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Exploratory drilling in Refugio and Aransas Counties, Texas, has demonstrated that the major structural axis of the Vicksburg-Jackson shale ridge, which was first identified in north-central Calhoun County, Texas, plunges southwestward in the subsurface across northeastern Refugio County into northern Aransas County. The length of the shale ridge is approximately 52 mi (84 km), extending southwestward from Lavaca Bay in Calhoun County to Copano Bay in Aransas County, Texas.
The southwestern limb of this regional structure is not as strongly uplifted in the subsurface of Refugio and Aransas Counties as it is in north-central Calhoun County. The structural characteristics of the ridge are steep dip to the southeast in the upper Frio, lower Frio unconformities associated with dip reversal to the north-northwest in the lower Frio section, and up-to-the-coast faulting.
Oil and gas discoveries in recent years along the southwestern extension of the shale ridge have been from structures similar to those found along the shale ridge in north-central Calhoun County. Two of the discoveries have been significant; additional exploration along the shale ridge should identify other drillable anomalies.
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