Table of Contents

1. Introduction
1.1 Introduction to Log Interpretation
1.2 Importance of Log Interpretation
1.3 Formation Evaluation
1.4 Reservoir Potential

2. Methods
2.1 Archie’s Equation and Ratio Method
2.2 Quick Look Technique
2.3 Bulk Volume Water (BVW)
2.4 Saturation Cross Plots
2.5 Log Derived Permeability
2.6 Shaly Sand Analysis

3. Interpretation
3.1 Significance of Ratio Method
3.2 Rxo/Rt
3.4 Apparent water resistivity (Rwa)
3.5 Conductivity-derived porosity
3.5 Wet resistivity (Ro)
3.6 Importance of Bulk Volume Water

4.0 Review

5.0 Conclusion

6.0 References

1
Log Interpretation
Introduction

Log interpretation refers to the extraction of desired information from the obtained log data. The
first and foremost priority of a petroleum geologist is to determine reservoir potential. This
refers to the volume of hydrocarbons present in the reservoir. From here on, the petroleum
geologist has to convert the obtained geological data into barrels of oil and standard cubic feet of
gas. This is a crucial process; it plays a vital role of making the big decision, i.e. whether a well
should be drilled in the particular area or not. It must be kept in mind that the petroleum industry
is a purely commercial industry, a well will only be drilled if a sufficient volume of
hydrocarbons is deemed to be present in the area of interest, and whether it will be economically
feasible to retrieve them.

There is no doubt that log interpretation is the foundation of well logging and ultimately
petroleum engineering, however the petroleum geologist also needs to look at other pieces of
information before arriving to a final decision. These are as follows:

1. Drill Stem Testing (DST)

 A drill stem test (DST) is a procedure for isolating and testing the pressure,
permeability and productive capacity of a geological formation during
the drilling of a well. It provides information on whether or not to complete the
well. The zone in question is sealed off from the rest of the wellbore by packers,
and the formations' pressure and fluids are measured. Data obtained from
a DST include the following:
 fluid samples
 reservoir pressure (P*)
 formation properties, including permeability (k), skin (S), and radius of
investigation (ri)
 productivity estimates, including flow rate (Q)

2
Log Interpretation
Fig 1: Schematic diagram of DST

Fig 2: Parts of a DST setup

3
Log Interpretation
2. Coring Analysis
 One way to get more detailed samples of a formation is by coring. Two
techniques commonly used at present. The first is the "whole core", a cylinder of
rock, usually about 3" to 4" in diameter and up to 50 feet (15 m) to 60 feet (18 m)
long. It is cut with a "core barrel", a hollow pipe tipped with a ring-shaped
diamond chip-studded bit that can cut a plug and bring it to the surface. Often the
plug breaks while drilling, usually in shales or fractures and the core barrel jams,
slowly grinding the rocks in front of it to powder. This signals the driller to give
up on getting a full length core and to pull up the pipe.

 Another, cheaper, technique for obtaining samples of the formation is "Sidewall

Coring. Advantages of this technique are low cost and the ability to sample the
formation after it has been drilled. Disadvantages are possible non-recovery
because of lost or misfired bullets and a slight uncertainty about the sample depth.
Sidewall cores are often shot "on the run" without stopping at each core point
because of the danger of differential sticking. Most service company personnel
are skilled enough to minimize this problem, but it can be significant if depth
accuracy is important.

Fig. 3: Coring Machine

4
Log Interpretation
3. Mud Logging
Mud logging (or Wellsite Geology) is a well logging process in which drilling mud and drill
bit cuttings from the formation are evaluated during drilling and their properties recorded on
a strip chart as a visual analytical tool and stratigraphic cross sectional representation of the
well. The drilling mud which is analyzed for hydrocarbon gases, by use of a gas
chromatograph, contains drill bit cuttings which are visually evaluated by a mudlogger and
then described in the mud log.

Fig. 4: Mud Logging Equipment

4. Seismic Exploration
Seismic exploration is the search for commercially economic subsurface deposits of crude
oil, natural gas and minerals by the recording, processing, and interpretation of artificially
induced shock waves in the earth. Artificial seismic energy is generated on land by vibratory
mechanisms mounted on specialized trucks.

