BASIC INFORMATION

NEEDED IN LOG
INTERPRETATION
1. Lithology

2. Temperature of Formation (Tf)

3. Resistivity of Different Fluids

(Rw, Rmf, Rm)
1. Lithology
- important as a requirement for porosity
logs before a zones' porosity can be
calculated (lithology or a matrix
constant)
- the formation factor (F), a variable used in
the Archie water saturation equation (Sw
= \iF*Rw/Rt), varies with lithology. Sw
changes as F changes.
2. Temperature of Formation (Tf)
- important in log analysis because
resistivities of the drilling mud (Rm), the
mud filtrate (Rmf), and the formation
water (Rw) vary with temperature.
- the Tf is determined by knowing:
a. formation depth (D)
b. bottom hole temperature (BHT)
c. total depth of the well (TD)
d. surface temperature
- these factors/data are used and by assuming a
linear geothermal gradient.
- the formation temperature is also calculated
(Asquith, 1980) by using the linear
regression equation:
y = mx + c
where: x = depth
y = temperature
m=slope - (geothermal gradient)
c = a constant - ( surface
temperature)
3. Resistivity of Different Fluids (Rw, Rmf, Rm)
- Correction of the Resistivities of different fluids (Rm,
Rmf or Rw) to formation temperature (Chart and
formula)
 
Arp's Formula:
Rtf = Rtemp * (Temp + 6.77) / (Tf + 6.77)
where:
Rtf = resistivity at formation temperature
Rtemp = resistivity at a temperature other than
formation temperature
Temp = temperature at which resistivity was
measured
Tf = formation temperature
 Methods of obtaining Rw
(formation water resistivity)

• analysis of water samples from a drill stem test

• a water producing well
• from a catalog of water resistivity values
• from the SP log
• in water zones by the apparent water resistivity (Rwa)
method
1. SPONTANEOUS POTENTIAL (SP) LOG
2. RESISTIVITY LOGS
3. GAMMA RAY LOGS
4. SONIC LOG
5. DENSITY LOGS
6. NEUTRON LOGS
SPONTANEOUS POTENTIAL (SP) LOG
• one of the earliest electric logs used in the industry.
• used to identify impermeable zones such as shale,
and permeable zones such as sandstone.
• it is a record of direct current (DC) voltage
differences between the naturally occurring
potential of a moveable electrode in the wellbore,
and the potential of a fixed electrode located at the
surface. It is measured in millivolts (mV).
• electric currents arising primarily from
electrochemical factors within the borehole create
the SP log response. These are brought about by
differences in salinities between Rmf and Rw
within permeable beds.
* because a conductive fluid is needed in the
borehole for the SP log to operate, it cannot be
used in non-conductive (oil-based) drilling
muds.
 
Uses:
1. detect permeable beds
2. detect boundaries of permeable beds
3. determine formation water resistivity
4. determine volume of shale in permeable beds
5. auxiliary use is the detection of HCs by the
suppression of the SP response.
STATIC SP (SSP)
 
Importance:
• SSP represents the maximum SP that a thick,
shale-free, porous and permeable formation can
have for a given ratio between Rmf/Rw.
• it is determined by formula or chart
• it is a necessary element for determining accurate
values of Rw and volume of shale Factors
influencing SP value measured in the borehole
1. Bed thickness — thin formation (<10 ft), SP value
is less than SSP.
whenever SP curve is narrow and pointed in
shape, SP should be corrected (by chart) for bed
thickness.
2. Bed resistivity — higher resistivities reduce the
deflection of the SP curves.
3. Borehole diameter & invasion — effects of these
are very small and in general, can be ignored
(Hilchie, 1978).
4. Shale contact — reduces the SP deflection in a
permeable formation.
in water-bearing zones the amount of SP reduction
is proportional to the amount of shale in the
formation.
in hydrocarbon-bearing zones the amount of SP
reduction is greater than the volume of shale and is
called "HC suppression" (Hilchie, 1978).
5. Ratio of Rmf/Rw — "shale baseline" a straight line
formed as the SP response of shales is relatively
constant.
SP curve deflections are measured from this shale
baseline.
permeable zones are indicated where there is SP
deflection from the shale baseline
• If SP curve moves to the left (negative deflection;
Rmf>Rw) or
• If SP curve moves to the right (positive deflection;
Rmf<Rw); permeable zones are present.
• Permeable beds boundaries are detected by the
point of inflection from the shale baseline.
• If Rmf = Rw, no SP deflection from the shale
baseline.
• The magnitude of deflection is due to the
difference in resistivity between Rmf and Rw and
not to the amount of permeability.
Resistivity of Formation Water (Rw) calculated
from the SP curve

