M. Bergsma, 1983. "Downthrown Fault Closure, Brazoria County, Texas", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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This seismic line is located in southwestern Brazoria County, Texas. The geologic setting is the Texas Gulf Coast Tertiary basin with the exploration objective in the Oligocene Frio. The Frio is characterized by a thick progradational sequence of sandstones and shales. Locally, the Frio was deposited in the downdip portion of the Houston delta system (Galloway et al, 1982). There are numerous trapping styles and features in the area, including salt domes, rollover anticlines into downthrown sides of growth faults, upthrown fault closures, and downthrown fault closures.
This seismic line illustrates a downthrown fault closure trap, with the potential producing zone in the lower Frio Textularia seligi (Texas, Mississippi) sand. On trend with the faults on this line is a field which produces from a similar downthrown trap in the Textularia seligi sand. The downthrown sandstone in the field is juxtaposed with a shale on the upthrown side of the fault. Locally, this sandstone is called the Helmuth sand.
The correlations on the interpreted seismic line are marked with stipling on the reflectors. The faults are marked with a dark line. Because of the change in reflection character across the faults of the prospective section, the correlation across the faults is an interpretation.
The quality of data on this seismic line is good — partly because it was recently acquired and processed with modern techniques, partly because the data quality in the area is generally good. Nothing out of the ordinary was done in obtaining this line, but the routine work was done carefully, an important approach that is often overlooked. This particular seismic line is much shorter than most of the example lines that will be included in this volume. Perhaps though, its length is more typical of what most geophysicists use in their work.