Interpreting geophysical well logs
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Transcript of Interpreting geophysical well logs
Lecture itemsHistorical Aspect
Types of Logsa) Gamma Ray
b) Sonic
c) Density/Neutron
d) Caliper
e) SP (spontaneous potential)
f) Resistivity (Induction)
Self Potential Log* Theory of measurement.
-Shale-base line& Sand line-SSP, PSP and SP log readings
* Factors affecting on log readings.* Applications.
-Resistivity Logs* Definition.
* Types
* Units& Presentation.* Theories of measurement.* Factors affecting on log readings.* Applications.
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Historical Aspect Schlumberger brothers, Conrad and Marcel, are credited
with inventing electrical well-logs.
On September 5, 1927, the first “well-log” was created in a
small village named Pechelbroon in France.
In 1931, the first SP (spontaneous potential) log was
recorded. Discovered when the galvanometer began
“wiggling” even though no current was being applied.
The SP effect was produced naturally by the borehole mud
at the boundaries of permeable beds. By simultaneously
recording SP and resistivity, loggers could distinguish
between permeable oil-bearing beds and impermeable
nonproducing beds.
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Types of Logs
a) Gamma Ray
b) Sonic
c) Density/Neutron
d) Caliper
e) SP (spontaneous potential)
f) Resistivity (Induction)
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Well LoggingIs a technique used for formation evaluation
to determine the size of the reservoir and the
amount of oil and gas in place.
The following parameters can be estimated
from different types of logging tools:
1) Borehole Hole Diameter.
2) Reservoir Thickness.
3) Porosity.
4) Water Saturation.
5) Rock Type (Lithology).
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Logging tools classification Based on the function, logging tools can be classified as follow:
1) Formation Fluid Indicators:
▪Induction
▪Laterolog
▪Microfocused and microresistivity devices
2) Formation property-lithology Indicators:
▪Acoustic
▪Density and lithologic density
▪Neutron
▪Gamma ray
3) Layer geometry Indicators:
▪Dipmeter
▪Borehole gravimeter
4) Auxiliary tools:
▪Spontaneous potential
▪Caliper
5) Specialty Tools:
▪Nuclear Magnetic Resonance
▪Dipole
▪Geochemical Tools
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a) Gamma Ray LogThe gamma ray measures the natural radioactivity of the rocks, and does not measure any
hydrocarbon or water present within the rocks.Shales: radioactive potassium is a common component, and because of their cation exchange
capacity, uranium and thorium are often absorbed as well. Therefore, very often shales will display high gamma ray responses, while sandstones and
limestone will typically show lower responses.
Shale is usually more radioactive than sand or carbonate, gamma ray log can be used tocalculate volume of shale in porous reservoirs. The volume of shale expressed as a decimalfraction or percentage is called Vshales.
Calculation of the Gamma Ray Index (IGR ) is the first step needed to determine the volume ofshale from gamma ray log.
The gamma ray log has several nonlinear empirical responses as well a linear responses. Thenon linear responses are based on geographic area or formation age. All non linearrelationships are more optimistic that is they produce a shale volume value lower than that fromthe linear equation. Linear response :
Where:
IGR =Gamma ray index
GRLog = gamma ray record from log
GRmin = gama ray for clean sand
GRmax = gamma ray for shale
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a) Gamma Ray
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The scale for GR is in
API (American
Petroleum Institute) and
runs from 0-125
units There are
often 10 divisions in a
GR log, so each
division represents 12.5
units.
Typical distinction
between a
sandstone/limestone
and shale occurs
between 50-60 units.
Often, very clean
sandstones or
carbonates will display
values within the 20
units range.
a) Gamma Ray➢For very hard compacted formation at depth of 8,000 ft
or more, gamma ray index is considered equal to shalevolume:
Vsh= IGR
➢For tertiary sediment rocks at depth of less than 4,000ft, the shale volume is:
Vsh= 0.083(23.7IGR
-1)
➢For older rocks at depth of 4,000-8,000 ft, the shalevolume is:
Vsh= 0.33(22IGR
-1)
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d) Caliper Log Caliper Logs record the diameter of the hole. It is very useful in relaying information about the quality of the
hole and hence reliability of the other logs. An example includes a large hole where dissolution, caving or falling of the rock wall occurred, leading to
errors in other log responses. Most caliper logs are run with GR logs and typically will remain constant throughout.
