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Well log is one of the most fundamental methods for reservoir characterization, in oil and gas industry, it is an essential method for geoscientist to acquire more knowledge about the condition below the surface by using physical properties of rocks. This method is very useful to detect hydrocarbon bearing zone, calculate the hydrocarbon volume, and many others. Some approaches are needed to characterize reservoir, by using well log data, the user may be able to calculate:

Well log is one of the most fundamental methods for reservoir characterization, in oil and gas industry, it is an essential method for geoscientist to acquire more knowledge about the condition below the surface by using physical properties of rocks. This method is very useful to detect hydrocarbon bearing zone, calculate the hydrocarbon volume, and many others. Some approaches are needed to characterize reservoir, by using well log data, the user may be able to calculate:

# shale volume (Vsh)

# shale volume (Vsh)

==Interpret the Lithology==

==Interpret the Lithology==

[[File:Well_Log_Analysis_Fig-3.png|thumb|300px|Figure 3-The use of gamma ray log to determine the lithology (Railsback, 2011).]]

[[File:Well_Log_Analysis_Fig-3.png|thumb|300px|Figure 3-The use of gamma ray log to determine the lithology.<ref>Railsback, 2011, Characteristics of wireline well logs in the petroleum industry.</ref>]]

The user will be able to interpret the lithology by using several logs, there are gamma ray, spontaneous potential, resistivity, and density log. Basically, a formation with high gamma ray reading indicates that it is a shaly or shale, when the low gamma ray reading indicates a clean formation (sand, carbonate, evaporite, etc.), lithology interpretation is very important in reservoir characterization because, if the lithology interpretation is already wrong, the other steps such as porosity and water saturation calculation will be a total mess.

The user will be able to interpret the lithology by using several logs, there are gamma ray, spontaneous potential, resistivity, and [[density log]]. Basically, a formation with high gamma ray reading indicates that it is a shaly or shale, when the low gamma ray reading indicates a clean formation (sand, carbonate, [[evaporite]], etc.), lithology interpretation is very important in reservoir characterization because, if the lithology interpretation is already wrong, the other steps such as porosity and water saturation calculation will be a total mess.

==Calculate the Shale Volume==

==Calculate the Shale Volume==

| Limestone || 2.710 || Salt Water || 1.15

| Limestone || 2.710 || Salt Water || 1.15

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| Dolomite || 2.877 || Methane || 0.423

| [[Dolomite]] || 2.877 || Methane || 0.423

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| Anhydrite || 2.960 || Oil || 0.8

| [[Anhydrite]] || 2.960 || Oil || 0.8

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| Salt || 2.040 || ||

| Salt || 2.040 || ||

==Calculate the Water Saturation==

==Calculate the Water Saturation==

There are so many methods to calculate water saturation, the user may use Archie’s (1942), Simandoux’s (1963), etc. which will use different formula for every one of them, but in this article, the author will use Simandoux’s (1963) method, to calculate the water saturation by using this method, the user will need to use the following formula:

There are so many methods to calculate water saturation, the user may use Archie’s,<ref>Archie, G. E., 1950, Introduction to petrophysics of reservoir rocks: AAPG Bulletin, v. 34, no. 5, p. 943-961.</ref> Simandoux’s (1963), etc. which will use different formula for every one of them, but in this article, the author will use Simandoux’s (1963) method, to calculate the water saturation by using this method, the user will need to use the following formula:

:<math>\frac{1}{Rt} = \frac{Sw^2}{F \times Rw} + \frac{Vsh \times Sw}{Rsh}</math>

:<math>\frac{1}{Rt} = \frac{Sw^2}{F \times Rw} + \frac{Vsh \times Sw}{Rsh}</math>

where Rt is the true resistivity of the formation (deep resistivity), Rw is the formation water resistivity, Vsh is the shale volume, Rsh is the resistivity of shale, Rwe is the formation water

where Rt is the true resistivity of the formation (deep resistivity), Rw is the formation water resistivity, Vsh is the shale volume, Rsh is the resistivity of shale, Rwe is the formation water





resistivity (without thermal effect), BHT is the bottom hole temperature, Rmf is the mud filtrate resistivity, SP is the spontaneous potential log reading, F is the formation volume factor, a is the tortuosity factor, m is the cementation exponent, φ is the porosity, and Sw is the water saturation. To acquire the value of a and m, the user will need to create a pickett plot, but according to Asquith (1980), the reference value is shown in table 3.

resistivity (without thermal effect), BHT is the bottom hole temperature, Rmf is the mud filtrate resistivity, SP is the spontaneous potential log reading, F is the formation volume factor, a is the tortuosity factor, m is the cementation exponent, φ is the porosity, and Sw is the water saturation. To acquire the value of a and m, the user will need to create a pickett plot, but according to Asquith,<ref name=Asquith>Asquith, G. B., Krygowski, D., & Gibson, C. R. (2004). Basic well log analysis(Vol. 16). Tulsa: American Association of Petroleum Geologists.</ref> the reference value is shown in table 3.