Procedure:
1. determine formation temperature
2. correct the resistivities of Rmf and Rw to
formation temperature.
3. to minimize for the effect of bed thickness, the
SP is corrected to static SP (SSP)

SSP represents the maximum SP a formation can

have if unaffected by bed thickness.
 Data necessary to correct SP to SSP (Chart 12)

• bed thickness
• resistivity from the shallow-reading resistivity tool
(Ri)
• resistivity of the drilling mud (Rm) at formation
temperature.

4. obtain value for the Rmf/Rw ratio (Chart 13)

5. obtain equivalent resitivity (Rwe) vale by dividing
Rmf by the Rmf/Rwe value (Chart 13)
6. correct Rwe value to Rw (Chart 14)
RESISTIVITY LOGS

 rock's matrix or grains are non-conductive and

since hydrocarbons are also non­conductive,
therefore the ability of the rock to transmit a
current is a function of water in the pores;
therefore, as the hydrocarbon saturation of the
pores increases, the rock's resistivity also
increases.
Uses:

• determine hydrocarbon versus water-bearing

zones (most important use)

• indicate permeable zones, and

• determine resistivity porosity

Determination of formation water saturation (Sw) from the
Archie Equation:

Where:
Sw= water saturation
F = formation factor (a/Фm)
a = turtuosity factor
m = cementation factor
Rw = resistivity of the formation water
Rt = true formation resistivity as measured by deep
reading resistivity tool
N = saturation exponent (most commonly 2.0)
Two (2) basic types of logs that measures formation
resistivity

1. Induction logs (most common, Dresser Atlas,

1975)
2. Electrode logs

Two types of Induction Devices

3. Induction Electric Logs

4. Dual Induction Focused Log
Classification of Resistivity Logs
I. Induction Logs (measures conductivity)
II. Electrode Logs (measures resistivity)
A. Normal Logs
B. Lateral Logs
C. Laterologs
D. Spherically Focused Logs (SFL)
E. Microlaterolog (MLL)
F. Microlog (ML)
G. Proximity Log (PL)
H. Microspherically Focused Log (MSFL)
 Depth of Resistivity Log Investigation

Flushed Zone Invaded Zone (Ri) Uninvaded Zone

(Rxo) 1. Short normal (Rt)
1. Microlog 2. Laterolog - 8 1. Long normal
2. Microlaterolog 3. Spherically 2. Lateral log
3. Proximity log Focused log 3. Deep Induction
4. Microsphericall 4. Medium log
y Focused log Induction log 4. Deep Laterolog
5. Shallow 5. Laterolog - 3
laterolog 6. Latero log - 3
7. g) Induction log
6FF40
*when Rmf is much greater than Rw, the Laterolog-8 and
Spherically Focused Lof will have a shallower depth of
investigation (closer to Rxo) than the medium induction,
shallow Laterolog, and the short normal.

Conductivity = 1000/Resistivity (milliohms/meter)

Resistivity = ohm-meters

NOTE:
•Induction log should be used in non-salt-saturated drilling
muds (i.e. Rmf>3 Rw) to obtain a more accurate value of
true resistivity (Rt).
•Electrode logs should be used for boreholes filled with salt-
saturated drilling muds ) to determine accurate Rt values.
INDUCTION ELECTRIC LOG

- composed of 3 curves; (1) short normal, (2)

induction, and (3) spontaneous potential obtained
simultaneously during logging of the well.
1) Short Normal = measures the resistivity of
invaded zone (Ri). When compared to Rt,
invasion is detected by the separation between the
short normal and induction curves.
-the presence of invasion is important because it
indicates a formation is permeable.