Fig. 5: Seismic Exploration

5
Log Interpretation
5. Decline Curve Analysis (DCA)
Decline curve analysis (DCA) is a graphical procedure used for analyzing declining
production rates and forecasting future performance of oil and gas wells. Oil and gas
production rates decline as a function of time; loss of reservoir pressure, or changing relative
volumes of the produced fluids, are usually the cause. Fitting a line through the performance
history and assuming this same trend will continue in future forms the basis of DCA concept.
It is important to note here that in absence of stabilized production trends the technique
cannot be expected to give reliable results.

 The basic assumption in this procedure is that whatever causes controlled the
trend of a curve in the past will continue to govern its trend in the future in a
uniform manner.

 Three types of declines are observed:

a. Exponential
b. Hyperbolic
c. Harmonic

6. Material Balance Equation

The material-balance equation is the simplest expression of the conservation of mass in a

reservoir. The equation mathematically defines the different producing mechanisms and
effectively relates the reservoir fluid and rock expansion to the subsequent fluid withdrawal.

7. Reservoir Simulation

 Simulation of petroleum reservoir performance refers to the construction and

operation of a model whose behavior assumes the appearance of actual reservoir
behavior. A model itself is either physical or mathematical. A mathematical model is
a set of equations that, subject to certain assumptions, describes the physical
processes active in the reservoir. Although the model itself obviously lacks the reality
of the reservoir, the behavior of a valid model simulates—assumes the appearance
of—the actual reservoir.

6
Log Interpretation
  The purpose of simulation is estimation of field performance (e.g., oil recovery)
under one or more producing schemes. Whereas the field can be produced only once,
at considerable expense, a model can be produced or run many times at low expense
over a short period of time. Observation of model results that represent different
producing conditions aids selection of an optimal set of producing conditions for the
reservoir.

Methods
The primary purpose of log interpretation is the accurate determination of water saturation and
porosity. It is also used to find out other properties such as cementation factor, matrix values and
formation permeability.
The following methods are used:

1. Archie’s Equation and Ratio Method

Water Saturation Determination:

Water saturation (Sw) determination is the most challenging of petrophysical calculations

and is used to quantify its more important complement, the hydrocarbon saturation (1 –
Sw). Complexities arise because there are a number of independent approaches that can
be used to calculate Sw. The complication is that often, if not typically, these different
approaches lead to somewhat different Sw values that may equate to considerable
differences in the original oil in place (OOIP) or original gas in place (OGIP) volumes.
The challenge to the technical team is to resolve and to understand the differences among
the Sw values obtained using the different procedures, and to arrive at the best calculation
of Sw and its distribution throughout the reservoir vertically and areally. In OOIP and
OGIP calculations, it is important to remember the relative importance of porosity and
Sw. A 10% pore volume (PV) change in Sw has the same impact as a 2% bulk volume
(BV) change in porosity (in a 20% BV porosity reservoir).

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Log Interpretation
 Archie developed his famous equation to calculate, from well log parameters, the water
saturation (Sw) of the uninvaded zone in a formation next to a borehole.
 Sw = [ (a / Fm)*(Rw / Rt) ](1/n)
 Sw = water saturation
 F = porosity
 Rw = formation water resistivity
 Rt = observed bulk resistivity
 a = a constant
 m = cementation factor
 n = saturation exponent
The ratio method identifies hydrocarbons from the difference between water saturations
in the flushed zone (Sxo) and the uninvaded zone (Sw). When the uninvaded zone form
of Archie's equation is divided by the flushed zone, the following results:

Where:
 Sw = water saturation in the uninvaded zone
 Sxo = water saturation in the flushed zone
 Rxo = formation's shallow resistivity from a measrement such as laterolog-8,
microspherically focused log, or microlaterolog
 Rt = true formation resistivity (i.e., deep induction or deep laterolog corrected for
invasion)
 Rmf = resistivity of the mud filtrate at formation temperature
 Rw = resistivity of formation water at formation temperature
When Sw is divided by Sxo, the formation factor (F = a/φm) is cancelled out of the
equation because the same formation factor is used to calculate both Sw and Sxo. This
can be very helpful in log analysis because, from the ratio (Rxo/Rt)/(Rmf/Rw) the
geologist can determine a value for both the moveable hydrocarbon index (Sw/ Sxo) and
water saturation by the ratio method without knowing porosity.

