Paralleling the Balcones fault zone, yet most numerous in the Uvalde salient and the Zavala syncline, are several dozens of ill-defined, olivine-basalt extrusives, most of which have been altered to serpentinite. Oil and gas production from rocks of Late Cretaceous to Tertiary ages exists on the crests and off the flanks of many of these domal features. There are many more domal extrusive complexes known from reconnaissance exploration, but, as yet, untested by the drill bit.

Age and rock type of the objective reservoir rocks are determined by the time when the underlying igneous body was extruded. Structural deformation or stratigraphic development above these effusive masses is primarily influenced by the presence of the extruded rock on an older rock surface. There are more clastic-rock reservoirs than domes. Oil or gas affinity for accumulation in either sandstone or limestone seems random. The serpentinite itself may serve as reservoir rock. Outpourings of igneous magma are concentrated in the Upper Cretaceous. Structure and stratigraphy of the overlying rocks into the Tertiary sediments are affected.

Hydrocarbon production from the domes ranges in depth from a few hundred feet to approximately 4,000 ft. The shallowest production is found above the shallowest extrusives, markedly demonstrating their structural and stratigraphic influence.

Exploration methods include surface geology, magnetic surveys, subsurface geology, and core drill near-surface geology. All methods are extremely effective where that particular geological method may be applied. From the standpoint of geology, land, and drilling, profitable objectives may be explored at minimum costs.

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