Borehole geometry is controlled by:
➢ Lithology
➢Mud type
➢ Formation Properties
➢ In-situ stresses
Borehole size can be determined from caliper log. Caliper log can be an indication to one of the following cases:
1) Gauged hole: diameter of hole is about equal to the bit size Hard well consolidated and impermeable formation. borehole diameter = drill bit size
2) Increased borehole diameter which means:a) Washout: general drilling wear, especially in shaly zones and dipping beds, both caliper larger than bit
size, considerable vertical extent .
b) Keyseat: asymmetric oval holes, formed by wear against the drill string at points where the borehole inclination changes (doglegs) .
c) Breakout: similar to keyseat but not due to doglegs, small brittle fractures due to existing stress regime of the country rock.
Unconsolidated formation borehole diameter > drill bit size
3)Decreased borehole diameter means:
a) Generally due to formation of mud cake
Mud cake thickness = (bit size diameter – caliper diameter reading)/2
b) mud cake formation indicates permeability and involves loss of mud filtrate into a permeable formation – invasion
Permeable formation borehole diameter < drill bit size
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b) Sonic (or Acoustic) LogSonic logs (or acoustic) measure the porosity of the rock.
Hence, they measure the travel time of an elastic wave through a formation (measured in ∆T- microseconds per meter).
Intervals containing greater pore space will result in greater travel time and vice versa for non-porous sections.
Wyllie’s time average equation can be used to determine porosity:
Where
t = log reading (s/ ft)
tma = transittime for the rock type (matrix)
tf =transittime for the fluid filling pores (usually189s/ft)
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Sonic logs are used to determine:
1) Determine porosity of reservoir
rock
2) Improve correlation and
interpretation of seismic records
3) Identify zones with abnormally
high pressures
4) Assist in identifying lithology
5) Estimate secondary pore space
6) Indicate mechanical integrity of
reservoir rocks and formations
that surround them (in
conjunction with density data)
7) Estimate rock permeability
Must be used in combination with
other logs, particularly gamma
rays and resistivity, thereby
allowing one to better understand
the reservoir petrophysics.
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HW Q.8:
From the gamma ray log, the record is 200 API, gamma ray
for shale zone is 120 API and gamma ray for clean sand is 40
API. Calculate the gamma ray index and shale volume if the
rocks at depth 3,500 ft and 7,500 ft.
Q.9: Sonic log reading t=100s/ft, tma = 80s/ft, tf=190s/ft. Calculate porosity.
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c) Density Log The density log belongs to the group of active nuclear tools, which contains a
radioactive source and two detectors. The Gamma Ray tool, which is a passive
nuclear tool, contains no source and can only measure the natural radiation in
the formation. The radioactive source is applied to the wellbore wall in a
shielded sidewall skid and emits medium gamma rays into the formation. The
gamma ray waves may be thought of as energy particles. As these energy
particles (photons) collide with the electrons in the formation, the gamma ray
loses some of its energy to the electron. This is called Compton scattering. The
denser the formation, the more electrons are presented, and more energy is
lost due to collisions. If the matrix density is known, then the energy loss is
directly related to porosity.
Density logs measure the bulk electron density of the formation, and is measured in kilograms per cubic meter (gm/cm3 or kg/m3).
Thus, the density tool emits gamma radiation which is scattered back to a detector in amounts proportional to the electron density of the formation. The higher the gamma ray reflected, the greater the porosity of the rock.
Electron density is directly related to the density of the formation (except in evaporates) and amount of density of interstitial fluids.
Helpful in distinguishing lithologies, especially between dolomite (2.85 kg/m3) and limestone (2.71 kg/m3).
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c) Density Log
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c) Density Log
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d) Neutron LogsdNucleus of all elements except hydrogen have neutrons. Neutrons have same mass as
protons but no charge. Their small size and electrical neutrality make neutrons ideal
projectiles for penetrating matter. Two categories of neutron sources are found in the
logging industry: chemical and pulsed sources. Chemical sources are composed of two
elements in intimate contact that continuously emit neutrons, usually Plutonium/Beryllium
or Americium/Beryllium. Such sources need to be heavily shielded when not in use.
Pulsed sources incorporate a neutron accelerator and a target, and can be activated by
simply switching on the accelerator. This source is used for pulsed neutron logging and in
tools that measure inelastic neutron collisions .
