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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.<ref>Railsback (2011). Characteristics of wireline well logs in the petroleum industry.</ref>]]

[[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

|-

|-

| Dolomite || 2.877 || Methane || 0.423

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

|-

|-

| Anhydrite || 2.960 || Oil || 0.8

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

|-

|-

| 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,<ref>Archie, G. E. (1950). Introduction to petrophysics of reservoir rocks. AAPG Bulletin, 34(5), 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:

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>

{| class = wikitable

{| class = wikitable

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

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

|-

|-

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

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

{| class="wikitable"

{| class="wikitable"

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

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

|-

|-

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

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

{| class="wikitable"

{| class="wikitable"

|-

|-

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

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

|-

|-

! 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.

==Sources==

==Sources==

* Ijasan, O., Torres-Verdín, C., & Preeg, W. E. (2013). Interpretation of porosity and fluid constituents from well logs using an interactive neutron-density matrix scale. Interpretation, 1(2), T143-T155.

* Ijasan, O., C. Torres-Verdín, and W. E. Preeg, 2013, Interpretation of porosity and fluid constituents from well logs using an interactive neutron-density matrix scale: Interpretation, v.1, no. 2, p. T143-T155.

* Tiab, D., & Donaldson, E. C. (2011). Petrophysics: theory and practice of measuring reservoir rock and fluid transport properties. Gulf professional publishing.

* Tiab, D., and E. C. Donaldson, 2011, Petrophysics: Theory and practice of measuring reservoir rock and fluid transport properties: Gulf Professional Publishing.

* Jorgensen, D. G. (1989). Using geophysical logs to estimate porosity, water resistivity, and intrinsic permeability.

* Jorgensen, D. G., 1989, Using geophysical logs to estimate porosity, water resistivity, and intrinsic permeability.

* Doveton, J. H. (1986). Log analysis of subsurface geology: Concepts and computer methods.

* Doveton, J. H., 1986, Log analysis of subsurface geology: Concepts and computer methods.

* Ellis, D. V., & Singer, J. M. (2007). Well logging for earth scientists (Vol. 692). Dordrecht: Springer.

* Ellis, D. V., and J. M. Singer, 2007, Well logging for earth scientists (Vol. 692). Dordrecht: Springer.

* Muammar, R. (2014). Application of Fluid Mechanics to Determine Oil and Gas Reservoir’s Petrophysical Properties By Using Well Log Data.  

* Muammar, R., 2014, Application of Fluid Mechanics to Determine Oil and Gas Reservoir’s Petrophysical Properties By Using Well Log Data.  

* Balan, B., Mohaghegh, S., & Ameri, S. (1995). State-of-the-art in permeability determination from well log data: part 1-A comparative study, model development. paper SPE, 30978, 17-21.

* Balan, B., S. Mohaghegh, and S. Ameri, 1995, State-of-the-art in permeability determination from well log data: part 1-A comparative study, model development: SPE paper 30978, p. 17-21.

==References==

==References==

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