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Mancini, E.A., Puckett, T.M., Parcell, W.C., and Llinas, J.C., 2001:Topical Reports 5-8, Smackover petroleum system (source, reservoir, seal and trap) and underdeveloped smackover reservoirs in the Mississippi Interior Salt Basin, U.S. Department of Energy Report, 442 p.
Abstract
The Smackover Formation, a major hydrocarbon-producing horizon in the Mississippi Interior Salt Basin (MISB), conformably overlies the Norphlet Formation and is conformably overlain by the Buckner Anhydrite Member of the Haynesville Formation. The Norphlet-Smackover contact can be either gradational or abrupt. The thickness and lithofacies distribution of the Smackover Formation were controlled by the configuration of incipient paleotopography. The Smackover Formation has been subdivided into three informal members, referred to as the lower, middle and upper members.
Structural or combination traps characterize Smackover hydrocarbon accumulations in the MISB area. Halokinesis is the principal process that formed these traps. Salt movement of the Jurassic Louann Salt has produced a complex array of salt-related structures in this basin. These structures include peripheral salt ridges, low relief salt pillows, salt anticlines and turtle structures, and piercement domes. The northern limit of the MISB is delimited by the regional peripheral fault trend. This trend includes a series of en echelon faults associated with the Pickens, Gilbertown, West Bend extensional fault and half graben systems. In the area immediately north and updip of the regional peripheral fault trend, Smackover traps consist of anticlinal structures associated with basement paleotopographic highs.
The major petroleum reservoirs in the Smackover Formation occur in the upper part of the formation. These upper Smackover strata consist of upward-shoaling mudstone to grainstone parasequence sets that accumulated as part of the late highstand systems tract deposits of an Upper Jurassic (Oxfordian to Kimmeridgian) depositional sequence. These deposits represent higher energy, normal, open-marine intertidal to subtidal facies. These lithofacies were deposited in shoal flank, shoal crest, lagoonal and subtidal marine environments of a carbonate shoal complex. Reservoir-grade rocks (greater than 6% porosity and more than 0.1 md permeability) are ooid and peloidal grainstone and packstone beds characterized by primary interparticle, grain-moldic, dolomite-intercrystalline and vuggy porosity. Coral-microbial and microbial reef facies of the upper Smackover also have significant reservoir potential. These strata consist of bafflestone and boundstone beds that accumulated as part of the early highstand systems tract deposits of the Upper Jurassic (Oxfordian to Kimmeridgian) depositional sequence. These lithofacies were deposited in reef crest, reef flank, back-reef, and forereef environments of a reef complex. Reservoir-grade rocks are bafflestone and bindstone beds characterized by primary shelter, fenestral enhanced, dolomite-intercrystalline and vuggy porosity.
Although the primary control on reservoir quality in Smackover reservoirs is the fabric of the depositional lithofacies, diagenesis plays a significant role in modifying reservoir architecture. Of the diagenetic events, multiple dolomitization and dissolution events probably had the greatest influence on reservoir development and quality. While the dolomitization created only minor amounts of intercrystalline porosity, it significantly enhanced permeability. It also stabilized the lithology which reduced the potential for later porosity loss due to compaction. The dissolution events enlarged primary (interparticle and shelter) and early secondary (moldic and intercrystalline) pores. While the dissolution did not create large amounts of new porosity, it did expand existing pore throats and enhanced permeability. Locally, the development of fracture systems also served to increase permeability. While diagenesis has a significant influence on Smackover reservoir quality, it generally does not alter the geographic distribution of reservoir-grade rock. The development and distribution of reservoir-grade rock is primarily a function of depositional processes.
The upper Smackover is capped by subaerial evaporites of the Buckner Anhydrite Member of the Haynesville Formation. The anhydrites conformably overlie the carbonate rocks of the Smackover. These evaporites act as a barrier to vertical petroleum migration, and thus are effective seal rocks.
The organic-rich laminated, lower to middle Smackover carbonate mudstones are the petroleum source rocks for Smackover hydrocarbons and are the principal source rocks for the petroleum source rocks for other reservoirs in the Mississippi Interior Salt Basin. Hydrocarbon expulsion from the Smackover source rocks commenced during the Early Cretaceous and continued into the Tertiary. Hydrocarbon migration associated with the Smackover source rocks in the basin was initiated in the Early Cretaceous and continued into the Late Cretaceous and Tertiary. The hydrocarbon migration model for the basin would suggest that reservoirs and traps first sourced from Smackover carbonates in the Early Cretaceous would contain immature oils, while reservoirs and traps sourced in the Late Cretaceous and Tertiary would contain mature oils and gas condensates.
To date, more than 259 million barrels of oil and 1.4 Tcf of natural gas have been produced from 88 Smackover fields in the salt anticline play of the MISB. Opportunities exist for continued exploration for similar undrilled salt features in the basin and for the evaluation of the reservoir potential of Smackover carbonates on salt structures that are productive from shallow Cretaceous horizons. Although Smackover reservoirs are productive in the MISB, these reservoirs are underdeveloped. Carbonate lithofacies associated with carbonate shoal complexes provide an excellent exploration target, especially in the deeper parts of the central portion of the basin. Microbial reef facies are potential reservoirs throughout the basin.
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