Natural gas is produced in the states of New York and Pennsylvania from rocks of Pennsylvanian, Mississippian, Devonian, Silurian, Ordovician, and Ozarkian age. The limits of production are defined in Pennsylvania by the highly folded and faulted area in the central part of the state and in New York by the Adirondack Mountains and the valley of the Hudson. From a historic standpoint this region is very interesting since the first natural gas well was drilled and the first natural gas transmission line was laid in this area. The stratigraphy of the area covers almost the entire Paleozoic system of rocks. These rocks were deposited in the northern end of the Appalachian geosyncline. The most important gas-producing horizons represent shore-line deposits or the fine sand deposits of deltas or partly enclosed bays. There are more than 50 gas-producing horizons and more than 200 separate gas fields in the area. The dominating structural features are the Appalachian geosyncline and the system of folds and faults which originated during the Taconic and Appalachian revolutions.
Gas occurrences in certain sandstones conform very closely to structural conditions but in general the character of the reservoir rocks is the most important factor in determining the accumulation of gas. The carbon ratio of the coals is an excellent index of the presence of oil or gas in the sandstones, but recent studies of certain of the more important reservoir rocks of the Chemung formation indicate that the permeability of the sandstone is of greater importance in limiting the area of oil production than the metamorphism of the rocks which was responsible for the progressive eastward devolatilization of the coals.
The supply of gas in this part of Pennsylvania at present exceeds market demands, whereas for several years New York's production has been supplemented by large amounts of imported gas. The proved reserves of the Pennsylvania fields combined with the large reserves of neighboring states should be ample to supply the demands of this territory for many years.
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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.