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High-pressure diapiric shales, commonly associated with domal structures along the Texas and Louisiana Gulf Coast, characteristically exhibit low values of resistivity, density, and acoustic velocity. Thus, well logs enable identification of these intrusive shale masses.
However, deep-water marine shales--of the types which are source beds for diapiric shales--also are high-pressure formations. These shales, in normal stratigraphic positions, exhibit log characteristics which are similar to those of diapiric shales. Therefore, although resistivity, density, and acoustic-velocity logs may indicate that a domal shale core may have been penetrated, additional data are required for confirmation.
Dipmeter surveys provide information to confirm or deny the intrusive nature of the shale. In addition, if the shale is found to be intrusive, dip information locates the well position with respect to the apex of the diapir. As the shale diapir is approached from above, dips (away from the apex) increase in magnitude--just as if a salt dome were being approached. Within the low-resistivity shale, the dips are relatively constant in both magnitude and azimuth, and dips approximate the dip of the contact between the bedded formations and the diapiric shale. This consistent dip within the domal core is distinctly different from the random dips found in gouge shale adjacent to piercement salt domes.
In an offshore field, resistivity values were used to map the top of a shale dome. None of the wells drilled on this structure penetrated salt. The deepest penetration into the domal shale was approximately 2,000 ft. Contour lines were drawn, using as a datum the depths where the various wells encountered a decrease in shale resistivity to 0.5 ohm-meter. The map indicates a minimum structural closure of 6,000 ft. Dips computed from the map agree closely with those measured within the domal shale by dipmeter surveys.
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