M. Bergsma, 1983. "Upthrown Fault Closure, Calhoun 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 Calhoun 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 within the Greta/Carancahua barrier/strand-plain system (Galloway et al, 1982). The dominant structural control is growth faulting with a number of fields producing from rollover anticlines. in Calhoun County, there occurs some counter-regional, up-to-the-coast, down-to-the-northwest faulting. This seismic line illustrates one of these faults.
This seismic line illustrates an upthrown fault closure trap, with the producing zone in the middle Frio, Nonion struma sands. Locally, two of the main productive sandstones are known as the G-1 and the K-2 sands. The correlations on the interpreted seismic line are marked with stipling on the reflectors and the faults are marked with a dark line. The trap shown on this seismic line is an upthrown fault closure on a down-to-the-northwest fault. Production from this field is from the G-1 sand, the trapping fault is at shot point 124 with the G-1 sand at approximately 1.910 secs on the seismic line. The throw on this fault is large, about 300 msec, or 366 m (1,200 ft). This throw is great enough to juxtapose the productive sandstone on the upthrown side of the fault with the younger shale zone on the downthrown side. There is some difficulty in correlating the upthrown side with the downthrown side of the large fault, so the correlation marked is an interpretation. The other smaller faults on the seismic line apparently are not important to the production of the field. There is K-2 sand production in the area, but so far none has been found in this trap.
The data quality on this seismic line is good. Since this is a good area for seismic data, careful acquisition and processing is enough to get good data. It is not necessary to do any special technique.
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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.