Changes

  | part    = Predicting the occurrence of oil and gas traps

  | part    = Predicting the occurrence of oil and gas traps

  | chapter = Predicting reservoir system quality and performance

  | chapter = Predicting reservoir system quality and performance

  | frompg  = 9-1

  | frompg  = 9-49

  | topg    = 9-156

  | topg    = 9-53

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | author  = Dan J. Hartmann, Edward A. Beaumont

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

  | link    = http://archives.datapages.com/data/specpubs/beaumont/ch09/ch09.htm

==Density log==

==Density log==

The [[Basic open hole tools#Density|density log]] measures the electron density of a formation. The logging device is a contact tool that emits gamma rays from a source. Emitted gamma rays collide with formation electrons and scatter. A detector, located a fixed distance from the tool source, counts the number of returning gamma rays. The number of returning gamma rays is an indicator of formation bulk density. The litho-density tool (LDT) also provides a photoelectron (P_e) cross section curve, an independent indicator of lithology.

The [[Basic open hole tools#Density|density log]] measures the electron density of a formation. The logging device is a contact tool that emits gamma rays from a source. Emitted gamma rays collide with formation electrons and scatter. A detector, located a fixed distance from the tool source, counts the number of returning gamma rays. The number of returning gamma rays is an indicator of formation bulk density. The litho-density tool (LDT) also provides a photoelectron (P_e) [[cross section]] curve, an independent indicator of lithology.

==Obtaining porosities from a density log==

==Obtaining porosities from a density log==

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Lithology

! Lithology || Density, g/cc || Average pe

! Density, g/cc

! Average pe

|-

|-

| Sandstone

| Sandstone || 2.65 || 1.8

| 2.65

| 1.8

|-

|-

| Limestone

| [[Limestone]] || 2.71 || 4.8

| 2.71

| 4.8

|-

|-

| Dolomite

| [[Dolomite]] || 2.876 || 3.0

| 2.876

| 3.0

|-

|-

| Anhydrite

| [[Anhydrite]] || 2.977 || 5.05

| 2.977

| 5.05

|-

|-

| Salt

| Salt || 2.032 || 4.6

| 2.032

| 4.6

|}

|}

==Obtaining porosities from a neutron log==

==Obtaining porosities from a neutron log==

Lithology, porosity, fluid type, and tool type affect neutron log response. When interpreting neutron logs, use the specific log for the specific tool, i.e., the charts in logging chart books that are specific to the sidewall neutron log (SNP) or the compensated neutron log (CNL).

Lithology, porosity, fluid type, and tool type affect neutron log response. When interpreting neutron logs, use the specific log for the specific tool, i.e., the charts in logging chart books that are specific to the sidewall neutron log (SNP) or the [[compensated neutron log]] (CNL).

To obtain porosity, read the value directly from the log. If the log is recorded in limestone units and the formation you wish to evaluate is sandstone or dolomite, then correct the log value by using the appropriate chart in a log interpretation chartbook.

To obtain porosity, read the value directly from the log. If the log is recorded in limestone units and the formation you wish to evaluate is sandstone or dolomite, then correct the log value by using the appropriate chart in a log interpretation chartbook.

==Example density-neutron log==

==Example density-neutron log==

[[file:predicting-reservoir-system-quality-and-performance_fig9-30.png|300px|thumb|{{figure number|1}}Example log recorded in sandstone units.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-30.png|300px|thumb|{{figure number|1}}Example log recorded in sandstone units. From Alberty;<ref name=Alberty>Alberty, M. W., 1994, [http://archives.datapages.com/data/specpubs/methodo1/data/a095/a095/0001/0150/0180.htm Standard interpretation; part 4—wireline methods], in D. Morton-Thompson and A. M. Woods, eds., Development Geology Reference Manual: [http://store.aapg.org/detail.aspx?id=612 AAPG Methods in Exploration Series 10], p. 180–185.</ref> courtesy AAPG.]]

The example log in [[:file:predicting-reservoir-system-quality-and-performance_fig9-30.png|Figure 1]] was recorded in sandstone units. Where the density and neutron logs nearly track together, the formation lithology normally is assumed to be sandstone (in the figure below). The slight separations may be due to changes in lithology as in more shale/clay. Where the density and neutron logs separate, either the lithology is different (neutron porosity > density porosity) from the recorded lithologic units (points 1 and 5) or gas is present (points 2, 3, and 4). A density–neutron crossplot resolves the separation problem (see [[:file:predicting-reservoir-system-quality-and-performance_fig9-31.png|Figure 2]]).

