Salt Diapir—Gulf of Mexico
M.T. Sunwall, K.A. McQuillan, C.J. Nick, 1983. "Salt Diapir—Gulf of Mexico", 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 example is representative of piercement salt domes from the Gulf of Mexico, Outer Continental Shelf area. It is located Offshore Louisiana in sediments of Tertiary and Quaternary age. The seismic profile A-A' (Figures 1, 2) illustrates the piercement structure and several important stratigraphic phenomena. This profile trends east to west and is oriented perpendicular to the salt-sediment interface and also perpendicular to the structural strike of the beds. It is worthy to note that the orientation of a seismic profile in this manner is essential to obtaining good quality data over a salt dome.
The most graphic seismic anomaly on the profile is the reflection free zone, between shotpoints 350-400, which contains the salt intrusive (Figure 2). The interpretation of the salt configuration, given in Figure 3, is based on a combination of well control, gravity, reflection, and refraction data.
The faulting associated with the dome is basically of two types consisting of small normal faults on the domal crest and numerous radial faults emanating from the salt perimeter. Though the crestal faults are readily apparent on the A-A' profile the radial faults are difficult to interpret because their strike is identical to that of the seismic line, thus the fault traces parallel the bedding planes.
The stratigraphic thinning, evident in the interval from 1.0 to 2.0 secs (Figure 2), indicates that this dome was a positive structural feature at the time of sediment deposition. Therefore, it is probable that the structural traps around this dome were formed contemporaneous with deposition and thus were in place, able to trap hydrocarbons upon their generation and migration from the basin.
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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.