--> Abstract: Sequence Stratigraphic Controls on Upper Jurassic Smackover Shoal and Reef Reservoir Heterogeneity, NE Gulf of Mexico, by E. A. Mancini and D. J. Benson; #90933 (1998).

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Abstract: Sequence Stratigraphic Controls on Upper Jurassic Smackover Shoal and Reef Reservoir Heterogeneity, NE Gulf of Mexico

Mancini, Ernest A., and D. Joe Benson - University of Alabama

The Upper Jurassic Smackover Formation of the northeastern Gulf of Mexico is a major hydrocarbon carbonate reservoir in the region. The Smackover depositional sequence is bounded by a lower unconformity with Jurassic Norphlet marine sandstones overlying Norphlet continental deposits and by an upper unconformity with Jurassic Haynesville subaqueous evaporates overlying Haynesville sabkha anhydrites. This depositional sequence includes shallow-water, shelf margin deposits (Norphlet marine sandstone), transgressive and condensed section deposits (lower to middle Smackover intertidal to subtidal carbonate mudstone, wackestone, and packstone, and reef boundstone and bindstone), and highstand deposits (upper Smackover subtidal to supratidal, shoaling-upward carbonate grainstone cycles, Buckner peritidal shoaling-upward evaporite cycles, and Haynesville peritidal carbonate, sandstone, shale, and evaporite). The carbonate mudstones of the transgressive systems tract are rich in marine amorphous and algal kerogen and have undergone favorable burial and thermal histories for hydrocarbon generation. The evaporates of the highstand systems tract are excellent seal rocks. Reservoir-grade Smackover carbonate rocks include microbial reef boundstones/bindstones of the transgressive systems tract and shoal grainstones/packstones of the regressive highstand systems tract. The reefs developed on paleohighs at the time of maximum Smackover transgression. The shoal grainstones/packstones accumulated as part of the late Smackover regression and progradation of peritidal carbonate lithofacies into basinal areas. The reef reservoirs can be dominated by primary shelter or by secondary dolomite intercrystalline and vuggy porosity. The diagenetically altered reef reservoirs exhibit excellent con nectivity and high porosity and permeability. The shoal reservoirs ran be dominated by primary interparticulate or by secondary grain-moldic, intercrystalline or vuggy porosity. The diagenetically altered (leached) shoal reservoirs exhibit less connectivity and lower permeability values. Carbonate grain dissolution, compaction and cementation play an important role in decreasing reservoir quality, while evaporite dolomitization is the key diagenetic agent in enhancing reservoir quality. The reef and shoal reservoirs exhibit moderate depositional (mesoscopic-scale) heterogeneity. The reef reservoirs have low diagenetic (microscopic-scale) heterogeneity, while the shoal reservoirs have high microscopic-scale heterogeneity. The intercrystalline pore system of the reef doloboundstones is characterized by moderate-sized pores that are well-connected by uniform pore throats, while the moldic pore system of the leached shoal grainstones is characterized by multi-sized pores that are poorly connected by narrow pore throats. Fluctuations in sea-level, variations in rates of carbonate productivity and changes in accommodation space exert the principal controls on carbonate reservoir architecture and heterogeneity through depositional and diagenetic processes.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil