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Gross volumes of oil, which must be kept in mind by resource estimators 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 8.2 trillion bbl of oil and heavy oil or tar in more than 35,000 accumulations in the world. About 2.6 trillion bbl of estimated undiscovered conventional oil added to the known volume of 8.2 trillion bbl yields a total of 10.8 trillion bbl known or reasonably estimated. Worldwide cumulative production of about 461 billion bbl of oil accounts for only 4% 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 1,850 billion bbl of oil. The smallest conventional fields in the distribution are about 1 billion bbl because the size cutoff 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 3 supergiant tar deposits which hold nearly half of the total 6,267 billion bbl.
Globally, the largest 3 accumulations, all heavy oil or tar, are in South and North America; the 2 largest conventional oil fields are in the Middle East. Prudhoe Bay and east Texas fields rank 25th and 35th respectively in descending size order.
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