Uses:
a) record a reliable value for resistivity from a bed
thickness of four (4) feet (Ri).
b) used to calculate a value for resistivity porosity if
a correction is made for unflushed oil in the
invaded zone.
2) Induction = measures electrical conductivity using
current generated by coils. These induced currents
are recorded as conductivity by receiver coils.
 
Modern devices have additional coils which focus the
current so that signals are minimized from adjacent
formations, the borehole, and the invaded zone, thus,

• a deeper reading of conductivity is taken, and

• more accurate values of the formation resistivity
(Rt) are determined
• it can measure a reliable value for resistivity down
to a bed thickness of five (5) feet.
DUAL INDUCTION FOCUSED LOG

 it consists of a deep-reading induction device (RILD

which measures Rt), a medium-reading device
(RILm which measures Ri) and a shallow reading
(Rxo) focused Laterolog (either LL-8 or SFL).
 it is used in formations that are deeply invaded by
mud filtrate.
 Because of deep invasion, Rim may not accurately
measure Rt, hence resistivity values from 3 curves are
used to correct deep resistivity (RH_ I)) to true
resistivity (Rt) using a Tornado Chart. This Tornado
Chart can also help determine the diameter of invasion
(di) and the ratio of Rxo/Rt.

RILD does not always record an accurate value for

deep resistivity in thin, resistive (where Rt>100 ohm-
m) zones. A technique called Rt minimum (Rtmin)
should be used to determine true resistivity (Rt).
Formula: Rt min = (LL-8 or SFL) * Rw/Rmf
 
Where: Rt min = true resistivity
Rmf = resistivity of the mud filtrate @
formation temperature
Rw = resistivity of the formation water
@ formation temperature
LL-8 = shallow resistivity Laterolog-8
SFL = shallow resistivity Spherically
Focused Log
RULE: The rule for applying Rt min is to determine Rt
from both the Dual Induction Focused Log Tornado
Chart and from the Rt min formula, and use
whichever value of Rt is the greater.
LATEROLOG
 
it is designed to measure true formation resistivity
(Rt) in borehole filled with saltwater muds (when
Rmf Rw).
 
Invasion Effect:
 
if Rmf Rw, when drilled with saltwater-based
muds, invasion does not strongly affect Rt values
from Laterolog.
if Rrnf > 3Rw, when drilled with freshwater-based
muds, Laterolog can be strongly affected by invasion.
Laterolog should not therefore be used,
 
Borehole Size and Formation Thickness Effect:
the effect is small enough wherein Laterolog
resistivity can be taken as Rt.

If saltwater-based mud is used, it gives a very poor

SP response, therefore, a natural gamma ray log is
used as a lithology and correlation curve.
 DUAL LATEROLOG-MICROSPHERICALL Y
FOCUSED LOG
 
 it consists of a deep reading (Rt) resistivity device
(RLLD) and a shallow reading (Ri) resistivity device
(Rns). The microspherically focused log is a pad
type, focused electrode log that has a very shallow
depth of investigation, and measures resistivity of
the flushed zone (Rxo).
 the three curves, RLLII RLLS and MSFL, are used
to correct (for invasion) the deep resistivity (RLLD)
to true formation resistivity. A Tornado Chart is
necessary to correct to Rt and to determine the
diameter of invasion (d,) and the ratio of Rt/Rxo.
MICROLOG (ML)
 
 it is a pad type resistivity device that primarily
detects mudcake (Hilchie,1978).
 
Two (2) resistivity measurements
 
1. Micro Normal = investigates 3 to 4 inches into the
formation (measuring Rxo).
2. Micro Inverse = investigates approximately 1 to 2
inches and measures the resistivity of the mudcake
(Rmc).
 
The detection of mudcake indicates that invasion
has occurred and the formation is permeable.
Permeable zones show up on the Microlog as positive
separation when the micro normal curves read higher
resistivity than the micro inverse curves.
 
