Most geologists who have worked the Oligocene-Frio trend of the Gulf Coast have encountered shale or salt diapirs in their subsurface mapping. Much has been written about salt diapirs or salt domes but little emphasis has been placed on pure shale diapirs.

The purposes of this paper are to acquaint the reader with a typical example of a shale diapir and to point out the possible economic and geologic significance of these features in future exploration for buried lower Frio structures. The North LaWard diapir was chosen because of the large number of wells drilled into the shale stock. This shale diapir has intruded into the lower Frio to a depth approximately 1,100 ft above the base of the Frio Formation. The intrusion appears to have taken place as late as late Frio time and most of the lower Frio reservoirs have been truncated and displaced by the shale. Shale diapirs that have not intruded the lower Frio are believed to have formed many of the lower Frio buried structures such as Una West, Potilla, and Copano fields.

The timing and extent of intrusion by the diapiric shale seem to be critical factors in determining the economic success or failure on these structures.

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