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many people. As the answers are sought eachindividual will possibly use the available datain a different manner. The common approachwill be in reading the logs and understandingthe various reactions produced by formationcharacteristics on our logging devices. Thefactors influencing log reading and the infor-mation they provide are what we wish to intro-duce to you in this course.
A.2 BASIC PETROLEUM GEOLOGYIn order to better understand log responses,
we should first review the types of rocks thatare encountered in the boreholes.
Common sedimentary rocks are:Sandstone, Siltstone, Shale, Lime-stone, Dolomite, Anhydrite.
In general, sedimentary rocks are depositedas either clastic sequences containing sand-stone, siltstones and shales or carbonate se-quences of limestone, dolomite, anhydrite andshale.
Clastic DepositionClastic rocks are formed from rock frag-
ments and weathered particles of pre-existingrocks. These sediments are transported bywind and water and are usually deposited inrivers, lakes and oceans as relatively flat-lyingbeds. Current and wave action later sorts thesediments such that in high energy environ-ments coarse-grained sands are deposited andin low energy environments fine-grained silts
and clays are deposited. The nature of the
deposition is such that cross-bedding struc-tures, channel patterns and gradational, rock types are common. In areas of fresh waterdeposition coal beds may occur indicating non-marine conditions.
After deposition and with deeper burial of thesequence, compaction occurs and the clasticgrains can become cemented together to formsedimentary rock.
Carbonate DepositionCarbonate deposition occurs in marine con-
ditions by the precipitation of limestone fromorganisms as fine particles, shells or massivegrowths. Limestones are deposited either asflat-lying beds on the ocean floor or as moundsor pinnacle reefs.
Barrier reef chains which grow in this man-ner may form restricted ocean basins land-ward, in which dolomite and anhydrite areprecipitated by the evaporation of sea water.
When limestones form near shore, there may
be mixing of limestone and eroded clastic ma-terial and in deeper ocean basins limestone andshale mixtures are common.
After deposition, later burial may causedolomitization of the limestone in which theactual composition of the rock is changed todolomite.
Because of their brittle nature compared toother sediments, limestones tend to fracturewith deformation which increases permeabilityand helps in the dolomitization process.
In many parts of the world multiple sequencesof clastic rocks overlie older carbonate se-
quences. Between each of the clastic and car-bonate groups, erosional inconformities arecommon and the nature of deposition withineach group is unique.
A.3 BASIC LOG INTERPRETATION CONCEPTS
Any given rock formation has numerousunique physical properties associated with it.Only those which can be measured and are use-ful will be considered in this course. They are:
a. Φ = Porosity: The void space betweengrains that is generally filled with liq-uids or gases.
b. S w
= Water Saturation: The percent-
age of the pore space filled with water(as opposed to hydrocarbon or air).
c. R = Resistivity: The resistance toelectrical current flow presented by aunit volume of rock.
d. RW
= Water Resistivity: The electrical
resistance of the water filling the pore
space in the rock. This value varieswith water salinity and temperature.e. k = Permeability: The ability of the rock
to pass fluids through it.
Consider the following unit cubes (FigureA2):
Cube A
If the porosity (Φ) is filled with water then,by definition, the water saturation S
W = 100%.
Cube BIf the porosity Φ is 70% filled with water and
Therefore the percentage volume of watersaturation
= Φ x Sw
For example: if Φ = 20% and Sw = 70%
Then: 14% of the bulk volume is water and70% of the pore space is water filled.
A.4 RESISTIVITY AS A BASIS FORINTERPRETATION - THE AR-CHIE EQUATION
In the previous section we introduced a num-ber of parameters used to evaluate rock forma-tions. If we could build on the effects of resistivity in conjunction with the other pa-rameters to develop a mathematical relationship,we would have an extremely useful tool for ourwork with potential hydrocarbon zones.
The remainder of this section will be devotedto developing such a formula.
The usefulness of resistivity logging rests onthe fact that:
- water is a conductor (low resistivity)- hydrocarbons and rocks are insulators
The cementation factor m relates to the po-rosity type and how it will transmit electrical
current to the actual rock (sometimes calledtortuosity).
Using the above equations
Recall Ro = FR
w(Equation 6)
aRw
Rt
= Ro = when S
w = 100%
Φm
if Sw
≠ 100%, then
aRw
1
Rtα x
Φm Sw
1
or Rtα R
ox
Sw
Ro
or Swα (8)
Rt
Through laboratory measurements, it wasfound that this relationship (8) is dependent onthe saturation exponent n as
Ro
Sw
n
=
Rt
FRw
or Sw
n
=
Rt
aRw
or Sw
n
= ( 9 )Φm R
t
Equation nine forms the Archie Relationshipthat is the basis for all conventional log inter-pretation techniques. Enhancements and re-finements may be applied for the morecomplicated rock types.
The remainder of this course is dedicated tomeasuring, evaluating and using porosity and
resistivity to calculate water saturation andhence hydrocarbon reserves using the conceptsof this equation.
A.5 DEFINITIONS
a) Formation Porosity (Φ)
Defined as the fraction of total volume occu-pied by pores or voids, where:
Pore Volume
Φ = x 100% Total Volume
When the pore space is intergranular it isknown as primary porosity. When the porosityis due to void space created after deposition,e.g. vugs or fractures in carbonates, the po-rosity is known as secondary porosity. Whenshale is present, the pore space occupied by thewater in the shale is included with the pore
space in the rock to give Total Porosity (ΦT).
If only the rock pore space is considered in a
shaly formation, the pore space is called Effec-tive Porosity (Φ
Is defined as the resistance offered by a for-mation to the flow of electrical current. It isexpressed in ohm-metre2 /metre.
We use several terms to describe formationresistivity under various circumstances of fluidcontent.
Rt: Describes the resistivity of a for-
mation undisturbed by the drillingprocess.
Ro: Describes a special form of R
t. It is
the resistivity of a clean formationwhen all pore space is filled withconnate water (R
w).
Rw: Is the symbol for the resistivity of
formation (connate) water.
c) Formation Factor (F)
For Resist ivity:
An important relationship exists between theresistivity of a fully water saturated formationand the resistivity of the contained water. Theratio of these two values is called Formation
Resistivity Factor (or more commonly, For-mation Factor) where:
Ro
F =R
w
F is a constant for the formation under con-
sideration. The value of F for any particularformation depends on:- formation porosity- pore distribution- pore size- pore structure
For Porosity
In a 1942 paper Gus Archie proposed thatthe relationship between formation factor andporosity could be described by the formula:
A.6 LOG SCALES AND PRESENTATIONSa. Well logs provide a continuous graph of
formation parameters versus depth.
Normal depth scales are:- 1:240 1 metre of log per 240
metres of measured hole depth.Each line is one metre, withheavy lines every 5 metres, andheavier lines every 25 m forease of reading. Depths are in-dicated every 25 metres(Figures A5 and A6).
- 1:600 1 metre of log per 600metres of measured hole depth.
Each line is 5 m, with heavylines every 25m. Depths are in-dicated every 25 metres (FigureA7).
- Other scales are available.These include 1:1200, 1:120,1:48 and 1:5.
- Log grids may be either loga-rithmic (Resistivity logs - Fig-ure A6) or linear (Porosity logs- Figure A5).
b. If a caliper device is present or the log be-ing generated is a type of sonic log, eventmarkers will be placed on each side of thedepth track integrating the quantity of holevolume or transit time recorded.
2) Integrated Cement Volume - Requirescaliper device plus future casing size- placed on the right side of the
depth track when space permits- and if sonic not present
- small marks indicate 0.1m3
while large marks represent
1.0m3
.
3) Integrated Transit Time - Requiressonic tool (Figure A5)
- placed on the right side of thedepth track
- small marks indicate 1 milli-second while large marks rep-resent 10 milliseconds of time.
If the log recorded is via Logging-While-Drilling methods, event markers on bothsides of the depth track (Figure A6) repre-sents the conversion from a time-basedsampling to a depth-based presentation.The markers therefore indicate the number
of data samples per unit depth. In otherwords, the larger the concentration of markers over a depth interval, the greaterthe number of data samples used to makethe log.
c. Logs also have headings and inserts- Log headings provide such information
as well depth, casing depth, mud pa-rameters, maximum temperature, andother comments pertinent to evaluationof log data (Figure A8 and A9).
- Inserts provide such information ascurve scaling, coding, date/time of ac-quisition, data curve first reading pointsand constants pertinent to the loggingrun following the insert. Curve codingon the log data indicates the deepestreading primary measurement (longdashed), to the shallowest reading pri-mary measurement (solid), when two ormore measurements are combined(Figure A10).