Shale zones are indicated by no separation or "negative
separation".
to detect zones of erroneous positive separation, a micro
caliper log is run, so that borehole irregularities are detected.
non-porous and impermeable zones have high resistivity
values on both the micro normal and micro inverse curves.
 
Hilchie (1978) states that resitivities of approximately ten
times (10X) the resistivity of the drilling mud (Rm) at
formation temperature indicate an impermeable zone.
MICROLATEROLOG AND PROXIMITY LOG
 
 designed to measure the resistivity of the flushed
zone (Rxo)
 since Microlaterolog is strongly influenced by
mudcake thicknesses greater than 1/4 inch
(Hilchie, 1978), it should be run only with
saltwater-based drilling muds.
 the more strongly focused Proximity log is
designed to investigate deeper so it can be used
with freshwater-based drilling muds where
mudcake is thicker.
RESISTIVITY DERIVED POROSITY

Since the ability of rock to transmit an electrical

current is almost entirely the result of the water in the
pore space, resistivity measurements can be used to
determine porosity. Normally, measurements of a
formation's resistivity close to the borehole (flushed
zone, Rxo, or invaded zone, Ri) are used to determine
porosity.
Water-bearing Formation..

When a porous and permeable water-bearing

formation is invaded by drilling fluid, formation
water is displaced by mud filtrate. Porosity in a
water-bearing formation can be related to shallow
resistivity (Rxo) by the following equations:

Sxo =

where: Sxo = 1.0 (100%) in water-bearing zones

1.0 =
Hydrocarbon-bearing Zones
In hydrocarbon-bearing zones, the shallow resistivity (Rxo)
is affected by the unflushed residual hydrocarbons left by
the invading mud filtrate. These residual hydrocarbons will
result in a very high Rxo values because hydrocarbons have
a higher resistivity than formation water. Therefore, the
calculated resistivity porosity in hydrocarbon-bearing zones
will be too low.

To correct for residual hydrocarbons in the flushed zone,

water saturation of the flushed zone (Sxo) must be known or
estimated. Then, a formations' shallow resistivity (Rxo) can
be related to porosity by the following:

Sxo =
 Table : Percentage of Residual Hydrocarbon
Saturation as a Function of Hydrocarbon Density and
Porosity (modified after Hilchie, 1978)

API°
Gravity RHS % Sxo %
Gas 40 to 5 60 to 95
High Gravity Oil 40 to 50 10 to 5 90 to 95
Medium Gravity
20 to 40 20 to 10 80 to 90
Oil
Low Gravity Oil 10 to 20 30 to 20 70 to 80
Porosity % RHS % Sxo %
25 to 35 30 70
15 to 20 15 85
GAMMA RAY LOGS

•it measures natural radioactivity in formations.

Uses:
1. used for identifying lithologies,
2. used for correlating zones, and
3. provide information for calculating the volume of
shale in a sandstone or carbonate horizon.

*Clean sandstone may also produce a high gamma

ray response if the sandstone contains potassium
feldspars, micas, glauconite, or uranium-rich waters.
Spectralog = can be run in addition to the gamma
ray log if there is a presence of glauconite,
micas, or potasium feldspars. It breaks the
natural radioactivity of a formation into the
different types of radioactive material; thorium,
potassium, and uranium.
VOLUME OF SHALY CALCULATION
Gamma ray logs can be used to calculate shale
volume in porous reservoirs and then, applied for
analysis of shaly sands.
*Calculation of Gamma ray index (first step)
(Formula from Schlumberger, 1974)

IGR = (GRlog – GRmin) (GRmax - GRmin)

where:
IGR = gamma ray index
GRlog = gamma ray reading of formation
GRmax = maximum gamma ray (shale)
GRmin = minimum gamma ray (clean sand or
carbonate)
POROSITY LOGS

A. SONIC LOG
- a porosity log that measures interval transit time (At) of
a compressional sound wave travelling through one
foot of formation.

Borehole Compensated (BHC) devices = modern sonic

logs that greatly reduces the spurious effects of borehole
size variations (Kobesh & Blizard, 1959), and also the
errors due to tilt of the sonic tool (Schlumberger, 1972).

Interval Transit time (Δt) in microseconds per foot is the

reciprocal of the velocity of a compressional sound wave
in feet per second.
- it is dependent upon both lithology and porosity,
hence, a formations' matrix velocity must be
known to derive sonic porosity either by chart or
by the following formula (Wyllie, et.al., 1958).

Фsonic = [Δtlog –Δtma) (Δtf - Δtma)]

where:
Ф = sonic derived porosity
Δtma = interval transit time of the matrix
Δtlog = interval transit time of formation
Δtf = interval transit time of the fluid in the
wellbore
(fresh mud - 189; salt mud = 185)
 Table: Sonic Velocities and Interval Transit
Time for Different Matrices

Δtma (µsec/ft)
Vma (ft/sec) Δtma (µsec/ft) Commonly
used
18,000-
Sandstone 55.5 -51.0 55.5 - 51.0
19,500
21,000 -
Limestone 47.6 - 43.5 47.6
23,000
23,000 -
Dolomite 43.5 - 38.5 43.5
26,000
Anhydrite 20,000 50.0 50.0
Salt 15,000 66.7 67.0
(Casing) Iron 17,500 57.0 57.0
Uses of Wyllie Formula:
 it can be used for determination of porosity in
consolidated sandstone and carbonates with
intergranular porosity (grainstone) or
intercrystalline porosity (sucrosic dolomites).
 porosity will be too low if calculated in
carbonates with vuggy or fracture porosity. It
records matrix porosity not vuggy or fracture
secondary porosity.
 the % porosity of these vuggy or fracture
secondary porosity can be measured by
subtracting the sonic porosity from the total
porosity.
 total porosity can be obtained from either density
or neutron logs.
B. DENSITY LOGS

- a porosity log that measures electron density of a

formation.
 
Uses: (Schlumberger, 1972)
• identify evaporite minerals,
• detect gas-bearing zones,
• determine hydrocarbon density,
• evaluate shaly sand reservoirs and complex
lithologies.
 it consists of a medium-energy gamma ray
source, either Cobalt-60 or Cesium-137, that
emits gamma rays into a formation.
 Compton Scattering = the interaction between
incoming gamma ray particles, that losses energy,
and electrons in the formation.
 the scattered gamma rays are counted as an
indicator of formation by the detector.
 electron density can be related to bulk density (ρb)
of a formation in gm/cc.
 because the modern density log is a compensated
log (dual detectors), the correction curve (Δρ)
records how much correction has been applied to
the bulk density curve (pi), due to borehole
irregularities.
 Whenever the Δρ exceeds 0.20 gm/cc, the value
of the ρb obtained from the ρb curve should be
considered invalid.
Formation Bulk Density (ρb) is a function of
matrix density, porosity (), and density of the fluid
in the pores (salt mud, fresh mud, or
hydrocarbons).
To determine density porosity, either by chart
or by calculation, the matrix density (Table) and
type of fluid in the borehole must be known.

The formula is:

Фden = [(ρb ρb) (ρb ρb)]

where:
Фden = density derived porosity
ρma = matrix density (table)
ρb = formation bulk density
ρf = fluid density (1.1 salt mud, 1.0 fresh mud and
0.7 gas)

Where invasion of a formation is shallow, low

density of the formations' hydrocarbons will increase
density porosity.
• oil does not affect significantly density porosity,
gas does (gas effect).
• if gas density is unknown, 0.7 gm/cc for fluid
density can be used as suggested by Hilchie
(1978).
Table. Matrix Densities of Common Lithologies
(after Schlumberger, 1972)

Lithology Pma (gm/cc)

