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The Arkoma basin is in east-southeastern Oklahoma and in west-central Arkansas. The basin was once a part of the large Ouachita geosyncline, and now is one of several structural basins that lie along the northern margin of the Ouachita mountain system, which traverses the southern and southeastern United States.
The basin exends for approximately 250 mi in an east-west direction, and is 20-50 mi wide from north to south. The deepest part of this arcuate trough is adjacent to the Ouachita mountain system where the sedimentary column is estimated to be 30,000 ft thick.
Rocks in the basin have been highly deformed by a combination of forces. The stability of the Ozark plateau on the north during basin subsidence caused tensional forces to develop which resulted in the evolution of major block faulting in the basin. Evidence indicates that some of these faults were growing contemporaneously with deposition of Lower Pennsylvanian beds.
Early Permian (Ouachita) mountain building on the south compressed Arkoma basin beds into a series of long, narrow, east-west anticlinal and synclinal folds. Overthrusting along anticlinal axes near the mountain front is common. Some of the folds have surface expressions extending 75 mi or more.
Sedimentary strata in the basin are predominantly dark-gray shale of the Lower Pennsylvanian Atokan Series (Pottsville). Most of the gas production in the Arkoma basin is from lenticular, fine-grained sandstone within the Atoka sequence.
Sandstone content in the Atoka increases from west to east in the basin, and the entire Atoka sequence is estimated to be 20,000 ft thick along the mountain front in Arkansas. Individual sandstone units in the Atoka are difficult to correlate because of their lenticularity. The basal Atoka sandstone, the Spiro, is an exception. It is the major gas-productive unit in the basin to date, and can be traced over a large area.
The Arkoma basin is essentially a dry-gas province with the gas being approximately 95 per cent methane. To date there are about 25 gas-producing zones in the basin, ranging in age from early Desmoinesian (Pennsylvanian) to Simpson (Ordovician).
Natural gas first was discovered in the basin in 1902, and sporadic development continued until 1959 when gas was found in the deep Red Oak and Spiro Sands (Atokan) in Latimer County, Oklahoma. This discovery sparked an intensive leasing and deep-drilling campaign.
The largest single gas reserve found to date in the basin is the Red Oak field in Latimer County, Oklahoma. The field is 20 mi long and six mi wide at the widest point. The principal productive units are the Red Oak sandstone found near 7,500 ft and the Spiro at about 12,000 ft.
Although trapping in some instances is structural, mounting evidence indicates that trapping is primarily stratigraphic in the Arkoma basin.
Lack of market and excessive drilling costs have beeen detrimental to development in the past. Major gas outlets now are available, and drilling problems and high expenses have been reduced greatly by use of air and gas as a drilling medium. Topographic problems warrant careful consideration in the planning of drilling operations, because the surface of most of the basin is rugged valley-and-ridge country.
It is speculated that the Ouachita mountain area just south of the Arkoma basin will become a new oil- or gas-productive province.
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