Neutron Logs measure the amounts of hydrogen present in the water atoms of a rock, and
can be used to measure porosity. This is done by bombarding the the formation with
neutrons, and determing how many become “captured” by the hydrogen nuclei.
Because shales have high amounts of water, the neutron log will read quite high
porosities- thus it must be used in conjunction with GR logs.
However, porosities recorded in shale-free sections are a reasonable estimate of the pore
spaces that could produce water.
It is very common to see both neutron and density logs recorded on the same section, and are often shown as an overlay on a common scale (calibrated for either sandstones or limestone’s).
This overlay allows for better opportunity of distinguishing lithologies and making better estimates of the true porosity.
* When natural gas is present, there becomes a big spread (or crossing) of the two logs, known as the “Gas Effect”. 20
d) Neutron Logs
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The following equation can be used to determine porosity from density log:
HW Q.10 :
The bulk density reading from density log is (2.2 gm/cc). The density of matrix is (2.45 gm/cc) and fluid density is (1.035 gm/cc). The density reading from neutron log is (15%). Calculate formation density.
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Example of dolomite overlying limestone, as
distinguished by the neutron/density.
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e) Resistivity Log Resistance is the opposition offered by a substance to the
passage of electric current. Resistivity is the resistancemeasured between opposite faces of a unit cube of thesubstance at specified temperature. Resistivity is measured inohm-meter2/meter, more commonly shortened to just ohm-meter.
Resistivity logs do not always measure resistivity directly.
➢ Some resistivity logs (actually induction logs) measuresconductivity instead which is the reciprocal of resistivity.
Induction logs are used in wells drilled with a relatively fresh-water mud (low salinity) to obtain more accurate value of trueresistivity.
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e) Resistivity (Induction) Resistivity logs record the resistance of interstitial fluids to the flow of
an electric current, either transmitted directly to the rock through an electrode, or magnetically induced deeper into the formation from the hole.
Therefore, the measure the ability of rocks to conduct electrical currents and are scaled in units of ohm-meters.
On most modern logs, there will be three curves, each measuring the resistance of section to the flow of electricity.
Porous formations filled with salt water (which is very common) have very low resistivities (often only ranging from 1-10 ohms-meter).
Formations that contain oil/gas generally have much higher resisitivities (often ranging from 10-500 ohms-meter).
With regards to the three lines, the one we are most interested in is the one marked “deep”. This is because this curve looks into the formation at a depth of six meters (or greater), thereby representing the portion of the formation most unlikely undisturbed by the drilling process.
One must be careful of “extremely” high values, as they will often represent zones of either anhydrite or other non-porous intervals.
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The resistivity of a rock (R) is given by:R = r (A / L)
Where:r = resistance (ohms) = E / I
A = Cross sectional area (meters2) L = Length (meters)R = resistivity (ohm-meters)E = Voltage (Volt)I = current (Amp)
Factors that influence Resistivity of Natural Porous Media:
1) Salinity of water2) Porosity3) Stress
4) Temperature
5) Pore geometry6) Rock Composition7) Wettability
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Formation Water Resistivity (Rw):
Formation water resistivity can vary widely from well to well. It can be estimated by thefollowing methods;
➢Chemical analysis of produced water
➢Direct measurement in resistivity cell
➢Using Empirical equations
The best method is direct measurement of resistivity.
Chemical analysis:
Resistivity of water is controlled by amount and type of ions present and temperature.Salinity is a measure of concentration of dissolved salts in water and is generally expressed asparts per million, grains/gallon or grams/liter.
1 grain/gallon = 17.118 ppm = 0.017118 grams/liter
NaCl is the most common dissolved salt in formation water; the concentration of otherdissolved ions is generally converted to equivalent concentration of sodium chloride;
Where;
C = equivalent concentration of NaCl.
Mi=weight multiplier (can be estimated from graph)
Ci= concentration of each ion.
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Based on equivalent
concentration of NaCl
and temperature,
formation water resistivity
can be determined using
graph.
The following equation
also can be used to
calculate (Rw).
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Formation Resistivity (Ro)
The resistivity of the formation saturated 100% with
formation water.
Archie equation:
Where:
FR = Formation factor
Formation porosity or the void space in the formation can
be determined from formation factor using the following
equation:
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HWQ.1
The chemical analysis of formation water as follow;
Room temperature=75oF
Calculate formation water resistivity at 75, 125 and 150oF.
