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Diagenesis and Reservoir Potential of the Upper Jurassic Smackover Formation of South Texas (1)
R. G. Loucks (2), D. A. Budd (3)
The hydrocarbon potential of the Smackover Formation in South Texas is virtually untested. Much of the section penetrated is impermeable; however, reservoirs as thick as 33 ft (10 m), with porosity ranging from 4 to 26 percent and permeabilities ranging from 0.1 to 6.5 millidarcys, have been cored at depths below 18,000 ft. Nearly complete dolomitization has resulted in the development of intercrystalline porosity in inner-shelf wackestones and shoal-complex grainstones. In addition, some grainstones have subsurface-derived oomoldic porosity.
In the grainstone facies four general stages of diagenesis affected porosity: Stage 1 (marine-phreatic environment)-precipitation of an isopachous carbonate cement and extensive grain micritization; Stage 2 (shallow-meteoric environment)-precipitation of very coarse-crystalline syntaxial calcite and fine-crystalline equant calcite, dissolution of argonitic skeletal grains, and incipient solution-compaction; Stage 3 (regional fluid-mixing environment)-intrapore precipitation of and grain/matrix replacement by fine- to medium-crystalline rhombic dolomite; and Stage 4 (subsurface environment associated with basinal fluid expulsion)-dissolution of ooids and dolomite, microstylolitization by solution compaction resulting from decarboxylation of kerogen, and precipitation of coarse-crystalline calcite and baroque dolomite. The magnitude of each general diagenetic stage varies regionally.
Porosity development in dolomitized wackestones is sporadic but within grainstones porosity is in part facies related. Hydraulically equivalent oolitic grainstone and oolitic quartzarenite facies are commonly dolomitized and have associated porosity development. In addition, dolomitization of oolitic facies is more extensive in the updip half of the shoal-complex trend. These facies/diagenetic relationships can be used as an exploration tool in the Smackover Formation in South Texas.
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