Sequence Stratigraphic Setting of Upper Smackover Carbonate Reservoirs in Southwestern Alabama

Abstract

The Upper Jurassic (Oxfordian) Smackover Formation is a regionally extensive, predominantly carbonate unit that includes significant hydrocarbon reservoirs in southwestern Alabama. Regional stratigraphic and sedimentologic data indicate that the most productive of these carbonate reservoirs occur within the lower part of the progradational, regressive highstand deposits of an unconformity-bounded, type-2 depositional sequence. This sequence, designated the LZAGC-4.1 sequence, is one of three unconformity-bounded units recognized for Jurassic strata in the eastern Gulf Coastal Plain. The LZAGC-4.1 sequence includes in ascending order, a basal type-2 unconformity, shelf margin deposits, transgressive deposits, condensed section deposits and progradational, regressive highstand deposits. A type-2 unconformity overlies the sequence. The LZAGC-4.1 sequence is interpreted to have been deposited in response to a transgressive-regressive cycle of relative sea-level change in the Oxfordian. Primary reservoir quality of these highstand deposits has been enhanced by favorable diagenetic alterations, which include dissolution and dolomitization. Diagenesis is, in part, related to subaerial exposure along the sequence boundary overlying the LZAGC-4.1 sequence. Establishment of a freshwater lens during exposure resulted in meteoric diagenesis, which includes leaching of unstable allochems and cements and mixing-zone dolomitization. These effects are particularly prevalent along the basin margin and around paleotopographic highs.