Sandstone 2.648
Limestone 2.710
Dolomite 2.876
Anhydrite 2.977
Salt 2.032
C. NEUTRON LOGS
 porosity log that measures the hydrogen ion
concentration in a formation.
 in clean formations (i.e. shale-free) where the porosity
is filled with water or oil, the neutron log measures
liquid-filled porosity.
 neutrons are created from a chemical source in the
neutron logging tool.
 the source may be a mixture of americium and
beryllium which will continuously emit neutrons.
 neutrons collide with the nuclei of the formation
material, and losses some of its energy. The maximum
loss occurs when the neutron collides with a hydrogen
atom and is a function of a formation's hydrogen
concentration.
 since hydrogen in a porous formation is concentrated
in the fluid-filled pores, energy loss can be related to
the formation's porosity.
Whenever pores are filled with gas rather than oil or
water, neutron porosity will be lowered. It occurs since
there is less concentration of hydrogen in gas compared to
oil or water. A lowering of neutron porosity by gas is
called "gas effect".
Variations in neutron log response:
• differences in detector types,
• spacing between source and detector,
• lithology.

 the variation in response can be compensated for by

using the appropriate charts.
 Neutron logs (unlike all other logs) must be
interpreted from the specific chart designed for a
specific log. Because neutron logs are not calibrated
in basic physical units (Dresser Atlas, 1975).
 
• Sidewall Neutron log = first modern neutron log.
• Compensated Neutron Log = most modern has a
neutron source and two detectors.
Advantage of CNL over SNL

• CNL are less affected by borehole irregularities.

• both CNL and SNL are recorded in apparent
limestone, sandstone, or dolomite porosity units.
• if a formation is limestone, and the neutron log is
recorded in apparent limestone porosity units,
apparent porosity is equal to true porosity.
However, when the lithology of a formation is
sandstone or dolomite, apparent limestone
porosity must be corrected to true porosity by
using the appropriate chart.
COMBINATION NEUTRON-DENSITY LOG
a combination porosity log.
 
Uses:
•to measures porosity,
•to determine lithology, and
•to detect gas-bearing zones.

- both curves are normally recorded in limestone

porosity units, however, sandstone and dolomite
can also be recorded.
 
 Procedures for obtaining true porosity: (Crossplot
Method)
1. Read apparent limestone porosities from the
neutron and density curves.
2. Crossplot values on a neutron-density porosity
chart.
 
Mathematical Calculations of Porosity from a
Neutron-Density Log
 
N-D = N
2
+ D /2 *(used for gas bearing formation)
2

 
where: ФN-D = neutron-density porosity
ФN = neutron porosity (limestone units)
ФD = density porosity (limestone units)
 COMBINATION GAMMA RAY-NEUTRON-
DENSITY LOG
 
 a powerful tool for subsurface geologist to
construct facies maps.
 It also illustrates the change in neutron-density
response between an oil- or water-bearing sand and
gas-bearing sand.
• Oil- or water-bearing sand has a density log
reading of FOUR (4) porosity units more than the
neutron log.
• Gas-bearing sand has a density reading of up to
TEN (10) porosity units more than the neutron log.
Gas Effect = where an increase in density porosity
occurs along with a decrease in neutron porosity in
a gas-bearing zone (created by as in the pores).
 
• Gas in the pores causes the density log to record
too much high a porosity (i.e. gas is lighter than oil
and water)
• Causes the neutron log to record too low a porosity
(i.e. gas has a lower concentration of hydrogen
atoms than oil and water).
 
*It is very important because it helps to detect gas-
bearing zone.
LOG INTERPRETATION
• Log parameters can help evaluate a zone and
determine whether a well completion attempt is
warranted.
Different methods which help establish these
important parameters:
1. Water Saturation of the uninvaded zone (Sw)
2. Water Saturation of the flushed zone (Sxo)
3. Bulk Volume Water
4. Pickett and Hingle crossplots
5. "Quick Look" analysis
6. Log derived permeability (Ke)
7. Shaly sand Analysis
• As important as log parameters are, however, they
should not be applied to the exclusion of another
data.
• Other relevant data are from DST, sample shows,
mud log analysis, nearby production, etc.

