An Integrated Interpretation of Magnetotelluric and Seismic Data: Mapping Hercynian Rift Clastics Below Acoustic Basement

Jeffrey H. Copley, Thomas C. Connally, and Ian Stewart

The pre-carbonate, deep Paleozoic clastic section in central Saudi Arabia is poorly defined by state of the art seismic methods. The "acoustic" basement is generally the base of the overlying carbonate sequence correlative with a regional unconformity. This sedimentary section forms an important source-reservoir sequence and can be a reservoir zone over large structural traps. A regional unconformity has removed this prospective section from the crestal portions of several known structures as mapped at "acoustic" basement and tested by boreholes. The primary object of the project was to determine if magnetotelluric (MT) data could be utilized to provide a thickness estimate of the deep clastic section above "geologic" basement.

Essential control on the shallower section was provided by seismic and well log data. The MT inversion process allowed the conductive layers to adjust in resistivity such that the higher frequency MT data were matched very closely. The laterally consistent lithology of the shallower (post carbonate) section allows one to expect that any lateral changes in the MT response observed would provide a thickness estimate of the deeper Pre-Khuff sediments.

The poster/workstation display provides a case history of an exploration project where multiple geophysical methods are used together. Detailed integration of seismic structural control resulted in a thickness estimate for the low resistivity clastic section at each MT site location. The utilization of workstatlon-based interpretive software provided the initial results in real-time, directly in the field, throughout the 8-month acquisition phase of the project.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California