L.R. Hamberg, 1983. "Seismic Profiles and a Stratigraphic Trap—East Texas Field", 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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The use of seismic profiles to recognize and map potential stratigraphic traps is becoming an increasingly important exploration tool, especially within the more mature hydrocarbon provinces of the world. Seismic stratigraphy often requires not only high resolution seismic data, but also considerable geologic input to interpret features indicative of possible stratigraphic traps. Fortunately, one of the most readily recognized traps of this nature, the simple stratigraphic pinchout, does not require specially acquired seismic data. The composite line illustrated in this text shows such a feature. Although this "pinchout" may not seem overly impressive, it demonstrates the trapping mechanism for the giant East Texas field located on the east side of the East Texas Tyler) basin in northeast Texas. This field was discovered in 1930, by random drilling, long before seismic was considered an effective exploration tool. It is obvious, however, that the trap is easily recognized on reasonably good seismic data, and could readily be mapped with such data.
The trap is basically a stratigraphic pinchout of the Upper Cretaceous Woodbine formation. The Woodbine sand-shale sediments were truncated by erosion on the west flank of the elevated Sabine Uplift. These sediments are unconformably overlain by the Austin Chalk which serves as the main top seal for this trap. The bottom seal is the open marine shales and limestones of Lower Cretaceous age. The illustrations provided with this text show the geologic setting and parameters associated with this classic stratigraphic trap.
The relatively low velocity sand-shale section of the Woodbine overlying the high velocity, predominantly open-marine, carbonates of the Lower Cretaceous results in an excellent seismic interface mapable over most of the East Texas basin. Velocities associated with the Austin Chalk are usually sufficiently fast enough to also result in a well defined seismic interforce at the base of the Chalk. The velocity contrast between these three lithologic sequences is great enough to allow these
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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.