helene Paquet, Geology Of Clays - Weathering, Sedimentology, Geochemistry Springer Verlag, Berlin (1970), ISBN 0-412-10050-9.
3. Gary Nichols, Sedimentology & Stratigraphy, Wiley-Blackwell, Malden, MA (1999), ISBN 0-632-03578-1.
4. Donald R. Prothero and Fred Schwab, Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy, W. H. Freeman (1996), ISBN 0-7167-2726-9.
5. Edward J. Tarbuck, Frederick K. Lutgens, Cameron J. Tsujita, Earth, An Introduction to Physical Geology, National Library of Canada Cataloguing in Publication, 2005, ISBN 0-13-121724-0
6. Juergen Schieber, John Southard, and Kevin Thaisen, "Accretion of Mudstone Beds from Migrating Floccule Ripples," Science, 14 December 2007: 1760-1763. See also "As waters clear, scientists seek to end a muddy debate," at PhysOrg.com (accessed 27 December 2007).
7. Joe H. S. Macquaker and Kevin M. Bohacs, "Geology: On the Accumulation of Mud," Science, 14 December 2007: 1734-1735.
8. Robert G. Loucks, Robert M. Reed, Stephen C. Ruppel,and Daniel M. Jarvie "Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale", Journal of Sedimentary Research, 2009, v. 79, 848-861.
9 "Geology of Oil," Steven Cooperman, Ph.D.
"Understanding Petroleum Exploration and Production,"
National Energy Foundation, Student Activity Guide
10."The Upstream: A Guide to Petroleum Exploration and Production," Exxon Corporation Informational Brochure