Q.2
Formation water contains 10,000 ppm of NaCL, 15,000 ppm of MgSO4 and 8,000 ppm of CaCl2. Calculate the resistivity at formation temperature 200oF.
Q.3
Calculate formation water resistivity at 150oF if the concentration of NaCl 50,000, 100,00 1nd 150,000 using graph and equations.
Q.4
If the formation resistivity in the above cases (Q.3) is 2.4 Ω-m at 225oF and the cementation factor is 2. Calculate the porosity for each case.
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Ion Concentration (ppm)
Na 14,000Cl 12,000Mg 10,000Ca 8,000SO4 11,000
True Resistivity (Rt):
The resistivity of the formation at any saturation of water less
than 100% when the hydrocarbon displaces some water
from pore space in the formation. The relationship between
formation resistivity (Ro) and true formation resistivity (Rt)
can be represented by resistivity index:
Where: IR = resistivity index
Water saturation (Sw) which is defined as the percentage of
the pore volume filled with water can be determined from the
following equation :
Where: n = saturation exponent ≈2
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HW Q.5
Calculate porosity and water saturation if the formation factor is 15, true formation resistivity 10 Ω-m and the concentration of the formation water at 75oF is 60,000 ppm. Use m=n=2 and formation temperature 200oF.
Q.6
The resistance cylindrical core having 3 in diameter and 10 in height saturated 100 % with formation water is 10 Ω. The resistance of the core is increased to 85 Ω when oil is injected to it. Calculate water saturation of the core after the injection of oil.
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f) SP (Spontaneous Potential)• The SP log records the electric potential between an electrode
pulled up a hole and a reference electrode at the surface.• This potenital exists because of the electrochemical differences
between the waters within the formation and the drilling mud. • The potenital is measured in millivolts on a relative scale only
since the absolute value depends on the properties of the drilling mud.
• In shaly sections, the maximum SP response to the right can be used to define a “Shale Line”.
• Deflections of the SP log from this line indicates zones of permeable lithologies with interstitial fluids containing salinities differing from the drilling fluid.
• SP logs are good indicators of lithology where sandstones are permeable and water saturated.
• However, if the lithologies are filled with fresh water, the SP can become suppressed or even reversed. Also, they are poor in areas where the permeabilities are very low, sandstones are tighly cemented or the interval is completely bitumen saturated (i.e., oil sands).
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f) SP (Spontaneous Potential) The spontaneous potential (SP) log is a measurement of the natural potential
difference or self-potential between an electrode in the borehole and a referenceelectrode at the surface . It represents a recording of naturally occurring physicalphenomenon in in-situ rocks.
The SP curve records the electrical potential (voltage) produced by the interaction offormation water, drilling mud and shale. Though relatively simple in concept, the SPcurve is quite useful for a number of things:1) Differentiates potentially porous and permeable reservoir rocks2) from nonpermeable shales3) Defines bed boundaries and correlation of beds4) Aids in lithology identification5) Detection of hydrocarbon from suppression of SP response6) Permits determination of formation water resistivity, Rw7) Gives semi-quantitative indication of bed shaliness
Three factors are necessary to produce an SP current:
1) a conductive fluid in the borehole,
2) a porous and permeable bed surrounded by an impermeable formation, and
3) a difference in salinity (or pressure) between the borehole fluid and the formation fluid.
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Resistivity of drilling mud filtrate (Rmf): ➢The resistivity of drilling mud filtrate
which is normally observed in thepermeable layers.
➢The SP deflection is a reflection ofcontrast between the mud filtrateand connate water resistivity.
➢The deflection is said to be normalor -ve when the mud filtrate is moreresistive than the connate waterand is reverse or +ve when the mudfiltrate is less resistive that theconnate water. It is quite commonto find fresh water in shallow sandsand increasingly saline water asdepth increases. Such a progressionis shown in the figure, where SPappears deflecting to left deep inthe well but is reversed near to thesurface.
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SP (spontaneous potential)
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1- Electrokinetic Potential (can be neglected)
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2- Electrochemical Potential 1) Membrane Potential
2) Liquid Junction Potential
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Shale Baseline and SSP: SP has no absolute values and thus treated quantitatively and qualitatively in terms of
deflection, which is the amount the curve moves to the left or to the right of a defined zero. Thedefinition of the SP zero, called shale baseline, is made on thick shale intervals where the SPcurve does not move. All values are related to the shale baseline.
