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Gas and some oil are produced from both clastic and carbonate units in the Jurassic of the East Texas basin. In the Smackover Formation, the reservoir facies are generally shallow marine carbonates that formed in shoal-water environments in the western and northern parts of the basin during late Smackover deposition. Productive intervals contain interbedded dolomites and oolitic grainstones. The dolomite beds are laterally persistent and contain the necessary porosity. Traps are found (1) over low-relief salt structures, (2) against faults and in fault closures, (3) in relatively shallow updip areas over basement structures, and (4) in the northeastern part of the basin, in Cass and Marion Counties, where there are deep basement ridges. In theory, there is potential for S ackover stratigraphic traps in many parts of the basin. However, increased exploration for such traps in East Texas will probably be sparked only after the first significant stratigraphic-trap discovery.
The Haynesville (Cotton Valley) limestone was deposited in carbonate-shelf environments in the western part of the basin and in shallow water along the western part of the Sabine platform. On the western edge of the East Texas basin, a distinct narrow carbonate shelf can be documented. The shelf edge has been encountered in McSwane and Branton fields as a narrow basement-supported feature. Landward, to the west, shallow lagoonal facies grade into evaporites and terrestrial red beds. In this western area, both structural and stratigraphic traps are present. In the eastern part of the basin, Haynesville production is distributed around the western edge of the Sabine platform. Reservoirs overlie both the platform and salt-supported highs just basinward of the platform. Several elongate n rth-south-trending gas fields have been established in this area. For the Haynesville limestone, continued development of known trends is still possible. In addition, this unit has not been extensively tested along the Mt. Enterprise fault system or in the central part of the basin.
Sandstones of the Cotton Valley Group on the Sabine platform produce gas with fracture stimulation at depths from 8,000 ft (2,450 m) to more than 10,000 ft (3,050 m). These sandstones can occur over an interval of as much as 1,400 ft (425 m); they generally have low porosity and permeability and are interbedded with gray to black shales, which probably serve as local source rocks. The underlying Bossier shales may also be a source of the hydrocarbons. Traps are stratigraphic with permeability pinch-out in individual beds. Gas-bearing Cotton Valley sandstones can be found almost anywhere on the Sabine platform, as well as other parts of the basin, but commercial production is typically dependent on the presence of multiple beds with significant porosities. The Cotton Valley sandstone c n be a favorable exploration target for the future with the development of appropriate pricing and a strong market for gas.
In places across the East Texas basin, thin sandstone or siltstone beds punctuate intervals of thick Bossier shale. These sandstone beds commonly release gas under relatively high initial pressures. Traps are stratigraphic with permeability pinch-out in individual beds, and confinement of the gas by thick shale above and below. The shales are also probably source beds. The sandstones are considered coarser grained facies of submarine fan systems that accumulated along the margins of the Bossier marine basin. Much of the Bossier production that has been developed to date is in structural lows in Haynesville reservoir trends. Presumably, the Bossier fans preferentially filled these lows, because structural position of the lows between Smackover-Haynesville structural highs had probably een established by the time of Bossier deposition, and paleobathymetry followed structure.
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