8
Log Interpretation
 Therefore, a geologist can still derive useful formation evaluation log parameters even
though porosity logs are unavailable. The moveable hydrocarbon index by the ratio
method is:

Where:
 Sw/Sxo = moveable hydrocarbon index
 Rxo = formation's shallow resistivity from a measrement such as laterolog-8,
microspherically focused log, or microlaterolog
 Rt = true formation resistivity (i.e., deep induction or deep laterolog corrected for
invasion)
 Rmf = resistivity of the mud filtrate at formation temperature
 Rw = resistivity of formation water at formation temperature
The cementation exponent (n) is assumed to be 2.0. If the ratio Sw/Sxo is equal to or
greater than 1.0, then hydrocarbons were not moved during invasion. This is true
regardless of whether or not a formation contains hydrocarbons.
Whenever the ratio Sw/Sxo is less than 0.7 for sandstones or less than 0.6 for carbonates,
moveable hydrocarbons are indicated.
To determine water saturation by the ratio method Swr you must know the flushed zone's
water saturation. In the flushed zone of formations with moderate invasion and average
residual hydrocarbon saturation, the following relationship normally works well:

Where:
 Sw = water saturation in the uninvaded zone
 Sxo = water saturation in the flushed zone

The ratio method water saturation (Swr) is:

9
Log Interpretation
 Where:
 Swr = Water saturation by ratio method.
 Rxo = shallow resistivity from measurements such as laterolog-8, microspherically
focused log, or microlaterolog.
 Rt = true formation resistivity (i.e., deep induction or deep laterolog corrected for
invasion).
 Rmf = resistivity of mud filtrate at formation temperature.
 Rw = resistivity of formation water at formation temperature.

2. Quick Look Techniques

The Rxo/Rt quicklook method can be used to identify hydrocarbon-bearing formations and
to indicate hydrocarbon movability (producibility). When Sw/Sxo is 1 in a permeable zone,
the zone will produce water or be nonproductive regardless of water saturation. A
value Sw/Sxo significantly less than 1 indicates that the zone is permeable and contains
some hydrocarbons, and that the hydrocarbons have been flushed (moved) by invasion.
Thus, the zone contains producible hydrocarbon.
The equation can be written as

which shows that an indication of Sw/Sxo can be obtained by

comparing Rxo/Rt with Rmf/Rw, where the subscript SP emphasizes that Rmf/Rw is derivable
from the SP. Equivalently, the comparison can be between log Rxo/Rt and the SP curve for
an indication of log Sw/Sxo.

The apparent water resistivity technique relies on the comparison of values of water
resistivity calculated at different intervals in a well. This comparison can be made
between different zones or within the same zone if a water-hydrocarbon contact (oil
water contact or gas water contact) is suspected in that zone. The assumption is that this
lowest value of Rwa is the most accurate value of true formation water resistivity (Rw) and
that values of Rwa greater than the minimum value are indicative of the presence of
hydrocarbons. Water saturation can also be calculated from the values of Rwa.

10
Log Interpretation
 Conductivity derived porosity technique determines porosity from Archie's equation,
using the form of the equation for complete water-bearing zones (where Sw =1). The
porosity values are plotted as a curve and are normally displayed in the same track as the
SP, shown as high porosity values on the left to low porosity values on the right.

The wet-resistivity curve (Ro) is one of the oldest quick-look techniques. Unlike the
other curves, which tend to be compared to the SP curve, it is plotted as an overlay on the
resistivity curve.

3. Bulk Volume Water

The bulk volume water is simply the product of the water saturation and porosity.
Mathematically:

BVW  S w *
Bulk volume water is one of the most important parameters in reservoir engineering. This
is because it indicates whether a reservoir is at irreducible water saturation or not. This
leads to a clear idea of reservoir potential, any such reservoirs will not produce water,
hence they are desirable.

4. Saturation Cross Plots

Saturation cross-plots, also called resistivity-porosity cross-plots are similar to the porosity cross-
plots. They are x-y plots of log data from which quantities of interest are derived; in this case,
that quantity is water saturation (Sw). These plots were created before the use of computers to aid
log interpretation. These techniques minimize the number of calculations that must be done and
aid the comparison of potentially productive zones. With the advent of early log-interpretation
software, their use declined, but this decline was reversed as people realized the power of the
techniques to use the ability of the human mind to recognize patterns in data.

