Abstract The Middle Pennsylvanian (Desmoinesian) Strawn Formation in the Trans-Pecos area of Texas was deposited during relative tectonic quiescence that prevailed before rapid infilling of the Val Verde Basin. It represents one of a series of backstepping carbonate ramps formed on the craton side of this foreland basin. Strawn Formation carbonate rocks in three cores—Conoco Anna McClung #3-1, Alex Mitchell #2-1R, and Creek Ranch #10-1—show several shallowing-upward sequences, each a few meters thick. The Creek Ranch core displays the deepest-water characteristics of the three cores; the lower part of this core is dominated by graded bedding. The Mitchell and McClung cores contain skeletal-rich carbonates. Both of these cores display characteristics of shallow-water bank or lagoonal environments. All three cores have approximately the same diagenetic history. Primary fluid inclusions indicate early porosity-occluding interparticle and mold-filling calcite precipitated from water with a narrow range of salinities. Modal salinities are that of seawater, but slightly lesser salinities (indicating mixing of seawater and meteoric water) and slightly greater salinities (indicating evaporative concentration of seawater) are also indicated. The influence of meteoric groundwater can be detected by stable-isotope analyses of the early cements at stratigraphic levels that correlate to the tops of the major shallowing-upward depositional sequences. However, subaerial exposure surfaces are not demonstrated in these cores but were likely to be present updip. Most porosity is cement-reduced vugs, dissolution-enlarged (and cement-reduced) molds (> 1/16 mm, < 4 mm), and fractures. Minor intraparticle, intercrystalline, and shelter porosity is also present. Reservoir porosity is caused by fracturing and a late-stage dissolution event. Dissolution in the Creek Ranch core is not as pronounced as in the other cores because of a dearth of skeletal material. Porous zones in the McClung and Mitchell cores are associated with open fractures spatially, which commonly interconnect with nearby molds and vugs. This complex porosity system occurred after stylolitization, as evidenced by “cuticles” of insoluble stylolitic residue that bridge across small dissolution-enlarged fractures. Porosity detected by wireline logs therefore is mostly effective porosity. The open-fracture network may have been caused by thrusting of the Strawn Formation, most likely in Permian time. Late-stage cement reduction of porosity occurs in two stages—first by calcite spar, then saddle dolomite. These cements are unevenly distributed. Both of these cements contain primary oil-filled fluid inclusions. Homogenization temperatures of primary aqueous fluid inclusions in saddle dolomites indicate that the Strawn Formation has been subjected to a temperature of at least 136°C (roughly 45°C over present formation temperature), which correlates to a vitrinite reflectance equivalent of 1.22%. Homogenization temperatures, in conjunction with oxygen isotope compositions, indicate that fracture-filling calcite spars and the later saddle dolomites precipitated from isotopically positive fluids, which were probably connate waters that had undergone extensive rock-water interaction. These observations suggest that thrusting of carbonate shelf strata, in a proximal foreland setting, was responsible for creation of latestage fracture porosity. In turn, tectonic expulsion of undersaturated, heated, connate water into the Strawn Formation enhanced the porosity. As this expulsed water cooled, it reached saturation with respect to calcite and dolomite, and these cements partly filled the available porosity. These processes of reservoir creation might be expected in other proximal foreland settings.

Abstract 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.

Abstract Stephens County, in north-central Texas, is situated on the axis of the Bend arch, one of the most important buried structural features in Texas. The most widespread occurrence of oil in the Bend series, of basal Pennsylvanian age, has been found in this county. Oil is also produced from sands in the Strawn formation, above the Bend. The control of oil accumulation is essentially anticlinal.

Abstract Oil-producing conditions in this shallow field are typical of those existing in. other fields in this part of Brown County, Texas. The sand producing the high-gravity oil from a depth of about 1,300 feet is lenticular. It is stratigraphically located about 50 feet below the unconformity between the Canyon and Strawn formations of lower Pennsylvanian age. The interruption of the normal northwest dip found in this field is thought to be due more to differential settling than to folding or faulting. Contour patterns based on the top of the Palo Pinto limestone and on the top of the producing sand are shown to bring out the conclusion that contours based on the top of a lenticular sand body do not show the true deformation but, instead, show only the shape and ex- tent of the sand body. Porosity and the amount of true sand in the producing horizon have been the most important factors in the accumulation here. Structural deformation is considered, secondary.