Post-Compaction, Subsurface Secondary Porosity Generation, and Occlusion in Upper Jurassic Smackover in Southern Arkansas: ABSTRACT

Abstract

The upper Smackover (Oxfordian) in southern Arkansas consists of a high energy, blanket sequence of ooid grainstones. It is a prolific gas and oil producer. Regional and field petrographic studies of reservoirs lead to the conclusion that porosity distribution is primarily controlled by post-compactional diagenetic processes. The most significant processes are calcspar cementation, occluding or reducing intergranular porosity, and dissolution, leading to evolution of vugs and enlargement of primary intergranular pores. The calcspar cement occurs as large poikilitic crystals, cementing several grains, or as blocky crystals with straight compromise boundaries. The cement binds crushed grains and particles of spalled-off oolitic laminae. Pressure solution microstylolites between adjacent grains do not extend into the cement crystals; cement generally pre-dates hydrocarbon emplacement, although some cements do contain hydrocarbon inclusions indicating that the timing of migration and cementation was nearly coincidental. The secondary pores consist of equidimensional or elongate vugs, from a few to several hundred microns in size. The dissolution post-dates all other diagenetic phases, excluding stylolitization. Both the calcite precipitation and dissolution are related to hydrocarbon migration, as well as to the updip movement of deep-seated brines, originating from the Louann Salt basins, evolving finally to calcium chloride brines, commonly found in the Smackover reservoir. End_of_Article - Last_Page 753------------