Abstract

The Outer Santa Cruz and Bodega basins of offshore central California are Neogene narrow-transform-margin basins, having areas of 1,300 square km (500 sq. ml.) and 3,500 square km (1,350 sq. ml), respectively. Both basins contain sections ranging in age from early Miocene to recent and are underlain by Paleogene and/or Cretaceous sequences that are more structurally complex than the overlying Neogene rocks. Exploratory tests were drilled by Shell Oil Company following the central and northern California lease sale of 1963; two in the Outer Santa Cruz basin and nine in the Bodega basin.

The Outer Santa Cruz basin contains rocks ranging in age from early Miocene through Pliocene and younger underlain by sandstones and ailtstones of probable Cretaceous age. Of primary interest are the middle and late Miocene age petroliferous siliceous shales, cherts and carbonates of the Monterey Formation. Structure consists of a series of elongate anticlines associated with northwest-southeast fault trends, concentrated primarily in the eastern half of the basin. In the two Shell exploratory wells, the Monterey section yielded shows of heavy oil with some associated gas. Based on the available data, we estimate that the Outer Santa Cruz basin contains a maximum recoverable reserve of 300 million metric tons of oil and 34 billion cubic meters of gas (1,2 billion barrels and 1.2 trillion cu ft).

In the Bodega basin early Miocene through Pliocene and younger rocks are present, underlain by more strongly deformed sedimentary sections of Paleogene age. The middle and late Miocene interval contains a siliceous Monterey section which is generally thinner and less chert-rich than the equivalent interval in the Outer Santa Cruz basin. Structural development consists primarily of north-northwest-trending anticlines which are formed in the upthrown block of the east-dipping Point Reyes thrust fault in the eastern portion of the basin, and an unnamed parallel thrust to the west near the basin center. Oil shows in the nine wells drilled in the northern half of the basin were variable, as was the thickness and degree of diagenetic alteration of the Monterey section. Gas shows were generally weak. Oil shows were noted in the late Miocene Santa Cruz mudstone and in the middle Miocene Laird sandstone. The Bodega basin is estimated to contain a maximum of 127 million metric tons (850 million bbis) of recoverable oil and 24 billion cubic meters (850 billion cu ft) of associated gas. Because of the variable nature of the Monterey Formation and apparently limited chert development, there is also a possibility that no economic reserves can be developed in this basin.

The eleven wells drilled by Shell provide a stratigraphic framework and guide for future exploration; however, because none of these wells were production tested and because of advances in the understanding of Monterey reservoirs gained through recent work in the offshore Santa Maria basin and Santa Barbara Channel, they cannot be considered to represent a complete exploration of these two basins. Only through a modern exploration program specifically targeted to the Monterey Formation and utilizing advanced logging and testing techniques can a definitive assessment of the hydrocarbon potential of these central California offshore basins be obtained.