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AAPG Bulletin

Abstract


Volume: 78 (1994)

Issue: 2. (February)

First Page: 166

Last Page: 180

Title: Regional Porosity Trends of the Upper Jurassic Norphlet Formation in Southwestern Alabama and Vicinity, with Comparisons to Formations of Other Basins

Author(s): James W. Schmoker, Christopher J. Schenk (2)

Abstract:

Sandstone porosity of the Upper Jurassic Norphlet Formation in southwestern Alabama and vicinity decreases systematically as depth and thermal maturity increase over a wide range. Median porosity is about 25% where equivalent vitrinite reflectance (Ro) is slightly over 0.7% in the northern part of the study area (Clarke County, Mississippi). Median porosity is reduced to 8% where Ro approaches 2.7% in the southern part of the study area (state waters of Mobile Bay).

Porosity of the cemented, tight zone at the top of the Norphlet in downdip locations is roughly 10% lower than porosities of facies underlying the tight zone, but nevertheless is slightly above the norm for other sandstones at similar Ro levels. Porosity of dune facies is consistently 2-5% higher than that of interdune facies, other factors being equal. Our data show 3-6% higher porosity in chlorite-dominated intervals relative to intervals where illite is the dominant clay mineral. Norphlet porosity has little or no correlation with position relative to the present-day hydrocarbon-water contact.

Based on comparisons at similar Ro levels, median (50th-percentile) Norphlet porosity exceeds porosities of "typical" sandstones in 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.

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