ABSTRACT

Gravity, magnetics, and seismic surveys combined with subsurface geological investigations resulted in very intriguing interpretations of an area east of Crystal City, Texas. The study area includes the south central part of Zavala County east to the Nueces River and the adjoining parts of Dimmit County to the south. The Elaine field is included in the study area.

Gravity and magnetic residuals were calculated using the least squares method, and the magnetics revealed several serpentine plugs. Some of the plugs were confirmed by the seismic interpretations. Although no geophysics were done over the Elaine field, subsurface study shows that the Elaine field is underlain by the largest of these plugs. The seismic studies also show that the Austin Chalk, on whose surface the lava that formed the domes was extruded, is highly fractured and faulted. The Austin Chalk is the lowest structural feature in the area.

The Anacacho Formation was deposited on the lava surface in the Elaine area, where it has a reef-like appearance. Isopachs of younger sediments show that they are draped and differentially compacted over the plugs. The Elaine plug affects sediments as young as the Escondido Formation. The isopachs resemble the structure maps.

Production in the area is mainly from the San Miguel, but significant amounts of hydrocarbons have also been produced from the Eagle Ford, Austin, Anacacho, and Olmos Formations.