The theoretical maximum deflection of the SP opposite permeable beds is called the static SPor SSP. It represents the SP value that would be measured in an ideal case with the permeablebed isolated electrically. It is the maximum possible SP opposite a permeable, water-bearingformation with no shale.
The SSP is used to calculate formation-water resistivity (Rw).
SP = -K log(Rmfe/Rwe)
SP= SP value: this should be the SSP
(Rmf)e = equivalent mud filtrate resistivity: closely related to Rmf
(Rw)e = equivalent formation water resistivity: closely related to Rw
K = temperature-dependent coefficient = 61+ 0.133 * T
T= formation temperature (°F)
SP value measured is influenced by:
• Bed thickness
• Bed resistivity (Rmf, Rw, )
• Borehole and invasion
• Shale content
• Ratio of Rmf/Rw (amplitude and sign)
• Temperature
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Factors affecting SP log measurements
Rmf/Rw (Salinity effect) Fresh mud: negative SP, Saline mud: positive SP.
Shale or clay content Shale reduces SP.
Permeability
Presence of hydrocarbon
Bed thickness: SP decreases when bed thickness decreases.
Invasion: Reduces SP.
Mud filtrate: The magnitude and direction of SP deflection from the shale baseline depends on relative resistivities of the mud filtrate and the formation water.
Resistive formations
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PSP (Pseudo-static SP):the SP value in the water–bearing shalysand zone read from the SP log. SSP (Static SP): the maximum SP value in a clean sand zone.
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Clean Laminated Structural Dispersed
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Q.7
Calculate water formation resistivity
and shale volume if SSP=40 mv and
PSP=15 mv. Reservoir temperature is
250oF and Rmf=0.5 Ω-m.
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Rw
calculation
from SP
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Mathematical Calculation of Rw from SSP (modified after Bateman & Konen, 1977)
Rmf at 75oF = Rmf temp
* x (temp + 6.77)/81.77
Correction of Rmf to 75o
K = 60 + (0.133 x Tf)
Rmfe / Rwe = 10 – SSP / K**
Rmfe = (146 x Rmf – 5) / (337 x Rmf + 77)
Rmfe formula if Rmf at 75oF < 0.1
Rmfe = 0.85 x Rmf
Rmfe formula if Rmf at 75o > 0.1
Rwe = Rmfe / (Rmfe / Rwe)
Rw at 75oF = (77 x Rwe + 5) / (146 – 377 x Rwe)
Rw at 75o formula if Rwe < 0.12
Rw at 75oF = - [0.58 – 10
(0.69 x Rwe –0.24)]
Rw at 75oF formula if Rwe > 0.12
Rw at formation temperature = Rw at 75o x 81.77 / (Tf + 6.77)
*Rmftemp = Rmf at a temperature other than 75oF
**The e subscript (i.e. Rmfe) stands for equivalent resistivity.
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Applications
Differentiation between shaly, clean and shale zones.
Differentiation between Permeable and non-permeable zones.
Calculation of Rw.
Determination of the volume of shale.
For correlation purposes
For sedimentological analysis and facies studies.
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Notice how the shale baseline shows a
distinctive drift with depth. This
characteristics is commonly caused by an
increases in relative oxidation of the rocks
that are close to the land surface. The
highest sandstone in the well has a muted
deflection on the SP log as compared with
the lower sandstones. This contrast is an
immediate indication that water in the upper
sandstone may be significantly fresher than
waters of the lower sandstone. In other
wells it is not uncommon to see sandstone
units where the SP deflection goes to the
right of the shale baseline. In these
instances, the drilling mud filtrate is salter
than the formation water.
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A good example of thisphenomenon is shown inthe figure attached. In theupper sandstone, "U", theSP log shows a deflectionto the right, indicatingformation water to befresher than the drillingmud, while in the lowersandstone, "L", thedeflection is to the left,showing the formationwater to be more saline.
Flow chart from oil-industry log
analysis to estimate formation
water resistivity, Rw, in deep
formations from the SP log. RMF
is mud filtrate resistivity measured
at temperature Tmf and recorded
on the log header; Tf is the
temperature of the formation,
generally estimated by
interpolating between the bottom-
hole temperature (BHT) at total
depth (TD) and mean annual
temperature at the surface; SSP is
the static self-potential measured
on the log between the "clean
line" and "shale line" in millivolts
(mv) and with associated sign
(positive or negative).
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