These techniques are currently regarded as necessary components of full-functioned computer-

based log-interpretation systems. Two saturation cross-plots are of wider use which are;

1. Pickett Cross-plot Method

2. Hingle Cross-plot Method

Their brief description and methodology is as given.

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Log Interpretation
 Pickett Cross-plot Method
The Pickett plot is one of the simplest and most effective cross-plot methods in use. This
technique estimates water saturation and can also help determine:

 Formation water resistivity (Rw)

 Cementation factor (m)

 Matrix parameters for sonic and density logs (Δtma and ρma)

The Pickett method is based on the observation that true resistivity (Rt) is a function of porosity
(φ), water saturation (Sw), and cementation exponent (m). It is actually a graphical solution of
Archie's equation in terms of resistivity, i.e.,

𝑎 ∗ 𝑅𝑤 1/𝑛
𝑆𝑤 = ( )
𝑅𝑡 ∗ 𝜑 𝑚

Taking log and putting 𝑆𝑤 = 1 forms;

𝑙𝑜𝑔(𝑅𝑡 ) = 𝑙𝑜𝑔(𝑎 ∗ 𝑅𝑤 ) – 𝑚𝑙𝑜𝑔(𝜑)

This form of the equation (y = b + mx) indicates that by plotting Rt on the y-axis (on a
logarithmic scale) against porosity (φ) on the x-axis (on a logarithmic scale as well), one can
determine the product (a*Rw) from intercept of the line (b), and the cementation exponent (m)
from the slope of the line (m). But if we plot Rt on x-axis and φ on y-axis then equation takes the
form of:

𝑙𝑜𝑔(𝑅𝑡 ) = 𝑙𝑜𝑔(𝑎 ∗ 𝑅𝑤 ) – (1/𝑚)𝑙𝑜𝑔(𝑅𝑡 ) − 𝑛𝑙𝑜𝑔(𝑆𝑤 )

Hingle Cross-plot Method

The oldest of the resistivity-porosity cross-plot methods that can be used to determine water
saturation (Sw) is the Hingle plot. As in other cross-plot techniques, a significant benefit of
Hingle's technique is that a value for water saturation (Sw) can be determined even if matrix
properties (ρma or Δtma) of a reservoir are unknown. This is also true if a reservoir's water
resistivity (Rw) is unknown. As with the Pickett plot, the Hingle plot seeks to plot resistivity
against porosity.

Plotting resistivity against porosity on linear scales produces a family of nonlinear trends. By
solving Archie's equation in the following form, the nonlinear trends become straight lines. i.e.,
1 1
 a  m  S wn  m
     * 
 Rt   w
R

12
Log Interpretation
While porosity is plotted on a linear scale, the resistivity is plotted on a very nonlinear scale. In
practice, the resistivity is plotted on a grid that has been constructed for specific values of
tortuosity factor (a) and cementation exponent (m). The grid constructed using a = 1.0 and m =
2.0 is usually used for sandstones, and the grid constructed using a = 0.62 and m = 2.15 is used
for carbonates. Using the constructed grids, resistivity values can be plotted directly on the
graph.

5. Log Derived Permeability

Log-derived permeability formulas are only valid for estimating permeability in

formations at irreducible water saturation (Schlumberger, 1977). When a geologist
evaluates a formation by using log-derived permeability formulas, the permeability
values, if possible, should be compared with values of nearby producing wells from the
same formation. Productivity estimates can be based on log-derived permeability if the
formation evaluated is compared with both good and poor production histories in these
nearby wells. By using comparisons of log-derived permeability from several wells, a
geologist is not using an absolute value for log-derived permeability.
Different methods for calculating log-derived permeability are discussed here. Before
these formulas can be applied, a geologist must first determine whether or not a
formation is at irreducible water saturation. Whether or not a formation is at irreducible
water saturation depends upon bulk volume water values (BVW = Sw*φ). When the bulk
volume water values of a formation are constant, the zone is at irreducible water
saturation. If the values are not constant, a zone is not at irreducible water saturation, and
the estimates of permeability are suspect.
The formulae for this are:

1. Wyllie & Rose Method

3 2

𝑘 = (250 ∗ 𝑆 ) (for medium gravity oils)
𝑤𝑖𝑟𝑟

And:

3 2

𝑘 = (79 ∗ 𝑆 ) (for dry gases)
𝑤𝑖𝑟𝑟

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Log Interpretation
 2. Timur Method
2.2 2

𝑘 = (93 ∗ )
𝑆𝑤𝑖𝑟𝑟

6. Shaly Sand Analysis

Shales are one of the more important common constituents of rocks in log analysis. Aside from
their effects on porosity and permeability, this importance stems from their electrical properties,
which have a great influence on the determination of fluid saturations.

