Unconventional opal-A and opal-CT diatomaceous reservoirs in the Monterey Formation of the San Joaquin Valley in California have produced for more than 30 yr. These opal-A reservoirs are characterized by high porosity (>50%) and low permeability (<10 md) making them a challenge to produce. Five diatomaceous reservoirs in the Monterey Formation are described, with locations at Lost Hills, Cymric, and McKittrick fields and two at Midway–Sunset field, all in the San Joaquin Valley.

Puzzling features of the diatomaceous reservoirs are considerable. Oil saturation is largely continuous through the reservoirs. Few or no “non-net pay” or non-oil saturated lithologies occur within the oil column. Surfaces corresponding to oil–water contacts for the diatomaceous reservoirs are poorly defined. Strong three-dimensional variations in oil saturation occur, and the edges of the reservoir appear wet at Lost Hills and for the Westates anticline and possibly at Cymric 1Y, implying a puzzling “lateral” oil–water contact. The transition from opal-A to opal-CT occurs at low formation temperatures, yet the fold amplitudes at Lost Hills, Cymric, and Westates are thousands of feet (several thousand meters), indicating that the crestal parts of these reservoirs have likely never been more deeply buried than they are today. The opal-A to opal-CT transition has been folded but more gently than bedding.

In general, the best-producing diatomaceous reservoirs have the best oil saturations (>45% [Cymric and McKittrick fields]) or are very thick (>1000 ft thick [Lost Hills field]). These insights provide general guidance about characterization of unconventional diatomaceous reservoirs and the identification of factors that most dramatically influence performance.