California Plio-Miocene Oils: Evidence of Early Generation
Published:January 01, 1987
Early generation of oil from Monterey and equivalent source rocks may well be the primary controlling factor on oil quality in the coastal basins of California. Commercial generation has occurred at reflectance levels as low as 0.3% RO, The wide range of chemical characteristics of these oils is comparable to source extracts from samples at low levels of organic metamorphism (LOM). At the same time, gross maturity parameters, such as extractable hydrocarbon concentration and hydrocarbon-to-organic-carbon ratios, ndicate that sufficient petroleum has been generated to consider these source rocks as commercially generative. Oil quality is a function of both source facies and of relative level of maturity. Production from these basins is considerable and demonstrates that sections with oil-prone kerogen can be commercially significant at levels of organic metamorphism much lower than the conventional threshold of 0.6-0.7% RO. Evaluation of such areas should be based on direct chemical maturity parameters as well as LOM measurements.
Figures & Tables
Exploration for Heavy Crude Oil and Natural Bitumen
Gross volumes of oil, which must be kept in mind to address the volume/size framework, may be thought of in order from largest to probably smallest volumes as follows: (1) generated; (2) dissipated; (3) degraded/ partially preserved; and (4) trapped and conventionally producible. Basic knowledge of these volumes may be from greatest to least in essentially reverse order.
The 332 largest known accumulations (less than 1% of the total number) account for more than three-quarters of the known 7.6 trillion bbl of oil and heavy oil or tar in more than 40,000 accumulations in the world. About 2.4 trillion bbl of estimated undiscovered conventional oil added to the known volume of 7.6 trillion bbl yields a total of 10 trillion bbl known or reasonably estimated. World-wide cumulative production of about 500 billion bbl of oil accounts for only 5% of the gross.
Oil in place must be estimated for conventional oil fields before comparison with heavy oil and tar accumulations. The size range of accumulations considered in the size distribution of the 332 largest known accumulations is from 0.8 to 1850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cut-off is 0.5 billion bbl of oil recoverable. The size distribution of the 332 largest known accumulations approaches log normal and is overwhelmed by the largest three supergiant tar deposits that hold nearly half of the total 5495 billion bbl.
Globally, the largest three accumulations, all heavy oil or tar, are in South and North America; the two largest conventional oil fields are in the Middle East. Prudhoe Bay and East Texas fields rank 18 and 34, respectively, in descending size order.