I. Archie Equation Sw and Sxo

Water Saturation (Sw) of a reservoirs’ uninvaded
zone is calculated by the Archie (1942) formula;

Sw = [(a/Фm) * (Rw/Rt)]1/n
where:

Sw = water saturation of the uninvaded zones

(Archie method)
Rw = resistivity of formation water at formation
temperature.
Ф = porosity
a = turtuosity factor
m = cementation exponent
n = saturation exponent which varies from 1.8 to
2.5, but normally equal to 2.0
• Sw is the most fundamental parameter used in log
evaluation.
• Additional information necessary:
 Water saturation should low enough for a
water-free completion.
 Hydrocarbons are moveable
 Zone is permeable
 Whether (volumetrically) these are economic,
recoverable hydrocarbon reserves
Water saturation of a formations’ flushed zone (Sxo)
based on the Archie equation:
 
Sxo = [(a/Фm) * (Rmf/Rxo)]1/n
where:
Sxo = water saturation of the flushed zone
Rmf = resistivity of the mud filtrate at formation
temperature
Rxo = shallow resistivity from LL-8, MSFL, or MLL

Sxo can be used as an indicator of hydrocarbon

moveability. If Sxo>>Sw, then hydrocarbons in the
flushed zone have probably been moved or flushed out
of the zone nearest the borehole by the invading drilling
fluids (Rmf).
II. Ratio Method
• it identifies hydrocarbons from the difference
between water saturations in the flushed zone
(Sxo) and the uninvaded zone (Sw).
• this can be very useful/helpful in log analysis
because, from the ratio of (Rxo/Rt)/(Rmf/Rw), a
value for both the moveable hydrocarbon index
(Sw/Sxo) and water saturation can be determine
without knowing porosity.
 
Formulas for calculating the moveable
hydrocarbon index and water saturation by the Ratio
Method are:
(Sw/Sxo) = [(Rxo/Rt)/(Rmf/Rw)]1/2

where:
Sw/Sxo = moveable hydrocarbon index
Rxo = shallow resistivity from LL-8, MSFL, or
MLL
Rt = true resistivity (Ril D or Rim corrected for
invasion)
Rmf = resistivity of mud filtrate at formation
temperature
Rw = resistivity of formation water at formation
temperature
• If Sw/Sxo is = to 1.0 or greater, then hydrocarbons
were not moved during invasion.
• If Sw/Sxo is <0.7 for sandstones or <0.6 for
carbonates, moveable hydrocarbons are indicated
(Schlumberger, 1972).
To determine water saturation (Sw) by the Ratio
Method, the flushed zones' water saturation must be
known.
Ratio Method formula for Water Saturation of
uninvaded zone
 
Swr=[(Rxo/Rt)/(Rmf/Rw)]5/8 or
Swr= (Rxo/Rt)/(Rmf/Rw)]0.625
Where: Swr = water saturation in uninvaded zone by
Ratio Method.
 After the Sw values of uninvaded zone has been
calculated by both methods, comparison should be
made and observations should be noted as follows:
 
1. if Sw (Archie) ≈ Sw (Ratio), 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 Sw (Archie) > Sw (Ratio), then the value for
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 may be
indicated and Sw (Archie) is considered a good value
for Sw.
3. if Sw (Archie) < Sw (Ratio), then the value for
Rxo/Rt is too high. Rxo is too high because of the
affect of adjacent, high resistivity beds, or Rim
(Rt) is too low because Rxo is less than Rt. Also,
an annulus type invasion profile maybe indicated
and/or Sxo<Sw1/5. Hence, a more accurate value
for water saturation can be estimated using the
following equation (from Schlumberger, 1977):
(Sw)cor = Swa * (Swa/Swr)0.25
where:
(Sw)cor = corrected water saturation of the uninvaded
zone.
Swa = water saturation of the uninvaded zone
Swr = water saturation of the uninvaded zone
 
4. if Sw (Archie) < Sw (Ratio), the reservoir may be
a carbonate with moldic porosity and low
permeability.

