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Sequence Stratigraphic Setting of Upper Smackover Carbonate Reservoirs in Southwestern Alabama

TEW, BERRY H., ERNEST A. MANCINI, and ROBERT M. MINK, Geological Survey of Alabama, Tuscaloosa, AL

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 (marine-reworked Norphlet Formation shoreface sandstones), transgressive deposits (lower Smackover intertidal to subtidal carbonate mudstones, wackestones, and packstones), condensed section deposits (middle Smackover subtidal carbonate mudstones), and progradational, regressive highstand deposits (upper Smackover subtidal to supratidal, upward-shoaling carbonate mudstone to grainstone cycles, Buckner Anhydrite Member of the Haynesville Formation, peritidal upward-shoaling evaporite cycles, and Haynesville Formation peritidal carbonates, sandstones, shales, and evaporites). 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.

The upper Smackover progradational, regressive highstand deposits of the LZAGC-4.1 sequence are prevalently characterized by stacked or shingled, upward-shoaling, grain-rich lithologies with ooids and peloids constituting the dominant allochems; early marine phreatic cements are present only as thin crusts. These strata are typical of parasequences deposited in a keep-up carbonate highstand regime. Keep-up systems are characterized by rapid sediment accumulation and short exposure to marine phreatic conditions. 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. Establi hment 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.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)