Abstract: Diagenesis of Passive Margin Reservoirs - Upper Jurassic Smackover Formation of the Eastern Gulf Coastal Plain, USA

Benson, D. J. and Mancini, E. A. ? University of Alabama

Smackover reservoirs in the Eastern Gulf Coastal Plain of the U.S. are extremely complex reflecting the influence of intense diagenesis. Diagenetic patterns, which vary dramatically over short distances, are principally controlled by paleotopography and eustatic sea level fluctuations. The general diagenetic history can be divided into five separate stages: (1) Marine Stage, (2) Meteoric Vadose Stage, (3) Meteoric Phreatic Stage, (4) Brine Reflux Stage, and (5) Burial Stage. The products of this complex diagenetic history are heterogeneous reservoirs which consist of one or some combination of primary interparticulate, moldic, dolomitic intercrystalline, and vuggy porosity. Primary interparticulate porosity is less common in the eastern than in the central and western portions of the Gulf Coastal Plain, largely because of extensive compaction and/or early cementation. Where present, however, interparticulate reservoirs are characterized by moderate to high porosity and good permeability. Moldic porosity is produced by early, fabric-selective dissolution of aragonitic allochems and is associated with areas of subaerial exposure. Moldic reservoirs commonly have high porosity, but low permeability and are generally productive only when permeability is enhanced by other pore types. Dolomitic intercrystalline porosity is common but intercrystalline porosity alone is seldom of reservoir quality and hydrocarbon production from dolomitized lithologies is generally dependent upon the coexistence of other porosity types. Vuggy porosity is the product of late, non-fabric selective dissolution. Vuggy pores are produced by solution enlargement of earlier formed interparticulate, moldic, or intercrystalline pores. Vuggy reservoirs are, by nature, composite and are characterized by moderate to high porosity and good permeability.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil