Regional Stratigraphy of Upper Jurassic Smackover Carbonate Rocks of Southwest Alabama: ABSTRACT

Abstract

ABSTRACT Upper Jurassic Smackover deposition in southwest Alabama was primarily controlled by the Mississippi interior salt basin and the Manila and Conecuh embayments and closely approximated carbonate sedimentation in the present-day Persian Gulf. These depositional sites are characterized by distinctive lithofacies and fossil assemblages. Early salt movement produced local variations in carbonate sediment distribution and pre-Jurassic paleo-highs, such as the Wiggins uplift and Conecuh arch, also modified carbonate sedimentation. Throughout much of southwest Alabama, the Smackover Formation consists of a lower predominately mudstone lithofacies which overlies the Norphlet sandstone and an upper lithofacies sequence dominated by grain-supported textures which is overlain by the Buckner anhydrite. Where present, the lower Smackover lithofacies include laminated mudstone and on occasion peloidal wackestone, peloidal-oncolitic packstone, or dolomite are present. The upper lithofacies sequence consists of oolitic and/or oncolitic grainstone, peloidal and/or oncolitic wackestone to packestone, and occasionally dolomite or mudstone. Petroleum traps in southwest Alabama are principally combination traps involving favorable stratigraphy and salt anticlines, faulted salt anticlines, or extensional faults associated with salt movement. Reservoir rocks include grainstones; leached and dolomitized wackestones, packstones, and grainstones; and dolomite. Porosity is facies-selective and is developed chiefly in lithofacies of the upper Smackover. Porosity includes primary interparticulate, secondary grain moldic, intercrystalline dolomite, vuggy, and fracture. The algal mudstones that characterize the lower Smackover and are interbedded with upper Smackover lithologies throughout most of southwest Alabama make excellent petroleum source rocks. The flanks of the Wiggins-Conecuh ridge and updip Smackover grainstones associated with salt structures are excellent areas for petroleum exploration in southwest Alabama. The key to successful prospecting is the delineation of traps associated with salt movement and recognition of either high to moderate energy lithofacies that have had their primary interparticulate porosity preserved or of lithofacies that have been dolomitized and/or leached with the development of intercrystalline dolomite and/or secondary grain moldic porosity.