Contents
A1.0 INTRODUCTION TO OPENHOLE LOG INTERPRETATION ................................. 1
A.1 USES OF LOGS ................................................................................................................................... 1
A.2 BASIC PETROLEUM GEOLOGY ........................................................................................................ 2
A.3 BASIC LOG INTERPRETATION CONCEPTS..................................................................................... 4
A.4 RESISTIVITY AS A BASIS FOR INTERPRETATION—THE ARCHIE EQUATION ........................... 5
A.5 DEFINITIONS ...................................................................................................................................... 7
a) Formation Porosity () ........................................................................................................................ 8
b) Formation Resistivity (R) ............................................................................................................................. 8
c) Formation Factor (F) .................................................................................................................................... 8
d) Water Saturation: S
w
...................................................................................................................................................................................................................................................................................................................
e) Hydrocarbons Saturation S
(
hy).....................................................................................................................................................................................................................................................................................
8
9
f) Clean Formations ........................................................................................................................... 9
g) Shaly Formations ............................................................................................................................. 9
h) Key Formulas ................................................................................................................................ 11
i) Key Symbols .................................................................................................................................. 11
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Introduction to Openhole Logging
A.6 LOG SCALES AND PRESENTATIONS ............................................................................................ 12
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A1.0 Introduction to Openhole Log
Interpretation
A.1 USES OF LOGS
A set of logs run on a well will usually mean
different things to different people. Let us
ex-amine the questions asked—and/or answers
sought by a variety of people.
The Geophysicist:
 Are the tops where you predicted?
 Are the potential zones porous as you have
assumed from seismic data?
 What does a synthetic seismic section
show?
The Geologist:
 What depths are the formation tops?
 Is the environment suitable for accumulation of hydrocarbons?
 Is there evidence of hydrocarbons in this
well?
 What type of hydrocarbons?
 Are hydrocarbons present in commercial
quantities?
 How good a well is it?
 What are the reserves?
 Could the formation be commercial in an
offset well?
The Drilling Engineer:
 What is the hole volume for cementing?
 Are there any keyseats or severe doglegs
in the well?
 Where can you get a good packer seat for
testing?
 Where is the best place to set a
whipstock?
The Reservoir Engineer:
 How thick is the pay zone?
 How homogeneous is the section?
 What is the volume of hydrocarbons per
cubic meter?
 Will the well pay-out?
 How long will it take?
The Production Engineer:
 Where should the well be completed (in
what zone(s))?
 What kind of production rate can be expected?
 Will there be any water production?
 How should the well be completed?
 Is the potential pay zone hydraulically isolated?
 Will the well require any stimulation?
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Introduction to Openhole Logging
 What kind of stimulation would be best?
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Log evaluation can be many things to many
people. As the answers are sought each individual will possibly use the available data in a
different manner. The common approach will
be in reading the logs and understanding the
various reactions produced by formation characteristics on our logging devices. The factors
influencing log reading and the information
they provide are what we wish to introduce to
you in this course.
A.2 BASIC PETROLEUM GEOLOGY
In order to better understand log responses,
we should first review the types of rocks that
are found in the boreholes.
Common sedimentary rocks are
sandstone, siltstone, shale, limestone,
dolomite and anhydrite
In general, sedimentary rocks are deposited
as either clastic sequences containing
sand-stone, siltstones and shales or carbonate
sequences of limestone, dolomite, anhydrite
and shale. (Figure A1).
deposition is such that crossbedding structures,
channel patterns and gradational rock types are
common. In areas of freshwater deposition coal
beds may occur, indicating non-marine
