Upper Jurassic Reefs of Smackover Formation, United States Gulf Coast

Abstract

The occurrence of reefs in the Smackover Formation, an Upper Jurassic (Oxfordian) sequence that has been recognized in the subsurface of the United States Gulf Coast, requires a revision of existing depositional models that have been based exclusively on subsurface data gathered from nonskeletal carbonate reservoirs. These reefal buildups are 10 to 130 ft (3 to 40 m) thick, commonly elongate, and several square kilometers in plan. They developed seaward of oolite shoals on paleostructures of three types that created subtle topographic highs on an otherwise ramplike sea floor: (1) basement ridges, (2) faulted basement highs, and (3) upthrown salt-cored fault blocks. Reef rubble zones have reservoir-quality porosity throughout the Smackover trend, but reef framework is a ta get only where diagenesis has been favorable. Although only a few reefs have produced commercial hydrocarbons, attractive porosities and permeabilities (mean ^phgr of 15%, mean K of 20 md) result from freshwater leaching, dolomitization, or fracturing. The biota forming Smackover reefs is similar to that described for sponge and algal buildups and patch reefs from the Late Jurassic of Europe. Reef framework is constructed by digitate and branching stromatolitic blue-green algae, Tubiphytes sp., and marine cements. The biota varies through the Smackover trend; reefs in southern Arkansas and northern Louisiana are more diverse. In these reefs, encrusters and binders produce constructional microframework cavities on and between corals (primarily Actinastrea), skeletal algae (Parachaetetes sp. and Cayeuxia sp.), lithistid and hexactinellid sponges, bryozoans, and hydrozoans. The cavities are commonly filled with marine cements and geopetal sediments. A vertical biotic zonation in cores of the reef interval suggests that the reef commu ity of organisms evolved through time. The reefs are commonly underlain and overlain by subtidal peloidal lime packstones containing oncolites and scattered fossils, and they can develop in close proximity to subtidal quartz sands.