III. Bulk Volume Water (BVW)

 the product of a formation's water saturation (Sw)

and its porosity (Ф)

BVW = Sw *Ф
Sw = uninvaded zone (Archie equation)
• If values for BVW, calculated at several depths are
constant or very close to constant, they indicate
that the zone is homogeneous and at irreducible
water saturation (Swirr).
• When a zone is at Swirr, water calculated in the
uninvaded zone (Sw) will not move because it is
held on grains by capillary pressure.
• Therefore, hydrocarbon production from a zone at
Swirr should be water-free (Morris & Beggs,
1967).
• A formation not at Swirr will exhibit wide
variations in bulk volume water values.
• Amount of water a formation can held by capillary
pressure increases with decreasing grain size, the
BVW also increases with decreasing grain size.
IV. Quick-Look Method

A. Rxo/Rt Curve
SP = -K * log (Rmf/Rw)
Where:
SP = spontaneous potential
K = 60 + (0.133*formation temperature)
Rmf = resistivity of mud filtrate at formation
temperature
Rw = resistivity of formation water at formation
temperature

In water zones (Sw = 1.0):

Rxo = F * Rmf and Ro = F * Rw
SP equation can be rewritten as:

SP = -K * log(Rxo/Ro)
Where:
Rxo — shallow resistivity from LL-8, MSFL, or
MLL
Ro — wet resistivity (Ro = Rt when Sw = 100%)
• In water-bearing zones, the measured values for
Rxo and Ro
• (Rt for Sw = 100%; RILD or RLLD) can be used
to calculate a value for SP.
• The presence of hydrocarbons results in Rt values
which are greater than Ro. This means that when
SP is calculated from the Rxo and Rt values, it will
be lower than the measured value of SP.
B. Rwa Curve
In water-bearing zones, the Archie equation for the
uninvaded zone can be rewritten as follows:
 
Sw = or
1.0 =

Rwa = apparent water resistivity (Rwa = Rw in

water-bearing zone)
F = formation factor (a/Фm); 1/Ф2 (CO3),
0.81/Ф2 (consolidated sands);
0.62/Ф2.I5(unconsolidated sands)
If Sw = 100%, the calculated Rwa value = Rw
If hydrocarbons are present, Rt > Ro, and Rwa >
Rw (Fertl, 1978)
- Rwa curve will deflect to the left in wet zones
- Rwa curve will deflect to the right in
hydrocarbon-bearing zones
- This deflection is similar to the behavior of the
RXO/Rt curve for hydrocarbon or wet zones.

 
Advantage of the Rwa curve: (over Rxo/Rt curve)
-The Rwa can be convertd to quantitative value for
water saturation (Sw).

The procedure is:

a) Sw =

Where:
Sw = water saturation of the uninvaded zone
Rw = resistivity of formation water at formation
temperature
Rwa = apparent formation water resistivity from
Rwa curve.

Note: when Rw = Rwa then Sw = 100%

Uses of Rwa:

1. for hydrocarbon detection

2. can be applied as a calculated value for Rw in
water-bearing zones
3. can be used as Rw to calculate Sw in
hydrocarbon-bearing zones if both zones have a
constant formation water resistivity.
D. Conductivity Derived Porosity Curve
1. A Dresser Atlas (1975) quick look curve along
with SP curve.
2. Resistivities of the uninvaded zone (Rt), rather
than Rxo, are applied to find resistivity porosity.
The formulas are as follows:
 
a. Water-bearing zones:
Ф= [(a*Rw)/Rt]1/m Sw = 100% and Rt = Ro

b. Hydrocarbon-bearing zones:
Ф = [(a*(Rw/Rt))/Sw2]1/m Sw < 1.0 and Rt > Ro
where: Ф = resistivity (conductivity) derived
porosity
Rw = resistivity of formation water at formation
temperature
Ro = formation resistivity when Sw = 100%
Rt = true formation resistivity (RILD or RLLD) Rt = Ro
if Sw = 100%
Sw= water saturation of the uinvaded zone
a = constant (Dresser Atlas uses 1.0 for CO3 & 0.62
for sandstone)
m = constant (Dresser Atlas uses 2.0 for CO3 & 2.15
for sandstone)