Very large volumes of oil in developed siliceous shale reservoirs of the southern San Joaquin basin, California, may be recoverable through carbon dioxide (CO2) flooding. More than 7 billion bbl of oil, trapped in the Miocene Monterey Formation, are present at relatively shallow depths in a series of large and giant fields discovered during the early part of the 20th century yet that have typical recoveries of less than 6%. Such low recoveries are due to the unique nature of the reservoir and to early completion practices. Siliceous shales consist mainly of biogenic silica (original diatom frustules) and varying amounts of clay and silt/sand. These rocks are thinly laminated and typified by high porosities (> 30%), very low permeabilities (< 1 md), extremely small pore throats, and variable degrees of fracturing. Early well completions, up through the 1950s, employed slotted liners and commingled production at high rates, thereby preventing interval evaluation and also causing rapid loss of reservoir pressure. To better understand these reservoirs, and to establish baseline criteria for evaluating CO2 flooding as an option for enhanced recovery, a multiyear study sponsored by the U.S. Department of Energy and Chevron U.S.A. was performed in Buena Vista Hills field, focused on the Antelope shale zone (upper Monterey Formation). A combination of advanced reservoir characterization, three-dimensional earth modeling, and flow simulation was performed. Resulting data provide essential new insight into the detailed nature of these reservoirs, for which mostly vintage log data (pre-1966) was previously available. Criteria established for CO2 flood evaluation include current oil saturation levels, reservoir pressure, lithologic heterogeneity, degree of fracturing, and well spacing. On the basis of these criteria, a site was chosen for a CO2 pilot test in Lost Hills field. This pilot is encompassed within the present Chevron-DOE study and will be covered in a future E & P Notes article.

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