Abstract: Comparison of Regional Trends of Sandstone and Carbonate Porosity

Timothy C. Hester, James W. Schmoker

Predictive porosity models should account for the effects on porosity of both burial history and rock properties. Plotting porosity against thermal maturity accounts for burial history, but because sandstones and carbonates are usually treated separately, relative effects of rock properties on porosity are largely undocumented. This report uses case examples of porosity-thermal maturity trends to illustrate an empirical approach to porosity forecasting that accounts for regional processes underlying porosity change and allows porosity variation to be linked with rock properties.

The dependence of porosity upon thermal maturity is generally lower for carbonates than for sandstones. For example, correlation coefficients for the Jurassic Smackover Formation (carbonate), Alabama, and for the Cretaceous "J" sandstone, Denver basin, Colorado, are -0.40 and -0.88, respectively. The lower correlation for carbonates reflects more porosity variability and suggests that carbonate porosity is more affected by local geologic factors than is sandstone porosity.

The coefficient, a, of the power function ^phgr=a(thermal maturity)^b also varies more for carbonates than for sandstones, whereas the exponent, b, is similar for both. This difference suggests that porosity may vary more between individual groups of carbonate data but that both lithologies are similarly affected by the overriding influence of thermal exposure.

Specific rock properties, such as depositional fabric, grain size, clay content, or the presence of carbonate cement, can affect porosity locally, but they are secondary effects in a regional sense. Thermal maturity usually accounts for regional porosity loss in the subsurface better than does any other single factor.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994