Shales are loose, plastic, fine-grained mixtures of clay-sized particles or colloidal-sized particles
and often contain a high proportion of clay minerals. Most clay minerals are structured in sheets
of alumina-octahedron and silica-tetrahedron lattices. There is usually an excess of negative
electrical charges within the clay sheets.

The positive surface charge is usually measured in terms of milli-ions equivalents per 100 grams
of dry clay minerals and is called the cation exchange capacity (CEC). When the clay particles
are immersed in water, the Coulomb forces holding the positive surface ions are reduced by the
dielectric properties of water. The counterions leave the clay surface and move relatively freely
in a layer of water close to the surface (the electrical balance must be maintained so that the
counterions remain close to the clay water interface) and contribute to the conductivity of the
rock.

The Archie water saturation equation, which relates rock resistivity to water saturation, assumes
that the formation water is the only electrically conductive material in the formation. The
presence of another conductive material (i.e., shale) requires either that the Archie equation be
modified to accommodate the existence of another conductive material, or that a new model be
developed to relate rock resistivity to water saturation in shaly formations. The presence of clay
also complicates the definition or concept of rock porosity. The layer of closely bound surface
water on the clay particle can represent a very significant amount of porosity.

However, this porosity is not available as a potential reservoir for hydrocarbons. Thus, a shale or
shaly formation may exhibit a high total porosity, yet a low effective porosity as a potential
hydrocarbon reservoir.

The way shaliness affects a log reading depends on the amount of shale and its physical
properties.

14
Log Interpretation
Calculating Shale Volume:
• The first step in shaly sand analysis is to calculate shale/clay volume.

• Common techniques include those that are derived from SP logs, Gamma Ray logs, and
neutron-density crossplot.

• The preferred approach is the use of Gamma Ray because it has several empirical
relationships that give it an edge over the other methods.

SP Log uses the following formula:

PSP
VShale  1.0 
SSP
Where:

 Vshale = volume of shale

 PSP = Pseudostatic spontaneous potential (Maximum SP of shaly formation)

 SSP = Static spontaneous potential of a nearby thick clean sand

For Gamma Ray logs, the gamma ray index has to be calculated first before shale volume can be
found out. It uses the following formula:

GRlog  GRmin
I GR 
GRmax  GRmin
Where:

 IGR = gamma ray index

 GRlog = gamma ray reading of formation

 GRmin = minimum gamma ray (clean sand or carbonate)

 GRmax = maximum gamma ray (shale)

In general, gamma ray index is equal to shale volume. However there are other cases as well:

 Larionov (1969) for Tertiary rocks:


Vshale  0.083 23.7 IGR 1 

15
Log Interpretation
 Steiber (1970):
I GR
Vsh 
3  2 I GR
 Clavier (1971):


Vsh  1.7  3.38  I GR  0.7 
2 0.5

 Larionov (1969) for older rocks:


Vsh  0.33* 22.0 IGR 1 

16
Log Interpretation
The calculated shale volumes are then corrected as per log:

17
Log Interpretation
Calculating Water Saturation:
The water saturation is then calculated using the corrected porosities. The discussion of these
methods is beyond the scope of discussion, they are mentioned below:

18
Log Interpretation
 19
Log Interpretation
Dual Water Model

20
Log Interpretation
 Interpretation
This section will talk about the results of the methods that have been discussed above.

1. Archie’s Equation and Ratio Method

To use the ratio method in an interpretation process, the water saturation of the
uninvaded zone should be calculated by both the Archie equation (Swa) and the ratio
method (Swr)- The following observations can be made:

1. If Swa ~ Swr the assumption of a step-contact invasion profile is indicated to be correct,

and all values determined (Sw, Rt, Rxo and di) are correct.

2. If Swa > Swr then the value iov Rxo/Rt is too low. Rxo is too low because invasion is very
shallow, or Rt is too high because invasion is very deep. Also, a transition-type invasion
profile might be indicated and Swa is considered a good value for the zone's actual water
saturation.

