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Drilling since 1954 across a wide area in South Texas has established a new and deeper trend of production which may be an important reserve of oil and gas. It extends from the Mexican border on the Rio Grande northeastward across Webb, LaSalle, McMullen, Atascosa, Karnes, DeWitt, and Lavaca counties, a distance of 200 miles, and has reached a width of 13 miles in McMullen County.
Several factors held back the early development of the play: it is deeper than most operators in the district usually drill; the gas encountered was sour; and the geology is more complex than anticipated. These factors are now fading into the background and additional development is expected. The large size of some of the reservoirs and their capability makes this a profitable operation.
The new development ranges in depth from 9,000 to below 14,000 feet, with the latter figure expected to be increased. Production is established in various structural features The Edwards limestone (Fredericksburg group of the middle Albian Cretaceous) is the main producing zone to date although additional drilling should prove other Cretaceous formations to be productive.
Structural patterns proved to date include piercement-type and deep-seated salt domes, folding along both down-to-the-coast and upthrown fault blocks, and possibly simple reef development in the limestone.
Across central Atascosa County is a series of up-to-the-coast fault blocks which have been tested and yielded several profitable oil and gas fields at depths of 7,000-8,000 feet in the Edwards limestone. This trend is now almost completely explored, and is not considered part of the new development area.
Seismic work has been largely responsible for most discoveries though some fields were mapped as surface structures as early as 1932. An unconformity at the top of the Cretaceous in some areas makes shallow subsurface misleading and the seismic interpretation difficult. Additional sample work is needed to determine newly found stratigraphic changes.
Well expenses have been reduced as contractors solve their local problems. A dry hole will range from $100,000 to $125,000, though some of the deeper tests have been extremely expensive. Completion costs are high due to the need of high-pressure connections and the sulphur content which makes corrosion-proof tubing and fittings necessary. The gas must be cleaned before sale and additional expensive plants will be required.
It is too early to estimate gross reserves for the district, but development to date indicates a major figure. The first opinions on the trends assumed that gas would be the only product, as the first ten fields yielded only minor condensate production. The discovery of the Persons field in Karnes County in 1959 changed that idea. One oil well was also completed on the north end of the Fashing field in Karnes County, with accumulation caused by a stratigraphic or permeability trap, within the gas reservoir.
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