Carlos Jorge Abreu, Paul Edwin Potter
Fifty-one cores were studied to determine the porosity and permeability of the sandstones of the Cretaceous Maceio Member of the Muribeca Formation in the northeastern half of the Sergipe-Alagoas basin of Brazil's passive continental margin. Here, the Maceio Member is a feldspar-rich sandstone that thickens offshore to more than 5000 m. The Maceio includes seven facies ranging from alluvial to submarine fans and includes lacustrine and evaporative environments.
One hundred and fourteen thin sections from plugs were used to systematically quantify 12 pore types and sizes and to identify a diagenetic sequence of 13 mineral phases. Stepwise regression helped identify the best predictors of plug permeability: totally open intergranular pores, intergranular pores with residual solids, and intergranually totally open pores with carbonate cement. Over 380 clay mineral analyses contributed to the understanding of diagenetic history.
Important conclusions include the following: everywhere in the Maceio, porosity and permeability are best at the top, and highest values occur where it is shallow and near the outcrop. Intergranular secondary porosity is predominant at the top throughout the Maceio and is the best predictor of permeability. Most of this intergranular porosity, especially where the Maceio is shallow, was formed by invasion of meteoric waters. However, minor porosities, possibly formed by organic acids in the deeply buried Maceio are common, whereas primary pores are rare.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990