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The Arkansas Valley is an east-west trending synclinorium bounded on the north by the Ozark uplift and on the south by the Ouachita Mountain anticlinorium. The Valley is an eastward extension of the McAlester basin of southeastern Oklahoma.
Pennsylvanian Atoka sediments form the principal outcrops in the Arkansas Valley. The Atoka measures 20,000 feet or more near the Ouachita front. Post-Atoka Pennsylvanian sediments (Hartshorne through Boggy) aggregate 3,000 feet in the western part of the Valley. Cambrian through Pennsylvanian Morrow sediments thicken southward from 5,000 feet or less in the Ozarks to an estimated 25,000 feet of correlatives in the Ouachitas. Little is known of the pre-Pennsylvanian section in the Arkansas Valley because of inadequate drilling densities.
Morrow shales, sandstones, and subordinate limestones were deposited mainly under shallow-water marine conditions. Morrow facies changes occur southeastward in the subsurface and eastward on the outcrop. Meagerly fossiliferous dark shales and lenticular sandstones constitute the Atoka in the Valley. Regional Atoka facies changes have not been observed.
Numerous east-west trending anticlines and synclines have surface expression in the Arkansas Valley. Many are faulted parallel with their axes. Steeply dipping flanks of the folds are indicated on the surface mostly by sharp ridges, and open synclines are expressed by flat-topped mountains. Folds in the southern part of the Arkansas Valley are tight and asymmetrical, with steeper north flanks. Dips of 50° or more are common. Progressively northward, the structures are less tightly folded and more nearly symmetrical. The folding was initiated during the Ouachita orogeny as early as Middle Pennsylvanian time and terminated after Boggy time.
A major fault system marks the structural boundary between the Valley and the Ozarks. These faults are normal and predominantly downthrown toward the south with displacements up to 3,000 feet. The structural boundary between the Arkansas Valley and the Ouachitas is more difficult to define. Faults near the Ouachita front are predominantly downthrown toward the north. High-angle reverse faults and low-angle thrusts are the common types in that region. Major faulting in the Arkansas Valley was not initiated until Atoka time or later.
Only dry gas is produced in the Arkansas Valley, chiefly from the Atoka but increasingly from the Morrow. The gas is sweet, generally high in methane and low in nitrogen. Currently there are 41 designated gas fields in the Arkansas Valley. Daily production averages 50 million cubic feet. Cumulative production since the first commercial well in 1902 totals 300 billion cubic feet. Reserves are in excess of 800 billion cubic feet.
Pennsylvanian production is expected to be expanded both eastward and southward in the Arkansas Valley. Pre-Pennsylvanian sediments, especially the Mississippian
and Arbuckle correlative sections, are prospective targets for oil or gas.
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