3. If Swa < Swr then the value for Rxo/ Rt is too high. Rxo is too high because of the effect of
adjacent, high-resistivity beds, or Rt estimated from the deep resistivity measurement is
too low because Rxo is less than Rt. Also, an annulus-type invasion profile might be
indicated and/or Sxo < Sw1/5 (from Equation 7.5). In this case, a more accurate value for
water saturation can be estimated using the following equation (from Schlumberger,
1977):

Where:
 (Sw)COR = corrected water saturation of the uninvaded zone
 Swa =: water saturation of the uninvaded zone (Archie method)
 Swr = water saturation of the uninvaded zone (ratio method)

4. If Swa < Swr the reservoir might be a carbonate with moldic (i.e., oomoldic, fossil-moldic.
etc.) porosity and low permeability.

21
Log Interpretation
2. Quick Look Techniques

a. Rxo/Rt

To interpret the Rxo/Rt quick-look curve, the impermeable zones must be eliminated
by reference to the SP, GR, or microlog curves or by resistivity ratios. Then, if the SP
and Rxo/Rt (actually –K log Rxo/Rt) curves coincide in a permeable zone, the zone will
most probably produce water. If, however, the Rxo/Rt curve reads appreciably lower
(i.e., to the right) than the SP, the zone should produce hydrocarbons. An Rxo/Rt value
less than the SP amplitude indicates movable hydrocarbons are present.

The Rxo/Rt quick-look technique is applicable to fresh mud conditions (Rxo > Rt) in
formations where invasion falls within the limits demanded by
the Rxo/Rt computation. In other words, water zones may appear to be hydrocarbon-
productive. This constitutes a safeguard against overlooking pay zones, and it is
considered a desirable feature in any quick-look approach.

The Rxo/Rt technique efficiently handles variations in formation water resistivity, Rw,
and in shaliness. Any change in Rw is reflected similarly into both the
computed Rxo/Rt and the SP amplitude. Thus, comparing the two curves still permits
formation-fluid identification. Shaliness also affects the two curves in a similar
manner. All other things remaining constant, shaliness reduces the Rxo/Rt value and
the SP amplitude. Finally, the Rxo/Rt quick-look technique does not require porosity
data, nor use of any F – ϕrelationships.

b. Apparent water resistivity (Rwa)

The zone with the lowest value of Rwa is the most likely to be water-bearing, and the
value of Rwa is closest to the actual value of Rw in the formation.

Zones with values of Rwa greater than the minimum observed are likely to have some
hydrocarbon saturation.

c. Conductivity-derived porosity
In water-bearing zones, the conductivity-derived porosity is high and approximately
equal to the true formation porosity.

In zones that contain hydrocarbons, the conductivity-derived porosity is low, lower than
the true formation porosity.

22
Log Interpretation
 d. Wet resistivity (Ro)
In water-bearing zones, Ro and the deep resistivity should overlay.

In hydrocarbon-bearing zones, the deep resistivity is higher than Ro, with the separation
increasing with increasing hydrocarbon saturation

3. Bulk Volume Water

A reservoir at irreducible water saturation exhibits bulk volume values that are constant
throughout, because the saturation is permanent and constant. This means that for a
particular region when the bulk volume water values are determined for different
intervals, the values should be equal or approximately the same.

4. Saturation Cross Plot

a. Picket Cross Plot

The salient features of this plot are:

1. Water-bearing points of different porosities plot along a straight line with a slope of (-
1/m) and an intercept (at porosity = 1.0) of (a*Rw).

23
Log Interpretation
 From this line, the cementation exponent (m) can be determined, and if the tortuosity
factor (a) is known, Rw can be predicted. This is the water-bearing, or Ro, line.

2. Hydrocarbon-bearing points lie away from the line, moved horizontally to the right from
the water-bearing line by their increased resistivity. The horizontal distance of a point
from the water-bearing line depends on the water saturation (Sw) of that point. If the
saturation exponent (n) is known the water saturation can be determined. Lines of
constant water saturation lie parallel to the water-bearing line as shown below.