conditions.
After deposition and with deeper burial of the
sequence, compaction occurs and the clastic
grains can become cemented together to form
sedimentary rock.
When limestones form near shore, there may
be mixing of limestone and eroded clastic
material. In deeper ocean basins, limestone and
shale mixtures are common.
After deposition, later burial may cause
dolomitization of the limestone in which the
actual composition of the rock is changed to
dolomite.
Because of their brittle nature compared with
other sediments, limestones tend to fracture
with deformation, which increases permeability and helps in the dolomitization process.
Clastic Deposition
Clastic rocks are formed from rock fragments
and weathered particles of preexisting rocks.
These sediments are transported by wind and
water and are usually deposited in rivers, lakes
and oceans as relatively flat-lying beds. Current
and wave action later sorts the sediments such
that
in
high-energy
environments
coarse-grained sands are deposited and in low
energy environments fine-grained silts and
clays are deposited. The nature of the
Carbonate Deposition
Carbonate deposition occurs in marine conditions by the precipitation of limestone from
organisms as fine particles, shells or massive
growths. Limestones are deposited either as
flat-lying beds on the ocean floor or as mounds
or pinnacle reefs.
Barrier reef chains that grow in this manner
may form restricted ocean basins landward, in
which dolomite and anhydrite are precipitated
by the evaporation of seawater.
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Introduction to Openhole Logging
Figure A1: Clastic Deposition vs. Carbonate Deposition
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In many parts of the world multiple sequences
of clastic rocks overlie older carbonate
sequences. Between each of the clastic and carbonate groups, erosional inconformities are
common and the nature of deposition within
each group is unique.
d.
re-
e.
A.3 BASIC LOG INTERPRETATION
CONCEPTS
Any given rock formation has numerous
unique physical properties associated with it.
Only those that can be measured and are useful
will be considered in this course. They are
a.
b.
c.
= porosity: the void space between
grains that is generally filled with liquids or gases.
S w = water saturation: the percentage
of the pore space filled with water (as
opposed to hydrocarbons or air).
R = resistivity: the resistance to
electrical current flow presented by a
unit volume of rock.
R W = water resistivity: the electrical
sistance of the water filling the pore
space in the rock. This value varies
with water salinity and temperature.
k = permeability: the ability of the
rock to pass fluids through it.
Consider the following unit cubes (Figure A2):
Cube A
If the porosity () is filled with water then, by
definition, the water saturation S W = 100%.
Cube B
If the porosity is 70% filled with water and
30% hydrocarbons, then, the water saturation
70
S W = _______ % = 70%
70 + 30
and hydrocarbons saturation
Cube “B”:
porosity = hydrocarbons and
water in
SW = 70%
Cube “A”: porosity =
waterfilled SW = 100%
Figure A2
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The usefulness of resistivity logging rests on
the facts that
 water is a conductor (low resistivity)
 hydrocarbons and rocks are insulators
(high resistivity)
S hy = 1 - S w = 30%
Therefore the percentage volume of water
saturation
=S
Consider the following unit cubes (Figure A3):
w
For example: if = 20% and Sw = 70%, then
Cube C
The resistivity Rt of the cube will vary with
water resistivity Rw (i.e. as Rw increases, Rt
in-creases and vice versa).
14% of the bulk volume is water and 70% of
the pore space is water filled.
A.4 RESISTIVITY AS A BASIS FOR
INTERPRETATION—THE ARCHIE
EQUATION
In the previous section we introduced a number
of parameters used to evaluate rock formations.
If we could build on the effects of resistivity in
conjunction with the other parameters to
develop a mathematical relation-ship, we would
have an extremely useful tool for our work with
potential hydrocarbon zones.
Therefore: Rt . R
w
(1)
Cube D
Replace 25% of the cube with rock (hence
= 75%) but maintain a constant Rw. Resistivity
R increases with decreasing porosity (i.e. as
t
decreases, Rt increases).
The remainder of this section is devoted to
developing such a formula.
Cube “C”
 Constant Current
 Porosity = 100%
 Sw = 100%
Cube “D”
 Constant Current
 Porosity = 75%
 Sw = 100%
Cube “E”
 Constant Current
 Porosity = 75%
 Sw = 70%
Figure A3
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Introduction to Openhole Logging
Therefore: R
1/.
Rw
(2)
R
t
(5)
o
Cube E
Replace 30% of remaining porosity with
hydrocarbons. Resistivity Rt increases with
decreasing water saturation Sw (i.e. as Sw
de-creases, Rt increases).
Now, let = 1, then Ro
Rw .
Now, let F = constant of proportionality
defined as the form ation factor.
Therefore: Ro = FRw
Therefore: R
1/ Sw .
(3)
t
By combining the above observations (1, 2
and 3), we can say
1
1
(6)
Returning to Equation 5 and introducing porosity as a variable, it is clear that
R R
t
Ro
or F = Rw
w
S
1
w
F
or
Rw
Rt
(4)
S
w
To solve for the constants of proportionality
let us first limit the equation as follows:
Let Sw= 100% (i.e. there is no hydrocarbon present and the porosity is 100%
water filled).
This is intuitively obvious as the relationship
