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Abstract

The sequence of deposition of the Upper Jurassic Smackover Formation in updip marginal positions, north of the regional peripheral fault trend in the eastern Gulf Coastal Plain is the result of the combined effects of many variables including paleotopography, eustasy, tectonic subsidence, and carbonate productivity, which have been highly influenced by environmental conditions. Tectonic subsidence is identified as a critical mechanism for the generation of the accommodation space required for the accumulation of thick Smackover sections measured in wells located in areas associated with basement paleohighs that were elevated during the time of Smackover deposition.

This study focuses on Vocation Field located in southwestern Alabama, in the eastern margin of the Manila sub-basin, along the western flank of the Conecuh ridge. The structure in Vocation Field is a Paleozoic basement high associated with the Triassic/Jurassic rifting event. The reservoir in the field is the Upper Jurassic Smackover Formation, which overlies siliciclastic deposits of the Norphlet Formation in the margins of the structure and onlaps crystalline basement rocks in updip positions. Based on core and thin section descriptions, four subenvironments have been interpreted for Smackover deposits: microbial reef complex, shallow lagoon, shoal complex, and sabkha-tidal flat. These subenvironments define an overall aggradational to progradational shallowing-upward marine cycle developed in an evaporate-carbonate setting. The reef and shoal complexes are the main and potential petroleum reservoirs. Significant boundstone accumulations have been deposited on the northeastern side of the basement structure due to restricted environmental conditions that favored the establishment and growth of the microbial reefs in that area.

Smackover deposition in Vocation Field was initiated as a result of a rapid relative sea level rise (transgressive event) that partially submerged the basement paleohigh and led to the development of microbial reef buildups on the northeastern flank of the structure during the “catch–up” phase of the carbonate system. Changes in the depositional environment and a decrease in the rate of relative sea level rise initiated the “keep-up” phase characterized by the aggradation and finally progradation of shallow subtidal and peritidal sediments of the upper part of the Smackover Formation.

According to the depositional model, the microbial boundstones are equivalent to the peloidal wackestones and laminated carbonate mudstones of the middle Smackover accumulated in downdip areas having greater water depths.

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