Jurassic Sequence Stratigraphy of the Eastern Gulf Coastal Plain: Applications to Hydrocarbon Exploration
TEW, BERRY H., ERNEST A. MANCINI, and ROBERT M. MINK, Geological Survey of Alabama, Tuscaloosa, AL
Based on regional stratigraphic and sedimentologic data, three unconformity-bounded depositional sequences associated with cycles of relative sea-level change and coastal onlap are recognized for Jurassic strata in the eastern Gulf Coastal Plain area. These sequences are designated, in ascending order, the LZAGC (Lower Zuni A Gulf Coast)-3.1, the LZAGC-4.1, and the LZAGC-4.2 sequences and include Callovian through Kimmeridgian Stage strata. The lowermost Jurassic sequence, the LZAGC-3.1, is bounded by a composite basal unconformity developed on Paleozoic and early Mesozoic rift-fill strata. The sequence includes Werner Formation and Louann Salt evaporites (transgressive deposits), Pine Hill Anhydrite Member of the Louann Salt anhydrites and shales (condensed section deposits), and Nor hlet Formation continental sandstones (progradational, regressive highstand deposits). This sequence represents the initial incursion of marine water into the opening Gulf of Mexico basin during the Callovian and relative sea-level fall subsequent to the deposition of the Norphlet during the Oxfordian. Sea-level fall is reflected in the type-2 unconformity at the base of the LZAGC-4.1 depositional sequence. This sequence comprises Norphlet marine-reworked sandstones (shelf margin deposits), lower Smackover Formation packstones, wackestones, and carbonate mudstones (transgressive deposits), middle Smackover carbonate mudstones (condensed section deposits), and upper Smackover carbonate mudstones to grainstones, Buckner anhydrites, and middle Haynesville Formation carbonates, siliciclastic , and evaporites (progradational, regressive highstand deposits). The LZAGC-4.1 depositional sequence consists of a basal type-2 unconformity, upper Haynesville peritidal sandstones (transgressive deposits), upper Haynesville carbonate mudstones and shales (condensed section deposits), and upper Haynesville anhydrites, shales, and continental sandstones (progradational, regressive highstand deposits). These sediments are overlain by marginal marine to continental strata of the Cotton Valley Group.
An understanding of the relationship of Jurassic reservoirs to sequence stratigraphy can serve as an aid to hydrocarbon exploration in the eastern gulf area. The most extensive and productive Jurassic hydrocarbon reservoirs in the study area occur within the progradational, regressive highstand deposits of the LZAGC-3.1 and LZAGC-4.1 depositional sequences. For example, the majority of Norphlet sandstone reservoirs in the onshore and offshore Alabama area are interpreted to have accumulated in eolian dune, interdune, and wadi (fluvial) depositional environments, which occurred in association with the highstand regressive system of the LZAGC-3.1 sequence. The most important Smackover reservoirs generally consist of partially to completely dolomitized ooid and peloid packstones and grai stones in the upper portion of the unit. These reservoirs occur in subtidal to supratidal, shoaling-upward carbonate mudstone to grainstone cycles in the highstand regressive system of the LZAGC-4.1 sequence. In addition, minor reservoirs that are discontinuous and not well developed are associated with the shelf margin and transgressive systems of the LZAGC-4.1.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)