Natural Gas in Bend Arch District, Texas
Gas development in the Bend Arch district of Texas has been in progress for about 20 years, but gas was not marketed to any extent until several years after its discovery. Prior to the entrance of gas pipe lines into the area, gas wastage was tremendous.
Production is obtained chiefly from the Strawn formation, of Upper Pennsylvanian age, and the Bend group of Lower Pennsylvanian age. Depths of gas wells vary from 160 feet to approximately 4,500 feet. Gas is widely distributed, geographically, in the area. Gas-producing horizons are numerous and widely distributed, stratigraphically, throughout rocks of Pennsylvanian age. Production is not obtained from the Bend arch as one large major structure, but from minor folds superimposed on its crest and flanks. Minor structures in the Bend formation are at places reflected in the structure of surface rocks, but in most places the relation is slight or entirely absent.
Gas occurrence is determined primarily by distribution of source and reservoir rocks, and secondarily by structure. The restricted area of the lenticular reservoirs, and in the Bend formation the scarcity of water, preclude gas migration for appreciable distances. Deep gas, especially that from the Bend, is much more valuable than shallow gas because of higher rock pressure, longer life, higher B. t. u., and gasoline content.
Total gas recoveries vary widely, even from the same sandstone zone at different localities. Several wells have produced more than 1 billion cubic feet of gas. Some wells beginning with good initial volume and pressure, however, have produced less than ten times the amount of their initial open flow.
A few deep tests have been drilled below the upper part of the Ellenburger limestone (Ordovician), two of which reached pre-Cambrian rocks.
Because of the large area of potential gas territory; indefinite relation, in many places, of gas occurrence to structure; possibility of developing heretofore neglected small gas showings in the Bend into commercial wells by shooting; and because of the fact that oil rather than gas was the objective during greatest drilling activity, the Bend Arch district is considered an important, but erratic, future gas reserve. If market demand warrants, the area will probably be drilled for gas for several years.
Figures & Tables
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.