Because of the distinctness of the type of structure that characterizes Gulf Coast salt domes and the remarkable intensity of the forces creating these features, the geology of the natural gas associated with salt domes is likewise exceptional.
Recurrence and irregularity of the upward movement of the shallow salt stocks or masses tend to divide the peripheral sands in some places into separated segments. Relatively intense movement during deposition of the sediments above or around salt domes has produced abrupt changes in the sands and later movements may have pinched out the sands against the salt. More recently discovered oil and gas fields that may be associated with very deeply buried salt masses show relatively much less deformation so that much larger deposits of gas occur.
There are only relatively small amounts of gas in most salt-dome fields in proportion to their oil. This relative scarcity of gas may be explained by the more fugitive character of gas which has permitted its escape into the atmosphere at different periods in the geologic history of the dome. The recurring periods of uplift and the creation of cracks and faults, because of their intensity, would foster such gas escapement. Commercial gas has been produced from only 7 salt-dome fields, although most of the fields have sufficient gas for local fuel needs. At present only 3 of the 68 producing salt-dome fields yield gas in quantities sufficient for use outside the field, although 4 other areas possess amounts, which, if developed, could easily supply outside commercial needs.
So few uniformly detailed analyses of coastal natural gas are available that no generalizations can be made regarding the relation of the content of coastal gases to their relation to oil on the domes, or to the position of the gases with regard to the structure.
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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.