Diagenesis of Fluvial Sands in Norphlet Formation (Upper Jurassic), Escambia County, Alabama: ABSTRACT

Abstract

The Upper Jurassic Norphlet Formation is an important hydrocarbon reservoir in Baldwin and Mobile Counties and offshore in Mobile Bay, Alabama. The formation is not productive in the Little Escambia Creek field, Escambia County, but underlies the productive Smackover Formation at a depth of approximately 15,500 ft (4725 m). The Norphlet sandstones examined in cores from two drill holes are largely fluvial in origin and consist of moderately to well-sorted, very fine to coarse-grained feldspathic sandstones extensively altered by a complex sequence of diagenetic reactions. Visible evidence of chemical and mechanical compaction is relatively minor in the sandstones. Paucity of compaction suggests that extensive early cementation by anhydrite and/or calcite reduced compaction; these cements were subsequently removed by migrating fluids. Porosity, both intergranular and intragranular, is generally well developed. Intergranular pores are due primarily to partial to complete dissolution of cements and mineral grains, especially feldspar. Intragranular pores are largely the result of partial leaching of rock fragments and of microporosity formed by precipitation of clay minerals in earlier dissolution pores.