The Monroe gas field, located in northeastern Louisiana, is about 425 square miles in area. It ranks third in area and initial reserves among the known gas fields, and has accounted for a large part of the natural gas production in Louisiana.
Gas is produced from two separate reservoir zones known as the “Gas rock” and the “Second sand.”
The Gas rock, or upper zone, occurs at depths ranging from 2,050 to 2,300 feet. In the western part of the field it lies within the Navarro (Upper Cretaceous) and consists of chalk and chalky sands of primary porosity. Because of erosion or non-deposition, the Navarro is absent over the central and eastern parts of the field where the Gas rock passes laterally into the eroded and leached part of older chalk beds, the Monroe gas rock formation (Upper Cretaceous).
The Second sand, or lower reservoir zone, occurs 100–250 feet below the Gas rock horizon. It is in part Woodbine (Upper Cretaceous) in age, and in part Comanche.
As of December, 1932, 876 gas wells have been completed in the field. Of approximately 70 wells testing the Second sand only 23 commercial wells have been obtained. The initial well-head pressure of each of the reservoirs was 1,020 pounds per square inch. The initial open-flow volumes from both the Gas rock and Second sand have ranged as high as approximately 50 million cubic feet. Approximately 1,592 billion cubic feet (metered and wasted) of gas have been taken from the reservoirs to the end of 1032.
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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.