The Arkansas basin in eastern Oklahoma, as in central Arkansas, is a westerly trending topographic and structural trough wherein enormous thicknesses of Paleozoic rocks have been preserved, at least 20,000 feet in the deeper parts. Within this region there are many long, narrow anticlines of major magnitude, generally faulted, and many faults not associated with anticlinal folds.
This region extends south from the northern and western limits of the outcrop of the Hartshorne sandstone to the Choctaw fault. Within it are several gas-producing structures, a few of major importance. Production is mostly from the Hartshorne sandstone (basal Allegheny) of Pennsylvanian age, but there are also gas-bearing sandstones higher in the Allegheny succession of rocks and lower in the Atoka and Wapanucka formations of the Pottsville subdivision. There are not less than 30 gas-bearing zones within the region. The gas-bearing sandstone reservoirs are commonly lenticular, and the producing areas do not necessarily conform with the structural form of the folds.
The most important gas-producing locality, at this time, is the Kinta-Quinton-Featherston district in Haskell and Pittsburg counties. It consists of gas-producing areas scattered along the axis of a west-southwest pitching anticline, faulted on the north flank.
Gas only has been produced within the confines of the region. Some attribute this phenomenon to the degree of metamorphism shown by the carbon ratio of coals, others to conditions of sedimentation prevailing prior to consolidation of the sediments.
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Geology of Natural Gas
Alberta is the only western Canadian province in which a production of natural gas and oil has been developed. Natural gas was discovered in 1885, and at present there are seven producing fields and 330 miles of main pipe lines.
Alberta is divided into eight structural provinces; four of these are gas-producing regions, one is prospective, and the others are of no interest as gas areas. The stratigraphic column has three persistent features, namely, the Palaeozoic limestone section, the profound unconformity superimposed on it, and the succeeding Mesozoic section of transgressive-regressive deposits.
The Turner Valley field is the only developed field producing from formations of Palaeozoic age, though there have been significant discoveries suggesting that other fields are present. A theory is advanced in this paper to explain a Palaeozoic origin for the heavy oil and bitumen in the basal sandstones of the Mesozoic. The gas accumulations in the basal sands were later derived from the bitumen and heavy oil. The reserves of gas in Palaeozoic rocks and the basal sands of Mesozoic age are large.
During Mesozoic time there were at least five marine transgressions of the seas, and there is a marked relation between the marine shales and the gas-bearing horizons in rocks of Mesozoic age. Gas is generally found in the sandstones immediately overlying, within, or immediately underlying the marine shales.
Gas is found in rocks of Jurassic age in the Southern Plains and the Southern Foothills. The reserves are estimated to be about 80 billion cubic feet. Only small amounts of gas are now produced from Jurassic horizons. Gas is found in marine formations of Comanche age in northern Alberta, but there are no developed fields, and the reserves are unknown. There are three gas-bearing horizons in the Colorado (Gulf series), with several fields, including the Foremost, Viking, and Medicine Hat fields. The possible reserves are large and are probably in excess of 600 billion cubic feet. The Lower Montana and Upper Montana rocks (Gulf series) produce gas over large areas, but the yields are small and the horizons are of minor importance. There are no marine rocks of post-Mesozoic age, and the only gas occurrences are small flows from lacustrine deposits.
The analyses of natural gases in Alberta when arranged according to geologic horizons and localities appear to show an increase in the proportion of higher hydrocarbons to methane in a westerly direction for a given gas-bearing horizon. This may be due to the effect on the source material of increasing metamorphism westward.