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Cearley Technology, Graham, TX

Abstract: Exotic Reservoirs of Zavala County

During Austin Chalk time, submarine volcanoes were extruded along the Balcones Fault in South Texas. These extinct volcanoes are known as the "serpentine plug" trend. On the upthrown side of the fault, the basalt core of one of the ancient volcanoes is exposed at Uvalde, Texas. On the downthrown side, the plugs are buried and comprise or control the post-Chalk oil & gas reservoirs of Zavala County. Many of the volcanoes broached sea level and are flanked by air-fall pyroclastics. A fringing carbonate reef, the Anacacho limestone, is often present. Above the plugs, the San Miguel sandstones are composed of reworked pyroclastics. Overlying Olmos and Escondido sandstones are draped over the plugs through differential compaction. All of the above formations, except for the basalt core of the plugs, are potential hydrocarbon reservoirs. The area is overpressured, with a gradient hovering around .500.

Although great reserves exist in Zavala County, easily located with subsurface contouring and 2-D seismic, not to mention surface methods, some severe problems exist with the development of the area. Huge tracts are controlled by individuals; many of these landowners have bad memories of unscrupulous operators. Gas transmission is at the same time a problem and an opportunity. Overpressure and gas bubble migration at shallow depth make each penetration a control problem. Drilling and completion operations must protect the clay-rich sandstones from swelling and emulsion block. Distance is a factor in every service company job, and road time comprises one-third of a typical invoice.

AAPG Search and Discovery Article #90918©1999 AAPG Southwest Section Meeting, Abilene, Texas