Geology of heavy crude oil and natural bitumen in the USSR, Mongolia, and China

The USSR, Mongolia, and China occupy an area of 33,385,390 km2, or a quarter of the earth's land area. Large reserves and resources of heavy crude oil and natural bitumen are present, especially on the Eastern European (Russian) and Siberian platforms, where at least 700 billion bbl is present (out of 1000 billion or more for the USSR as a whole). Chinese and Mongolian resources, in contrast, are of the order of 100 billion bbl. Thus, the heavy oil and natural bitumen reserves and resources of the Siberian platform comprise one of the three largest accumulations in the world, the other two being the Western Canada basin and the Eastern Venezuela basin. Most of the USSR reserves and resources are Paleozoic and Proterozoic, unlike those of most of the rest of the world, which are Mesozoic and Tertiary. Moreover, carbonate-platform deposits predominate, in contrast to the near-shore clastic environments common elsewhere. Two lineages of heavy oil and bitumen are well developed. The first is the kir, or regressive lineage, characterized by the loss of the light fractions from a paraffinic or paraffinic-naphthenic parent oil and the formation of surface stratiform deposits, kirized bitumens, and asphalt lakes. Deposits of this lineage form in short periods of time, rarely longer than 10 million years. Kirs are found in active tectonic belts (e.g., mobile belts, active horst-and-graben areas, mud-volcano fields, etc.). The second, and quantitatively more important, platform or progressive lineage is characterized by thermal metamorphic changes during evolution and high concentrations of sulfur, nickel, vanadium, copper, uranium, and other elements derived from a largely naphthenic- aromatic parent oil. Such deposits form during time periods that usually exceed 100 million years. One apparent end product of platform-lineage bitumen development can be the creation of rich and economic concentrations of sedimentary, stratabound, metallic ore deposits. Heavy-oil and bitumen deposits of platform lineage commonly are found in ancient mobile belts. In the Soviet Union and China, such deposits are found in several Paleozoic fold belts, especially in volcanic or eugeosynclinal facies. The presence of platform-lineage deposits in volcanic/eugeosynclinal belts commonly indicates that those strata are allochthonous. Thus, the presence of sizable heavy-oil and bitumen deposits in tectonized volcanic belts may provide a valuable clue to the identification of a major overthrust belt. Future exploration programs for normal petroleum accumulations must take into account the possible significance of heavy-oil and natural bitumen deposits in incompatible geochemical settings.