between Ro and Rw is related to that particular
unit cube of rock and its porosity characteristics.
Through empirical measurements, it was
determined that
a
F = ____
(7)
m
Then, define Ro = Rt (ie: Ro is the wet resistivity of the formation for the condition Sw =
100%):
where
a = constant
m = cementation factor
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The cementation factor m relates to the porosity type and how it will transmit electrical
current to the actual rock (also called tortuosity).
Using the above equations
Recall Ro = FRw (Equation 6)
aR
R= R=
t
w
when Sw = 100%
o
w
(9)
w
m
R
t
Equation 9 forms the Archie relationship that
is the basis for all conventional log interpretation techniques. Enhancements and refinements
may be applied for the more complicated rock
types.
The remainder of this course is dedicated to
measuring, evaluating and using porosity and
resistivity to calculate water saturation and
hence hydrocarbons reserves using the concepts of this equation.
if Sw 100%, then
aR
aR
or Sn =
1
w
R
t
S
m
w
A.5 DEFINITIONS
1
a) Formation Porosity ()
Defined as the fraction of total volume occupied by pores or voids, where
or R R
t
o
S
w
pore volume
R
o
or S
(8)
w
R
=
100%
total volume
t
Through laboratory measurements, it was
found that this relationship (8) is dependent on
the saturation exponent n as
R
t
FR
w
or Sn = _______
w
When the pore space is intergranular it is
known as primary porosity. When the porosity
is due to void space created after deposition,
(e.g., vugs or fractures in carbonates), the porosity is known as secondary porosity. When
shale is present, the pore space occupied by the
water in the shale is included with the pore
space in the rock to give total porosity (T ). If
only the rock pore space is considered in a
shaly formation, the pore space is called efec-
tive porosity (e ).
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b) Formation Resistivity (R)
Defined as the resistance offered by a formation to the flow of electrical current. It is
expressed in ohm-meter 2 /meter.
For Porosity
We use several terms to describe formation
resistivity under various circumstances of fluid
content.
a
R:
Describes the resistivity of a fort
mation undisturbed by the drilling
process.
In a 1942 paper Gus Archie proposed that
the relationship between formation factor and
porosity could be described by the formula
F=m
where
a = empirical constant.
m = cementation factor.
Some recommended F and relationships are
R: Describes a special form of Rt. It
o
is the resistivity of a clean formation when all pore space is filled
with connate water (Rw ).
R:
Is the symbol for the resistivity of
w
formation (connate) water.
0.62
F = _____________
(for sands)
2.15
0.81
F = _____________
(for sands)
2
c) Formation Factor (F)
1
F = _____________
For Resistivity
(for carbonates)
2
An important relationship exists between the
resistivity of a fully water saturated formation
and the resistivity of the contained water. The
ratio of these two values is called formation
resistivity factor (or more commonly, formation factor) where:
R
o
w
F=
R
F is a constant for the formation under consideration. The value of F for any particular
formation depends on:
- formation porosity pore distribution pore size
- pore structure.
Chart Por-1 (figure A4) in the Log Interpretation Chart book is based on several different F- relationships.
d) Water Saturation (Sw )
Defined as the fraction of pore volume filled
with water where
water filled pore volume
100%
s=
w
total pore volume
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Introduction to Openhole Logging
e) Hydrocarbons Saturation ( S hy)
g) Shaly Formations
Defined as the fraction of pore volume filled
with hydrocarbons where:
This describes formations where some of the
formation void space (porosity) is filled with
shale.
hydrocarbon-filled pore volume
Shy =
total pore volume
or
100%
Shy = 1 – Sw .
f) Clean Formations
The term clean formation refers to those that
are shale free.
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Shale distribution is considered to be:
- Laminated: The formation is built up
of thin laminae of sand and shale.
- Dispersed: The shale particles are dispersed in the pore space.
- Structural: The shale replaces matrix.
Formation Resistivity Factor versus Porosity
5
10
20
50
100
200
500
1000
2000
5000
10,000
5
This chart gives a variety of formation resistivity factor-to-porosity conversions. The proper choice is best
determined by laboratory measurement or experience in the area. In the absence of this knowledge,
recommended relationships are the following:
0.62
For Soft Formations: Humble Formula: Fr =
0.81
or Fr =
2.15
2
0.62
For Hard Formations: Fr =
with appropriate cementation factor, m.
m
EXAMPLE: is 6% in a carbonate in which a cementation factor, m of 2 is appropriate
Therefore, from chart, Fr = 280.
Chart Por-1
Figure A4
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Introduction to Openhole Logging
h) Key Formulas
R
o
R
FR
w
where
n is usually taken as 2
w
R
xo
R mf
Archie’s formula: Sw n =
i) Key Symbols
BHT - bottom hole temperature in degrees
Celsius
di
- average diameter of invaded
h
zone (Di)
IDPH
 bed thickness in meters
 resistivity from the deep phasor inIMPH
duction
 resistivity from the medium Phasor
SFL
induction
 resistivity from the Spherically Focused Log
Rm