{| class = wikitable

{| class = wikitable

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|+ Table 3-Tortuosity factor (a) and cementation exponent (m) reference table (Asquith, 1980).

|+ Table 3. Tortuosity factor (a) and cementation exponent (m) reference table.<ref name=Asquith />

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! Lithology || a (tortuosity factor) || m (cementation exponent)

! Lithology || a (tortuosity factor) || m (cementation exponent)

Defined as the rock’s ability to transmit fluid, higher permeability shows that the rock is able to transmit fluid easiliy and it means that the more hydrocarbon that can be produced daily, it is affected by many factors, such as shale volume, effective porosity, and many other else. There are so many methods that can be used to calculate the permeability, but in this article, the author will use Coates’s (1981) method, the formula is listed below:

Defined as the rock’s ability to transmit fluid, higher permeability shows that the rock is able to transmit fluid easiliy and it means that the more hydrocarbon that can be produced daily, it is affected by many factors, such as shale volume, effective porosity, and many other else. There are so many methods that can be used to calculate the permeability, but in this article, the author will use Coates’s (1981) method, the formula is listed below:

()

:<math>\text{k} = 100 \times \frac{\mathit{\Phi}^2 \times (1 - Swirr)}{Swirr}</math>

where k is the permeability, φ is the porosity, and Swirr is the irreducible water saturation (the author use 0.3 as the assumption for this variable). From the formula above, we can conclude that if the irreducible water saturation is at 1, then the permeability will be zero.

where k is the permeability, φ is the porosity, and Swirr is the irreducible water saturation (the author use 0.3 as the assumption for this variable). From the formula above, we can conclude that if the irreducible water saturation is at 1, then the permeability will be zero.

There are so many kinds of elastic properties of a rock, there are Acoustic Impedance (AI), Shear Impedance (SI), Poisson Ratio (σ), etc. and most of them depend on the wave velocity and density.

There are so many kinds of elastic properties of a rock, there are Acoustic Impedance (AI), Shear Impedance (SI), Poisson Ratio (σ), etc. and most of them depend on the wave velocity and density.

where Vp is the P-Wave velocity and Vs is the S-Wave velocity. According to Castagna et al (1985), Vp and Vs can be calculated by using this formula:

 

where Vp is the P-Wave velocity and Vs is the S-Wave velocity. According to Castagna et al,<ref>Castagna, J. P., Batzle, M. L., & Eastwood, R. L. (1985). Relationships between compressional-wave and shear-wave velocities in clastic silicate rocks.Geophysics, 50(4), 571-581.</ref> Vp and Vs can be calculated by using this formula:

 

where φs is the sonic-derived porosity, Vclay is the clay volume, Δtlog is the sonic log reading (DT), Δtmatrix is the matrix transit time (see table 4 for reference value), and Δtfluid is the fluid transit time (see table 4 for reference value). Theoretically, a formation with high density will has lower transit time (Δtlog) which will cause the seismic wave to travel faster in that formation. An anomaly in density and sonic log (Δt) in a formation may indicates the presence of fluids in that formation (see section 9).

where φs is the sonic-derived porosity, Vclay is the clay volume, Δtlog is the sonic log reading (DT), Δtmatrix is the matrix transit time (see table 4 for reference value), and Δtfluid is the fluid transit time (see table 4 for reference value). Theoretically, a formation with high density will has lower transit time (Δtlog) which will cause the seismic wave to travel faster in that formation. An anomaly in density and sonic log (Δt) in a formation may indicates the presence of fluids in that formation (see section 9).

{| class="wikitable"

{| class="wikitable"

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|+ Table 4-Matrix and fluid transit time reference table (Schlumberger, 1972).

|+ Table 4. Matrix and fluid transit time reference table.<ref>Schlumberger Limited, 1984, Schlumberger log interpretation charts.</ref>

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! Lithology !! Value (μs/ft) !! Fluid !! Value (μs/ft)

! Lithology !! Value (μs/ft) !! Fluid !! Value (μs/ft)

==Reflectivity Coefficient==

==Reflectivity Coefficient==

The reflectivity coefficient could be derived from density and sonic log then the user may complete this method simply by using the AI difference between every formation which shows the reflectivity coefficient (R) which shows the rock’s ability to reflect the seismic wave to the surface, the formula is listed below:

The reflectivity coefficient could be derived from density and sonic log then the user may complete this method simply by using the AI difference between every formation which shows the reflectivity coefficient (R) which shows the rock’s ability to reflect the seismic wave to the surface, the formula is listed below:

where ρ1 is the density of the rock in the first formation, ρ2 is the density of the rock in the second formation, Vp1 is the P-Wave velocity in the first formation, and Vp2 is the P-Wave velocity in the second formation. The reflectivity coefficient is very related with seismic, it represents how good is the rock’s ability to reflect seismic wave, if the reflectivity is high, then more seismic wave will be reflected back to the surface which will be shown by the presence of bright spot, but if the reflectivity is very low, it is called dim spot, both of them could be used as hydrocarbon indicator.

where ρ1 is the density of the rock in the first formation, ρ2 is the density of the rock in the second formation, Vp1 is the P-Wave velocity in the first formation, and Vp2 is the P-Wave velocity in the second formation. The reflectivity coefficient is very related with seismic, it represents how good is the rock’s ability to reflect seismic wave, if the reflectivity is high, then more seismic wave will be reflected back to the surface which will be shown by the presence of bright spot, but if the reflectivity is very low, it is called dim spot, both of them could be used as hydrocarbon indicator.

