SCHMOKER, JAMES W., U.S. Geological Survey, Denver, CO; and CHRISTOPHER J. SCHENK, U.S. Geological Survey, Denver, CO

ABSTRACT: Porosity of the Norphlet Formation (Upper Jurassic), Southwestern Alabama and Vicinity

The purposes of this study are: (1) to develop a predictive empirical model for the porosity and porosity range of sandstones of the Norphlet Formation in southwestern Alabama and vicinity, and (2) to examine the degree to which Norphlet porosity is anomalous relative to that of sandstones in general. The overall reduction of Norphlet porosity during burial is correlated to thermal history as represented by equivalent vitrinite reflectance (R<o>). Porosity variability at a particular Ro level is represented by the 10th, 25th, 50th, 75th, and 90th percentiles of core-plug porosity measurements.

Norphlet porosities decrease systematically as R<o> increases over a wide range. Median (50th-percentile) porosity is about 25% where R<o> is 0.7% in the northern part of the study area (Clarke County, Mississippi). Median porosity is reduced to 8% where R<o> approaches 2.7% in the southern part of the study area near Mobile Bay. Where R<o> is less than 2.7%, average Norphlet reservoir porosity is well represented (and might be predicted) by the regression line of median porosity versus R<o> derived from core-plug measurements. However, in offshore areas where R<o> exceeds 2.7%, extrapolation of these porosity R<o> trends will underpredict some (but not all) Norphlet porosity distributions.

Based on comparisons at similar R<o> levels, median Norphlet porosities exceed those of typical sandstone units of other basins by more than a factor of two throughout the study area; even the lower (10th-percentile) Norphlet porosities are higher than median porosities of sandstones in general. Interpretations of Norphlet burial diagenesis must therefore account for porosities that are high, not just in some offshore areas, but throughout southwestern Alabama and vicinity.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.