Abstract: Sedimentary Model Prediction of High-Pressure Shales and Oil-Spill Potential on Atlantic Outer Continental Shelf

William B. Travers

Sedimentary modeling of the U.S. Atlantic Outer Continental Shelf (OCS) versus the Gulf of Mexico and the Santa Barbara region of California suggests that oil spills will not occur during drilling through unexpected high-pressure shales.

In the intensively drilled regions of the Texas-Louisiana Gulf Coast and offshore California, abnormally high subsurface pressures have led to oil spills. The pressures were caused by the rapid deposition of thick sections of volcanogenic clays which lack continuous permeable interbeds of sand or limestone. Near Santa Barbara, California, active recent faulting also contributes to overpressuring of shallow strata by fluid migration from deeper strata. The Atlantic and Santa Barbara shelves have very different geologic histories. Thus, a Santa Barbara-type oil spill is virtually impossible along the U.S. Atlantic OCS.

Models of the depositional and the important postdepositional history of the Gulf Coast and the Atlantic OCS are shown to be somewhat alike, but with critical differences. The two areas contain sediments of similar age and similar depositional environments. However, the supply and depositional rate of volcanogenic clays (mainly montmorillonite clays) of the Atlantic OCS was low. Montmorillonite clay was carried to the U.S. Atlantic by an ancestral Gulf Stream, but in small quantities. Recent studies suggest that much of this montmorillonite was carried into bays and estuaries and, thus, only small amounts were deposited offshore on the OCS. Therefore, only thin and discontinuous layers of volcanogenic clays will be found and these will be too small to develop and maintain abnormally h gh pressures.

Thus, in spite of the small quantity of subsurface data from the Atlantic OCS a reliable sedimentary model can be made to evaluate the possibilities of abnormal subsurface pressures. In fact, the petroleum prospects of the Atlantic OCS appear to be reduced substantially by the paucity of shale layers to act as sealing rock for petroleum traps.

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC