This paper presents the mathematical premises on which are based the estimation of natural gas reserves. The data used in the estimation of natural gas reserves are the records of (1) initial and current reservoir pressures, (2) volume of gas withdrawn, (3) the physical properties of gases, and (4) the physical properties and areal extent of the gas-bearing reservoir. Equations are given which include corrections for the deviation of gases from Boyle's law, and the determination of bottom-hole pressures from well-head pressures in closed wells. Three cases common to the determination of bottom-hole pressures are described. Six cases concerned with the estimation of original and remaining natural gas reserves in a reservoir are set forth.
Not many years past, natural gas wells were valued according to their ability to produce gas—a sum of dollars for each million cubic feet of daily open-flow capacity. Because many in the industry, even as late as 1919, held that the supply of natural gas in the United States had reached its climacteric and would soon be exhausted, little interest was taken in the determination of natural gas reserves within a reservoir, but the determination of reserves became necessary with the enactment of the Federal Tax laws of 1917. The discovery of large gas supplies, mostly west of Mississippi River and far removed from adequate markets, increased the need for precise reserve estimates. To utilize these supplies in distant markets, large capital expenditures were necessary. To insure the safety of these expenditures, as far as
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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.