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Delineation of a Gas Sand Anomaly in Southeast Texas by Electrotelluric Method

Arnon Sugar

A Catahoula (Oligocene) gas sand anomaly in the Texas Gulf Coast area, originally located by bright spot seismic prospecting, was the objective of a six-station electrotelluric (ET) survey. The ET method uses naturally occurring electromagnetic pulses that originate from planes of contrasting conductivity in the subsurface. The frequency of these signals can be related to their depth by the skin-depth equation, as used in other telluric methods. Interpreting changes in the electric field associated with these signals at the surface over a range of frequencies (depending on the depths of interest) yields information on the depth and thickness of subsurface resistivity anomalies. The sand body was overlain by 250 ft of shale and presented a discrete subsurface resistivity a omaly that was well suited to ET prospecting. One existing well was used as a control point to establish the local relationships between lithologic properties, depth, and ET signal characteristics. The ET survey yielded information on the depth, thickness, and hydrocarbon content of the sand body. Borehole penetration subsequent to the survey confirmed this information at two points.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.