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Approximately 270 giant fields located in 60 basins account for the principal world energy sources. To compare the geologic and historical development characteristics of giant fields, one of several possible basin classifications has been proposed. Three general basin types based on the different crustal thicknesses in cratons, oceans, and zones intermediate between the two are the basis of a classification of 8 types of basins. There appears to be a relation between the classified basin types and both their hydrocarbon characteristics and, to some extent, their historical development patterns.
Cratonic basins are typified by taphrogenetic, block structures out to the mobile zone where the intermediate crustal zone basins are developed. In general, cratonic basins have high-gravity, low sulfur crude and contain over three fourths of the world's gas and the great majority of known Paleozoic hydrocarbons. They have moderate oil recovery per cubic mile of sediments and are relatively predictable in hydrocarbon character. Intermediate basins are more or less directly related to "sea-floor spreading" and commonly display structural trends at angles to cratonic trends. Depending on the tectonics of the various leading edges of worldwide plates these basins are either intensely or relatively moderately deformed. They are commonly subject to high heat flow, at some time during their development. As a result of their tectonic history they are less predictable and their hydrocarbon characteristics are much more variable than those of cratonic basins. Ocean basins are little known and in water too deep for commercial prospect at present.
Normally accepted geologic conditions for the formation of hydrocarbons are enhanced by several special factors including the presence of evaporites, unconformities, regional arches, and suitable geothermal gradients resulting in giant and supergiant accumulations. The lack of significant reserves in Paleozoic rocks may be related to the advent of post-Permian "sea-floor spreading."
When the history of the world's oil basin development patterns over the last 100 years is analyzed it is noted that: (1) more producing basins are being found, but the industry is experiencing a lower success rate in its search; (2) although half of the producing basins contain giant fields, the odds are that only 1 of 5 or 6 basins have prospects of major reserves; (3) the time required to discover oil and develop it in a basin appears to be related to when it was explored, its size, and its geologic character. These factors are modified by terrain and market relations; (4) there has been a tendency to develop basins more rapidly in recent years.
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