The example log in [[:file:predicting-reservoir-system-quality-and-performance_fig9-30.png|Figure 1]] was recorded in sandstone units. Where the density and neutron logs nearly track together, the formation lithology normally is assumed to be sandstone (in the figure below). The slight separations may be due to changes in lithology as in more shale/clay. Where the density and neutron logs separate, either the lithology is different (neutron porosity > density porosity) from the recorded lithologic units (points 1 and 5) or gas is present (points 2, 3, and 4). A density–neutron crossplot resolves the separation problem (see [[:file:predicting-reservoir-system-quality-and-performance_fig9-31.png|Figure 2]]).

==Using a density–neutron crossplot==

==Using a density–neutron crossplot==

To determine lithology or correct porosities for lithologic or gas effects from a density–neutron crossplot, follow the steps listed in the table below.

To determine lithology or correct porosities for lithologic or gas effects from a density–neutron crossplot, follow the steps listed below.

: 1. Use the table below to determine how to enter a neutron porosity value.

| 1 || Use the table below to determine how to enter a neutron porosity value.

{| class = "wikitable"

{| class = "wikitable"

|-

|-

| Neutron porosity is in sandstone or dolomite units || Enter the chart on the sandstone or dolomite line. Project up or down to a density value.

| Neutron porosity is in sandstone or dolomite units || Enter the chart on the sandstone or dolomite line. Project up or down to a density value.

|}

|}

: 2. Use the table below to determine how to enter a density porosity value

| 2 || Use the table below to determine how to enter a density porosity value

{| class = "wikitable"

{| class = "wikitable"

|-

|-

| Density log scale is bulk density || Enter the y-axis with the log bulk density value and intercept the neutron projection.

| Density log scale is bulk density || Enter the y-axis with the log bulk density value and intercept the neutron projection.

|}

|}

: 3. Use the table below to determine formation lithology and porosity.

| 3 || Use the table below to determine formation lithology and porosity.

{| class = "wikitable"

{| class = "wikitable"

|-

|-

|-

|-

| Point falls away from appropriate diagonal line|| Move down and to the right parallel to the nearest dashed line until a diagonal line is intersected. Read the value for porosity at that point. The lithology is a combination of the lithologies of the lines on either side of the point of intersection. Gas is present if the original point is northwest of the appropriate diagonal lithology line.

| Point falls away from appropriate diagonal line|| Move down and to the right parallel to the nearest dashed line until a diagonal line is intersected. Read the value for porosity at that point. The lithology is a combination of the lithologies of the lines on either side of the point of intersection. Gas is present if the original point is northwest of the appropriate diagonal lithology line.

|}

|}

===Example density–neutron crossplot===

===Example density–neutron crossplot===

[[file:predicting-reservoir-system-quality-and-performance_fig9-31.png|300px|thumb|{{figure number|2}}Example density-neutron crossplot.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-31.png|300px|thumb|{{figure number|2}}Example density-neutron cross plot. From Alberty;<ref name=Alberty /> courtesy AAPG.]]

[[:file:predicting-reservoir-system-quality-and-performance_fig9-31.png|Figure 2]] is an example density-neutron crossplot. Points 1–5 are from the log (Figure 9-30). Points 2, 3, and 4 are from a zone that shows crossover. Crossover occurs when the density log reads higher than the neutron log in a zone of the same lithology as the log matrix lithology, i.e., sandstone. Point 2 has the greatest crossover; Point 4, the least. Once completed, the well from which this log was taken showed Point 2 to be in a gas reservoir, Point 3 to be in light oil reservoir, and Point 4 to be in a zone with residual oil.

[[:file:predicting-reservoir-system-quality-and-performance_fig9-31.png|Figure 2]] is an example density-neutron crossplot. Points 1–5 are from the log ([[:file:predicting-reservoir-system-quality-and-performance_fig9-30.png|Figure 1]]). Points 2, 3, and 4 are from a zone that shows crossover. Crossover occurs when the density log reads higher than the neutron log in a zone of the same lithology as the log matrix lithology, i.e., sandstone. Point 2 has the greatest crossover; Point 4, the least. Once completed, the well from which this log was taken showed Point 2 to be in a gas reservoir, Point 3 to be in light oil reservoir, and Point 4 to be in a zone with residual oil.

Porosities corrected for gas effect are 24%, 25%, and 28%. Points 1 and 5 are in shale zones, even though they plot as dolomite. They are shale reference points for this interval of the log.

Porosities corrected for gas effect are 24%, 25%, and 28%. Points 1 and 5 are in shale zones, even though they plot as dolomite. They are shale reference points for this interval of the log.

* [[Determining Rt]]

* [[Determining Rt]]

* [[Calculating Rw from SP logs]]

* [[Calculating Rw from SP logs]]

* [[Constructing a Pickett plot]]

* [[Pickett plot construction]]

 

==External links==

==External links==

[[Category:Predicting reservoir system quality and performance]]

[[Category:Predicting reservoir system quality and performance]]

[[Category:Pages with badly formatted tables]]

[[Category:Pages with badly formatted tables]]