The siliceous shales of the Monterey Formation, San Joaquin Basin, California are estimated to contain over 7 billion barrels of oil in place, of which 6 percent has been produced. This jointly funded Chevron/US Department of Energy project represents the first comprehensive study of the siliceous shale in the San Joaquin Basin using advanced coring, logging, geochemistry, seismic, and fracture characterization techniques. The goal of this reservoir study is to establish the viability of CO2 enhanced oil recovery in the siliceous shale.

In 1996, 952 feet of core was taken and analyzed in Buena Vista Hills well 653Z-26B as part of the comprehensive siliceous shale reservoir characterization study. The core had an average permeability less than 1 millidarcy, an average porosity of 29 percent, and an average oil saturation less than 14 percent. Hundreds of thin beds of sand with high porosity and permeability are dispersed throughout the siliceous shale, though the sum total of sand comprises less than 5 percent of the core. Also, based on capillary pressure data, these sands can contain oil saturations greater than 50 percent at current reservoir pressure. Open fracture networks are not prevalent, although where they are present, the fractures do aid in the producibility of the reservoir along with the thin sands.

Based on our detailed reservoir characterization, we have developed hydraulic flow units, a log-based lithologic model and a water saturation algorithm for the siliceous shale at Buena Vista Hills. This geologic model will form the basis for future reservoir simulation and feasibility studies.