H. E. Nagle, E. S. Parker, 1971. "Future Oil and Gas Potential of Onshore Ventura Basin, California", Future Petroleum Provinces of the United States—Their Geology and Potential, Volumes 1 & 2, Ira H. Cram
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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 result 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 technologic 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.
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The geology of the entire United States, including the continental shelf and slope, was studied by petroleum geologists to determine its petroleum potential. Prospective areas of the 11 regions were assessed qualitatively and, usually, quantitatively.
The prospective basinal area covers approximately 3.2 million sq mi (statute; 8.3 million sq km) and contains approximately 6 million cu mi (25 million cu km) of sedimentary rock above basement or 30,000 ft (9,144 m). Other less prospective areas are, in the aggregate, large.
The prospective area has not been explored adequately. Many high-potential areas are indicated by the geology and extent of exploration, particularly in parts of Alaska, California, Colorado, Louisiana, Mississippi, Montana, New Mexico, North Dakota, Oklahoma, Texas, Utah, and Wyoming, and in parts of the offshore of Alaska, California, Louisiana, and Texas. The prospective Atlantic, Florida, and Alaska continental shelves, and the entire continental slope, barely have been touched by drilling, and other prospective areas and depths on land and the continental shelf remain largely unexplored.
Estimates of potential crude oil reserves of the basinal area only, exclusive of known reserves, range from 227 to 436 billion bbl of original oil in place. The potential probably exceeds the mean of 332 billion bbl. Approximately 32 percent of the oil in place would be recoverable at known rates of recovery. Ultimately, the rate of recovery may reach 60 percent.
Estimates of potential natural gas reserves exclusive of known reserves range from 595 to 1,227 trillion cu ft of recoverable natural gas. The gas potential also probably exceeds the mean of 911 trillion cu ft.
The ultimate petroleum potential of the United States, including known reserves, may exceed 432 billion bbl of crude oil, 1,543 trillion cu ft of natural gas, and 49 billion bbl of natural gas liquids.
Finding and developing the large petroleum potential will require a great amount of drilling because a significant percentage of the visualized undiscovered crude oil and natural gas is in stratigraphic traps, combination stratigraphic and structural traps, reefs, and complex structural situations. Estimates of future domestic demand call for accelerated exploration. To the extent that policies of industry and government militate against accelerated exploration, particularly drilling, a high percentage of the petroleum resources of the United States will not be reduced to possession.