Frontier Exploration in Offshore Northern California--Lessons Learned and New Strategies

Steven B. Bachman, James K. Crouch

A series of exploratory wells were drilled in the northern California offshore basins in the 1960s following leasing of federal tracts off northern California, Oregon, and Washington. The drilling, while encountering numerous oil shows, was considered at the time to indicate low prospectivity in an area that extended as far south as the offshore Santa Maria basin. However, subsequent major discoveries in this decade in the offshore Santa Maria basin, such as the Point Arguello field, indicate that many of these basins are highly prospective exploration targets. Many of the features that make the offshore Santa Maria basin productive are also present in the other offshore basins of northern California and require a reevaluation of these basins.

The lessons learned in offshore Santa Maria (and in other areas with Monterey production) is that the presence of several key features can indicate the potential for substantial reserves: (1) the presence of potential Monterey source and reservoir rocks; (2) diagenesis of siliceous rocks to opal-CT and quartz grades; (3) deep burial of basinal sections to enhance higher gravity oil generation; and (4) complex faulting and folding adjacent to areas of deep burial. These features are present in such northern California offshore areas as the Outer Santa Cruz and Point Arena basins and suggest that these basins may have substantially more reserve potential than previously considered. Exploration strategies can thus be formulated based on the experiences in offshore Santa Maria. In additio to Monterey plays, more conventional clastic source and reservoir rocks have exploration potential in the offshore Eel River basin, as well as in the other northern California basins.

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