ABSTRACT

Southern portions of the Coastal Bend Area of Texas from southeastern Calhoun County to southern Aransas County are well south and east of the fairway sand development of the Frio formation. Down-dip Frio formation sands in these counties are usually dirty, shaley thin-bedded sands; or there is no sand development whatsoever.

Located ten to twelve miles inland from the coast, there are a series of large growth faults, striking generally parallel to the coast, which control and rejuvenate sand deposition on the downthrown blocks of these regional down to the coast faults, causing a significant increase and expansion in the amount of Frio formation deposition due to movement on the fault contemporaneously with deposition. Sand deposition was the result of longshore currents, moving clastics parallel to the coast, and depositing them in a littoral to neritic environment.

On the downthrown block of these hinge line growth faults in areas of local, anticlinal development, the crest of the structures are devoid of sand. As the section is traversed in the subsurface northwest from the anticlinal crest into the trace of the growth fault plane, there is a substantial increase in the amount and quality of the sands deposited, most of which contain commercial hydrocarbon deposits when associated with local structure. These sands vary in physical characteristics from shaley, thin-bedded sands to moderately permeable, well developed sands varying in thickness from a few feet to several feet. Northwest Powderhorn Field in Calhoun County at the northeastern extremity of the area and the Nine Mile Point Field at the southwestern extremity of the area in Aransas County are two examples of this type of down dip Frio deposition and structures associated with producing gas-condensate fields.