The Circum-Pacific belt, more than 50,000 km long and from 600 to 5,000 km wide, is a complex of crustal structures which were developed at different times since the late Precambrian, have different makeup, and are confined to the common ring zone.

About 40% of all basins known in the world are within the Circum-Pacific belt. The sedimentary basins

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within the belt are of different size, age, and relief. Some contain oil and gas and account for almost one third of world oil production. These basins contain thick sedimentary sequences which form lens-like bodies. Sedimentary basins in which the sediments are more than 3-3.5 km thick generally contain oil and gas.

Tectogenesis is the leading process in the origin and formation of sedimentary basins and their transformation to oil and gas basins. Therefore, when classifying them, one should be guided by the tectonic setting of basins.

All basins in the Circum-Pacific belt can be divided into five groups. The first group includes basins at the junction of the Circum-Pacific belt with ancient platforms. Such basins are composed of thick Mesozoic, Cenozoic, and some Paleozoic sequences and contain oil and gas.

The second group is connected with intermountain areas. These basins usually are on continental-type crust and contain Mesozoic and Cenozoic sequences 3-5 or more km thick. This is the most numerous group including more than 70 basins.

The third group includes pericontinental folded basins at the junction of folded continental structures and oceanic floor. They are mainly along the American Pacific Coast and contain great thicknesses of predominantly Cenozoic deposits.

The fourth group includes perioceanic basins connected with island arcs. These basins may be between an island-arc uplift and an oceanic floor or in a deep-sea depression.

The fifth group includes intraplatform basins, which are rare and are chiefly within the East Australian Paleozoic folded belt.

The main criterion for oil and gas content is the thickness of sedimentary rocks. The function of the thickness is the degree of katagenetic transformation of dispersed organic matter in the subaqueous part of the sedimentary section. All sedimentary basins more than 3.5 km thick contain oil and gas fields, irrespective of their hypsometric position.

The thicknesses of the rock sequences, rather than faults, control oil and gas content. No distinct relation has been observed between oil and gas accumulation and the position of lithospheric plates defined by "the new global tectonics" concept.

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