Indications of gas are found accompanying seepages of oil, at the surface, from an area south of the Rio Soto la Marina to the vicinity of the Rio Cazones. Combustible gas is found, accompanying oil, in wells in the Northern and Southern fields of the Tampico Embayment. In a large part of the Northern fields the gas is non-inflammable, due to an excess proportion of carbon dioxide. Natural gas is found in wells in association with oil in the fields of the Isthmus of Tehuantepec and Tabasco.
The writer is indebted to E. DeGolyer for the notes on the Tanhuijo well. Acknowledgments are due to the following for most of the gas analyses and other data: S. A. Grogan, chief geologist of the Mexican Gulf Oil Company, E. H. Ritter, chief geologist of Compania Mexicana de Petroleo “El Aguila,” and to D. Trumpy, former chief geologist of the same company. Sam Katz, of Compania Explotadora “La Imperial,” and P. J. Reilly, of Carbonic Products Corporation, supplied the gas analyses and some of the data concerning the Quebrache field. A. H. Seabrook of the Kern Mex Oilfields, Ltd., supplied the gas analyses from the fields of La Palma and Salinas. Paul Weaver reviewed the original manuscript and made several suggestions which have been included in the text. W. L. Steed, formerly general superintendent of the Mexican Petroleum Company at Ebano, drew the writer's attention to the occurrence of gas associated with salt water in the Ebano field.
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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.