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  = 9-76

  | topg    = 9-156

  | topg    = 9-77

  | 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

  | isbn    = 0-89181-602-X

  | isbn    = 0-89181-602-X

}}

}}

Diagenesis alters the original pore type and geometry of a sandstone and therefore controls its ultimate [[porosity]] and [[permeability]]. Early diagenetic patterns correlate with environment of deposition and sediment composition. Later diagenetic patterns cross facies boundaries and depend on regional fluid [[migration]] patterns (Stonecipher and May, 1992). Effectively predicting sandstone quality depends on predicting diagenetic history as a product of depositional environments, sediment composition, and fluid migration patterns.

[[Diagenesis]] alters the original pore type and geometry of a sandstone and therefore controls its ultimate [[porosity]] and [[permeability]]. Early diagenetic patterns correlate with environment of deposition and sediment composition. Later diagenetic patterns cross facies boundaries and depend on regional fluid [[migration]] patterns (Stonecipher and May, 1992). Effectively predicting sandstone quality depends on predicting diagenetic history as a product of depositional environments, sediment composition, and fluid migration patterns.

==Diagenetic processes==

==Diagenetic processes==

* Compaction

* Compaction

Cementation destroys pore space; grain leaching creates it. Compaction decreases porosity through grain rearrangement, plastic deformation, pressure solution, and fracturing.

Cementation destroys pore space; grain leaching creates it. Compaction decreases porosity through grain rearrangement, plastic [[deformation]], pressure solution, and fracturing.

==Diagenetic zones==

==Diagenetic zones==

Surdam et al.<ref name=ch09r61>Surdam, R., C., Dunn, T., L., MacGowan, D., B., Heasler, H., P., 1989, Conceptual models for the prediction of porosity evolution with an example from the Frontier Sandstone, Bighorn basin, Wyoming, in Coalson, E., B., Kaplan, S., S., Keighin, C., W., Oglesby, L., A., Robinson, J., W., eds., Sandstone Reservoirs: Rocky Mountain Association of Geologists, p. 7–21.</ref> define diagenetic zones by subsurface temperatures. Depending on geothermal gradient, depths to these zones can vary. The table below summarizes major diagenetic processes and their impact on pore geometry.

Surdam et al.<ref name=ch09r61>Surdam, R., C., Dunn, T., L., MacGowan, D., B., Heasler, H., P., 1989, Conceptual models for the prediction of porosity evolution with an example from the Frontier Sandstone, Bighorn basin, Wyoming, in Coalson, E., B., Kaplan, S., S., Keighin, C., W., Oglesby, L., A., Robinson, J., W., eds., Sandstone Reservoirs: Rocky Mountain Association of Geologists, p. 7–21.</ref> define diagenetic zones by subsurface temperatures. Depending on [[geothermal gradient]], depths to these zones can vary. The table below summarizes major diagenetic processes and their impact on pore geometry.

{| class = "wikitable"

{| class = "wikitable"

|-

|-

! Zone

! rowspan = 2 | Zone || rowspan = 2 | Temperature || colspan = 2 | Major diagenetic processes  

! Major diagenetic processes

|-

|-

| Shallow

! Preserves or enhances porosity || Destroys porosity

|

|-

 

| * Grain coatings (inhibit later overgrowths)  * Nonpervasive carbonate cements that can be dissolved later

|  

 

* Grain coatings (inhibit later overgrowths)   

| * Clay infiltration  * Carbonate or silica cement (in some cases irreversible)  * Authigenic kaolinite  * Compaction of ductile grains

* Nonpervasive carbonate cements that can be dissolved later  

 

* Clay infiltration   

* Carbonate or silica cement (in some cases irreversible)   

* Authigenic kaolinite   

* Compaction of [[Ductility|ductile]] grains

|-

|-

| Intermediate

| Intermediate || 80-140&deg;C or 176–284&deg;F  

| [[temperature::80-140&deg;C]] or 176–[[temperature::284&deg;F]]

|  

| * Carbonate cement dissolved  * Feldspar grains dissolved

* Carbonate cement dissolved   

 

* Feldspar grains dissolved

| * Kaolinite, chlorite, and illite precipitate as a result of feldspar dissolution  * Ferroan carbonate and quartz cement

 

* Kaolinite, chlorite, and illite precipitate as a result of feldspar dissolution   

