Changes

A pitfall of using porosity–depth plots for porosity prediction is that [[Correlation and regression analysis|regression]] relationship averages out anomalies and complicates predictions of unusually porous sandstones. Use porosity–depth plots for porosity prediction with caution. If enough porosity data are available to make a meaningful plot, keep the “data cloud” on the plot in order to view the ranges of porosity at different depths. In a frontier exploration setting, the usefulness of porosity–depth plots may be limited if global data sets must be used.

A pitfall of using porosity–depth plots for porosity prediction is that [[Correlation and regression analysis|regression]] relationship averages out anomalies and complicates predictions of unusually porous sandstones. Use porosity–depth plots for porosity prediction with caution. If enough porosity data are available to make a meaningful plot, keep the “data cloud” on the plot in order to view the ranges of porosity at different depths. In a frontier exploration setting, the usefulness of porosity–depth plots may be limited if global data sets must be used.

[[:file:predicting-reservoir-system-quality-and-performance_fig9-54.png|Figure 1]] is an example of regression porosity–depth plots for different formations along the U.S. Gulf Coast. Unfortunately it does not include the raw data, so we cannot see porosity variations within each formation. Formations on the left side of the plot, like the Vicksburg, tend to be quartz cemented. Formations on the right side, like the Frio (areas 4-6). tend to be clay cemented.

[[:file:predicting-reservoir-system-quality-and-performance_fig9-54.png|Figure 1]] is an example of regression porosity–depth plots for different formations along the U.S. Gulf Coast. Unfortunately it does not include the raw data, so we cannot see porosity variations within each formation. Formations on the left side of the plot, like the Vicksburg, tend to be [[quartz]] cemented. Formations on the right side, like the Frio (areas 4-6). tend to be clay cemented.

==Equation for porosity prediction==

==Equation for porosity prediction==

Scherer<ref name=ch09r53>Scherer, M., 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0005/0450/0485.htm Parameters influencing porosity in sandstones: a model for sandstone porosity prediction]: AAPG Bulletin, vol. 71, no. 5, p. 485–491.</ref> studied the cores of 428 worldwide sandstones and listed the most important variables for predicting sandstone porosity:

Scherer<ref name=ch09r53>Scherer, M., 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0005/0450/0485.htm Parameters influencing porosity in sandstones: a model for sandstone porosity prediction]: AAPG Bulletin, vol. 71, no. 5, p. 485–491.</ref> studied the cores of 428 worldwide sandstones and listed the most important variables for predicting sandstone porosity:

* Percentage of quartz grains

* Percentage of [[quartz]] grains

* [[Core_description#Maturity|Sorting]]

* [[Core_description#Maturity|Sorting]]

* Depth of burial

* Depth of burial

* [[Porosity]] = percent of bulk volume

* [[Porosity]] = percent of bulk volume

* In quartz = percent of solid-rock volume

* In [[quartz]] = percent of solid-rock volume

* Sorting = Trask sorting coefficient

* Sorting = Trask sorting coefficient

* Depth = meters

* Depth = meters

* In age = millions of years

* In age = millions of years

The equation can be used with a high degree of confidence in uncemented to partly cemented sandstones. But if the reduction of porosity by cement exceeds 2.1% bulk volume, then corrections need to be made based on local sandstone quality characteristics. Numbers for percent solid volume quartz and sorting may be difficult to obtain. Use 75% for percent solid volume quartz and 1.5 for sorting when these values are not known.

The equation can be used with a high degree of confidence in uncemented to partly cemented sandstones. But if the reduction of porosity by cement exceeds 2.1% bulk volume, then corrections need to be made based on local sandstone quality characteristics. Numbers for percent solid volume [[quartz]] and sorting may be difficult to obtain. Use 75% for percent solid volume quartz and 1.5 for sorting when these values are not known.

The table below shows numbers that Scherer<ref name=ch09r53 /> developed by his analysis of reservoir sandstones.

The table below shows numbers that Scherer<ref name=ch09r53 /> developed by his analysis of reservoir sandstones.

| Depth || Meters || 0–5,960 || 2,230 || 1,150.0

| Depth || Meters || 0–5,960 || 2,230 || 1,150.0

|-

|-

| Quartz || Percent solid rock volume || 12–97 || 75 || 23.0

| [[Quartz]] || Percent solid rock volume || 12–97 || 75 || 23.0

|-

|-

| Sorting || Trask coefficient || 1.1–4.2 || 1.5 || 0.6

| Sorting || Trask coefficient || 1.1–4.2 || 1.5 || 0.6

</gallery>

</gallery>

Sandstone porosity prediction is a matter of estimating original composition and subsequent diagenesis. Use the list below to predict sandstone porosity.

Sandstone porosity prediction is a matter of estimating original composition and subsequent [[diagenesis]]. Use the list below to predict sandstone porosity.

# Estimate the original composition of the sandstone from provenance (use [[:file:predicting-reservoir-system-quality-and-performance_fig9-55.png|Figure 2]]) and depositional environment.

# Estimate the original composition of the sandstone from provenance (use [[:file:predicting-reservoir-system-quality-and-performance_fig9-55.png|Figure 2]]) and depositional environment.

[[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]]