The onshore part of the Ventura basin, California, contains marine sedimentary rocks ranging in age from Late Cretaceous to Pleistocene. All these units and the nonmarine Oligocene Sespe Formation are oil productive to some extent. The most prolific oil-producing sequences, of Pliocene and Miocene age, have been only partially evaluated. Pre-Oligocene rocks have been evaluated to a very limited extent. Structural cross sections traversing the Ventura basin at five places illustrate the tectonic and stratigraphic features and the wide variety of petroleum accumulations. The area is subdivided into 10 subprovinces of different structural type, stratigraphy, or tectonic history. Each of these areas is thought to have potential for stratigraphic and srucural entrapment as a r sult of the depositional and tectonic history.

The volume of marine sedimentary rocks in the Ventura basin is 6,600 cu mi (27,500 cu km), of which 2,700 cu mi (11,250 cu km) is marine shale. Ultimate recoverable oil reserves of 1,750 million bbl have been developed to date, 85 percent of which is in Pliocene and Miocene marine beds, and 14 percent of which is nonmarine Sespe reservoirs. Future potential, in terms of accumulated oil in place, is estimated to be 3,650 million bbl in Pliocene and Miocene strata, 450 million bbl in Sespe reservoirs, and 850 million bbl in rocks older than Sespe. Future exploration will be limited by the economic factors of cost, price, and changing land usage and availability. Included in the future reserves is approximately 900 million bbl of low-gravity oil which will be made available only through echnologic advances.

Present economic factors in the Ventura basin are sufficiently attractive to assure continued exploration. The complexity of structure and operational difficulties related to topography or stratigraphy in many parts of the basin have limited the usefulness of seismic techniques in the past. New developments in seismic technology may yield improved results and permit more accurate definition of structural trends.