ABSTRACT

The process of impact cratering results in the instantaneous formation of unique structures characterized by extensive fracturing and brecciation of the target rock. This process can be conducive to economic hydrocarbon accumulations (Red Wing Creek with 130 million bbl of oil in place and Viewfield containing 100 million bbl in place--both in the Williston Basin) provided these impact features can be recognized in the subsurface. Geologists generally are unfamiliar with cratering mechanics and the high temperature, high strain-rate, and high pressure effects that cause quartzofeldspathic rocks to undergo mineral transformations that are often misinterpreted as being "volcanic" in origin.

Reservoirs associated with astroblemes usually are limited to a highly deformed central uplift in larger craters, or to the fractured and brecciated rim facies. The presence of reservoirs and trapping mechanisms is largely dependent upon the preservation state of the crater in the subsurface.

A probable impact crater recently has been identified in South Texas. The poorly preserved, roughly circular crater outline has rim uplifts that consist of well-cemented ferriginous Carrizo sandstone that is overturned in places. Large allochthonous blocks of Carrizo sandstone litter the area outside the impact site, and thrust faults are present in the Indio shale outcrops along the Nueces River adjacent to the impact area. A test well was drilled in the center of the impact area to investigate a strong gravity maximum that had been observed previously. The well was drilled to a total depth of 1200 ft and was dry; however, subsequent wells drilled along the crater rim periphery have been completed as producers from depths as shallow as 200 ft. Lyles Ranch field, which lies in the immediate vicinity of the impact crater site, may represent another hydrocarbon accumulation associated with an astrobleme.