{| class="wikitable"

{| class="wikitable"

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|+ Table 5-Petrophysical properties reference of some sedimentary rocks.

|+ Table 5. Petrophysical properties reference of some sedimentary rocks.

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! Lithology !! Gamma Ray (API) !! Spontaneous Potential (mV) !! Resistivity (Ωm) [If shale resistivity is 8] !! Density (gr/cm3)

! Lithology !! Gamma Ray (API) !! Spontaneous Potential (mV) !! Resistivity (Ωm) [If shale resistivity is 8] !! Density (gr/cm3)

File:Well_Log_Analysis_Fig-4B.png|Figure 4B-Determining a bad hole based on bit size and caliper log response.

File:Well_Log_Analysis_Fig-4B.png|Figure 4B-Determining a bad hole based on bit size and caliper log response.

File:Well_Log_Analysis_Fig-5A.png|Figure 5A-Lithology interpretation of South Barrow 18 well, the author use the combination of GR-SP- Resistivity-RHOB logs to interpret the lithology (NPHI log is present here to aid the author in locating a hydrocarbon bearing zone.

File:Well_Log_Analysis_Fig-5A.png|Figure 5A-Lithology interpretation of South Barrow 18 well, the author use the combination of GR-SP- Resistivity-RHOB logs to interpret the lithology (NPHI log is present here to aid the author in locating a hydrocarbon bearing zone.

File:Well_Log_Analysis_Fig-5B.png|Figure 5B-Reservoir A (upper) lithology interpretation.

File:Well_Log_Analysis_Fig-5B.png|Figure 5B-Reservoir A (upper) lithology interpretation.

File:Well_Log_Analysis_Fig-6.png|Figure 6-The calculation result of Vshale, Sw, φ, and k in South Barrow 18 well.

File:Well_Log_Analysis_Fig-6.png|Figure 6-The calculation result of Vshale, Sw, φ, and k in South Barrow 18 well.

File:Well_Log_Analysis_Fig-7.png|Figure 7-The calculation result of AI, SI, Vp/Vs, and σ in South Barrow 18 well.

File:Well_Log_Analysis_Fig-7.png|Figure 7-The calculation result of AI, SI, Vp/Vs, and σ in South Barrow 18 well.

File:Well_Log_Analysis_Fig-8.png|Figure 8-The result of reflectivity coefficient calculation, a very high or very low R value is usually caused by the presence of hydrocarbon or big difference of density and wave velocity between two formations.

File:Well_Log_Analysis_Fig-8.png|Figure 8-The result of reflectivity coefficient calculation, a very high or very low R value is usually caused by the presence of hydrocarbon or big difference of density and wave velocity between two formations.

File:Well_Log_Analysis_Fig-9A.png|Figure 9A-The relation between log data and reflectivity coefficient, from this figure, we can see that the detection zone of interest (red and black circle) can also be done by looking onto the R, a formation that contains hydrocarbon usually has very low or very high R (purple lines).

File:Well_Log_Analysis_Fig-9A.png|Figure 9A-The relation between log data and reflectivity coefficient, from this figure, we can see that the detection zone of interest (red and black circle) can also be done by looking onto the R, a formation that contains hydrocarbon usually has very low or very high R (purple lines).

File:Well_Log_Analysis_Fig-9B.png|Figure 9B-The technique to detect hydrocarbon bearing zone by using RHOB-NPHI, resistivity, and gamma ray log.

File:Well_Log_Analysis_Fig-9B.png|Figure 9B-The technique to detect hydrocarbon bearing zone by using RHOB-NPHI, resistivity, and gamma ray log.

File:Well_Log_Analysis_Fig-10A.png|Figure 10A-Crossplot between depth and acoustic impedance (AI).

File:Well_Log_Analysis_Fig-10A.png|Figure 10A-Crossplot between depth and acoustic impedance (AI).

File:Well_Log_Analysis_Fig-10B.png|Figure 10B-Crossplot between depth and acoustic impedance (AI), the black circles show the acoustic impedance anomaly.

File:Well_Log_Analysis_Fig-10B.png|Figure 10B-Crossplot between depth and acoustic impedance (AI), the black circles show the acoustic impedance anomaly.

File:Well_Log_Analysis_Fig-11.png|Figure 11-Crossplot between velocity ratio (Vp/Vs) and acoustic impedance (AI), by using this crossplot, we can determine the formation orientation whether it contains hydrocarbon or not, how about the pressure, etc.

File:Well_Log_Analysis_Fig-11.png|Figure 11-Crossplot between velocity ratio (Vp/Vs) and acoustic impedance (AI), by using this crossplot, we can determine the formation orientation whether it contains hydrocarbon or not, how about the pressure, etc.

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==References==

==References==