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.
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.