The water saturation of a point plotting away from the water-bearing line can be
determined by the equation:
1
R  n
S w   o 
 Rt 

24
Log Interpretation
 b. Hinge Cross Plot
This plot has following attributes.

1. Water-bearing points of different porosities plot along a straight line. The x-intercept of
the line (where conductivity is zero and resistivity infinite) occurs at the point where
porosity is zero. If the bulk density or acoustic travel time of the formation, instead of the
porosity, is plotted along the x-axis, the .x-intercept predicts the matrix value (matrix
density or matrix travel time) of the formation.

2. Hingle plot allows the interpreter to predict some of the parameters from the logs rather
than estimating them by other methods. The formation water resistivity (Rw) can be
estimated by choosing any point along the water-bearing line. The point's resistivity (Ro)
and porosity can be read from the plot, and values of tortuosity factor (a) and cementation
exponent (m) are assumed based on the chart that is chosen. Formation water resistivity is
then:

Ro * m
Rw 
a

25
Log Interpretation
 5. Log Derived Permeability

The formulae for this are:

1. Wyllie & Rose Method

3 2

𝑘 = (250 ∗ 𝑆 ) (for medium gravity oils)
𝑤𝑖𝑟𝑟

And;
3 2

𝑘 = (79 ∗ 𝑆 ) (for dry gases)
𝑤𝑖𝑟𝑟

2. Timur Method
2
2.2

𝑘 = (93 ∗ )
𝑆𝑤𝑖𝑟𝑟

6. Shaly Sand Analysis

1. SP Logs
 Deflection is decreased with respect to the shale base line, giving a lower SP
reading than actual.

 The value of formation water resistivity and subsequently, water saturation

decreases.

2. Gamma Ray Logs

 The presence of shale/clays increases the radioactivity, hence the gamma ray log
reading also increases.

26
Log Interpretation
3. Sonic Logs
 The obtained porosity value is higher than the actual value, because shales and
clays have a higher interval transmit time.

4. Neutron Logs
 The obtained porosity value is higher than the actual value, due to the presence of
clay bound water which is adsorbed to the surface.

5. Density Logs
 The obtained porosity value is higher than the actual value. This is because
shale/clay has a low matrix density.

6. Resistivity Logs
 Shale/clay is conductive. As a result, the obtained formation water resistivity
value is higher than the actual value, which eventually increases the water
saturation value from it’s true value.

27
Log Interpretation
Review
The following points can be gathered and be ascertained as the crux of the report:

1. Different methods have explained how to calculate water saturation and porosity.

2. Archie’s equation is widely used as it can easily accommodate uninvaded and flushed
zones.

3. The ratio method is derived from Archie’s equation, not only does it give a quick
indication of hydrocarbon movability, it also holds the advantage of being used when
there is no formation or porosity data available.

4. Bulk volume water indicates whether water will be produced along with hydrocarbons
from potential payzones.

5. Quick look methods can be used to determine potential payzones.

6. Hingle and Pickett crossplot techniques are straightforward and quick-time methods that
give some important parameters required for reservoir analysis.

7. Log derived data can provide a reliable estimate of permeability, given that the
formation is at irreducible water saturation.

Conclusion
The following points can be concluded from the report:

• Log interpretation is the foundation of well logging.

• The first step towards the determination of a reservoir potential lies in the determination
of water saturation and porosity.

• Log interpretation devises methods that give the maximum accuracy in the calculation of
the respective parameters, hence being the basis of ascertaining reservoir potential.

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Log Interpretation
References
http://petrowiki.org/Log_analysis_in_shaly_formations

http://petrowiki.org/Material_balance_in_oil_reservoirs

http://petrowiki.org/Reservoir_simulation

http://petrowiki.org/Production_forecasting_decline_curve_analysis

http://petrowiki.org/Formation_evaluation_during_mud_logging

http://wiki.aapg.org/Drill_stem_testing

https://www.spec2000.net/14-swirr.htm

http://petrowiki.org/Well_log_interpretation

https://www.spec2000.net/01-rockfluidmodel.htm

http://wiki.aapg.org/Archie_equation

https://www.spec2000.net/14-swrat.htm

http://wiki.aapg.org/Water_resistivity_determination

http://petrowiki.org/Water_saturation_determination

http://www.glossary.oilfield.slb.com/Terms/a/archie_equation.aspx

http://archives.datapages.com/data/specpubs/carbona2/data/a054/a054/0001/0000/0013.htm

http://wiki.aapg.org/Determination_of_water_resistivity

29
Log Interpretation