resistivity of the mud
R mf R mc

resistivity of the mud filtrate
Rw
 resistivity of the mudcake
R wa  resistivity of the formation water
 apparent resistivity of the formation
water
Rt
- resistivity of the formation
(uncontaminated zone)
R
- resistivity of the formation when
o
100% water filled

resistivity of the flushed zone
R
R
R
R
xo
Rsh
F
S
w
(close to borehole)
resistivity of the shales
- formation resistivity factor

porosity in percent
- water saturation, percent of pore
space occupied by water in uncontaminated zone
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-
a
S
xo

water saturation, as above, in
flushed zone
Shc
- hydrocarbons saturation as percent of
pore space occupied by water
K
- coefficient in the sp formula
SSP
- static spontaneous potential - the
maximum possible for a particular
Rmf / Rw
PSP - pseudostatic spontaneous potential—the SP
found in a thick shaly sand
k
- permeability in millidarcies
pore volume
 porosity =
100%.
total volume
 sonic porosity
 density porosity
 neutron porosity
DN

total porosity




effective porosity
secondary porosity
volume of shale
photoelectric index
+
2
A complete list of symbols and subscripts is
included in Section J (Miscellaneous).
A.6 LOG SCALES AND PRESENTATIONS
a) Well logs provide a continuous graph of formation parameters versus depth.
Normal depth scales are
 1:240—1 m of log per 240 m of measured
hole depth. Each line is 1 m, with heavy
lines every 5 m, and heavier lines every 25
m for ease of reading. Depths are indicated
every 25 m (Figures A5 and A6).
 1:600—1 m of log per 600 m of measured
hole depth. Each line is 5 m, with heavy
lines every 25 m. Depths are indicated
every 25 m (Figure A7).
 Other scales are available. These include
1:1200, 1:120, 1:48 and 1:5.
 Log grids may be either logarithmic
(resistivity logs—Figure A6) or linear
(porosity logs—Figure A5).
b) If a caliper device is present or the log being
generated is a type of sonic log, event markers are
placed on each side of the depth track integrating
the quantity of hole volume or transit time recorded.
1. Integrated hole volume—requires caliper device
(Figure A5)
 placed on the left side of the
depth track
3
 small marks indicate 0.1 m whereas large
marks represent
3
1 . 0 m.
2. Integrated cement volume—Requires
caliper device plus future casing size
 placed on the right side of the depth track
when space permits— and if sonic not
present
3
3. Integrated transit time—Requires sonic
tool (Figure A5)
 placed on the right side of the
depth track
 small marks indicate 1 msec
whereas large marks represent 10
msec of time.
If the log is recorded using logging-whiledrilling
methods, event markers on both sides of the
depth track (Figure A6) represent the conversion
from time-based sampling to a depth-based
presentation. The markers there-fore indicate the
number of data samples per unit depth. In other
words, the larger the concentration of markers
over a depth interval, the greater the number of
data samples used to make the log.
c) Logs also have headings and inserts.
 Log headings provide such information as
well depth, casing depth, mud params,
maximum temperature and other comments
pertinent to the evaluation of log data
(Figures A8 and A9).
 Inserts provide such information as curve
scaling, coding, date/time of acquisition,
data curve first-reading points and constants pertinent to the logging run fo
l-lowing the insert. Curve coding on the
log data indicates the deepest reading primary measurement (long dashed) to the
shallowest reading primary measurement
(solid) when two or more measurements
are combined (Figure A10).
 small marks indicate 0.1 m while large
marks represent
3
1 . 0 m.
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Introduction to Openhole Logging
Figure A5: Linear Grid 1/240 Scale
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Logarithmic Grid 1/240 Scale
Data Sample Event Markers for L WD Curves
Figure A6
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Introduction to Openhole Logging
Figure A7: Linear Grid 1/600 Scale
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Figure A8: Log Heading (page 1)
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Introduction to Openhole Logging
Figure A9: Log Heading (page 2) and Log Tail
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Introduction to Openhole Logging
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