* Ferroan carbonate and [[quartz]] cement

|-

|-

| Deep

| Deep || > 140&deg;C or 284&deg;F

| > [[temperature::140&deg;C]] or [[temperature::284&deg;F]]

|  

| * Feldspar, carbonate, and sulfate minerals dissolved

* Feldspar, carbonate, and sulfate minerals dissolved

 

| * Quartz cement (most destructive)  * Kaolinite precipitation  * Illite, chlorite form as products of feldspar dissolution  * Pyrite precipitation

* Quartz cement (most destructive)   

 

* Kaolinite precipitation   

* Illite, chlorite form as products of feldspar dissolution   

* Pyrite precipitation

|}

|}

==Effect of temperature==

==Effect of temperature==

[[file:predicting-reservoir-system-quality-and-performance_fig9-46.png|300px|thumb|{{figure number|1}}. Copyright: Wilson, 1994a; courtesy SEPM.]]

[[file:predicting-reservoir-system-quality-and-performance_fig9-46.png|300px|thumb|{{figure number|1}}Porosity–depth plot for sandstones from two wells with different geothermal gradients. Copyright: Wilson;<ref name=ch09r66 /> courtesy SEPM.]]

Depending on geothermal gradient, the effect of temperature on diagenesis can be significant. Many diagenetic reaction rates double with each [[temperature::10&deg;C]] increase (1000 times greater with each [[temperature::100&deg;C]]).<ref name=ch09r66>Wilson, M., D., 1994a, Non-compositional controls on diagenetic processes, in Wilson, M., D., ed., [[Reservoir quality]] Assessment and Prediction in Clastic Rocks: SEPM Short Course 30, p. 183–208. Discusses the effect that variables such as temperature and pressure have on diagenesis of sandstones. A good reference for predicting sandstone reservoir system quality.</ref> Increasing temperatures increase the solubility of many different minerals, so pore waters become saturated with more ionic species. Either (1) porosity–depth plots of sandstones of the target sandstone that are near the prospect area or (2) computer models that incorporate geothermal gradient are probably best for porosity predictions.

Depending on geothermal gradient, the effect of temperature on [[diagenesis]] can be significant. Many diagenetic reaction rates double with each [[temperature::10&deg;C]] increase (1000 times greater with each [[temperature::100&deg;C]]).<ref name=ch09r66>Wilson, M., D., 1994a, Non-compositional controls on diagenetic processes, in Wilson, M., D., ed., [[Reservoir quality]] Assessment and Prediction in Clastic Rocks: SEPM Short Course 30, p. 183–208. Discusses the effect that variables such as temperature and pressure have on diagenesis of sandstones. A good reference for predicting sandstone reservoir system quality.</ref> Increasing temperatures increase the solubility of many different minerals, so pore waters become saturated with more ionic species. Either (1) porosity–depth plots of sandstones of the target sandstone that are near the prospect area or (2) computer models that incorporate geothermal gradient are probably best for porosity predictions.

[[:file:predicting-reservoir-system-quality-and-performance_fig9-46.png|Figure 1]] is a porosity–depth plot for sandstones from two wells with different geothermal gradients. The well with the greater geothermal gradient has correspondingly lower porosities than the well with lower geothermal gradient. At a depth of [[depth::7000 ft]], there is a 10% porosity difference in the trend lines.

[[:file:predicting-reservoir-system-quality-and-performance_fig9-46.png|Figure 1]] is a porosity–depth plot for sandstones from two wells with different geothermal gradients. The well with the greater geothermal gradient has correspondingly lower porosities than the well with lower geothermal gradient. At a depth of [[depth::7000 ft]], there is a 10% porosity difference in the trend lines.

==Effect of age==

==Effect of age==

In general, sandstones lose porosity with age. In other words, porosity loss in sandstone is a function of time. According to Scherer,<ref name=ch09r53 /> a Tertiary sandstone with a Trask sorting coefficient of 1.5, a quartz content of 75%, and a burial depth of [[depth::3000 m]] probably has an average porosity of approximately 26%. A Paleozoic sandstone with the same sorting, quartz content, and burial depth probably has an average porosity of approximately 13%.

In general, sandstones lose porosity with age. In other words, porosity loss in sandstone is a function of time. According to Scherer,<ref name=ch09r53 /> a [[Tertiary]] sandstone with a Trask sorting coefficient of 1.5, a [[quartz]] content of 75%, and a burial depth of [[depth::3000 m]] probably has an average porosity of approximately 26%. A Paleozoic sandstone with the same [[Core_description#Maturity|sorting]], quartz content, and burial depth probably has an average porosity of approximately 13%.

==See also==

==See also==

[[Category:Predicting the occurrence of oil and gas traps]]  

[[Category:Predicting the occurrence of oil and gas traps]]  

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

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