The geochemistry of the organic matter of the Paleozoic rocks of the south and southeast parts of the West Siberian platform has been studied extensively. Information has also been gathered on the Middle Ob, Krasnoleninsk arch, and individual exploration areas of the central and north of the platform. Most of the wells penetrated only the upper part of the Paleozoics. In only a few wells has it been possible to trace variations in geochemical parameters of organic matter with depth.
The Paleozoic sediments were deposited under a wide range of litho-facies, climatic, and tectonic conditions, as reflected in the significant variations in geochemical parameters. For example, the content of organic carbon ranges from 0.01 to 11 percent. Shales carry the greatest amount of organic matter with average content of 2.1 percent. Value for marl is 0.54 percent, and that for limestone is 0.37 percent. These values indicate that content of organic carbon in these Paleozoic sediments exceeds substantially clarke levels. Sapropelic material usually predominates in the carbonate rocks, and humic in the clastics.
The content of bituminous components in the rocks is small, ranging from hundredths to thousandths of a percent. This is apparently the result of significant alteration of the organic matter of these Paleozoic sediments. At high stages of catagenesis after loss of volitles the bitumoids gradually harden, converting to practically insoluble material. Tars predominate at 36-53 percent. Oils and asphaltenes are subordinate.
Ninety Paleozoic samples were investigated using the Rock-Eval method to estimate their residual oil-generating potential, quantitative expression of which is the hydrogen index. A decrease in hydrogen content in the kerogen is due to generation of hydrocarbons during catagenesis of the organic matter. The greatest change in chemical composition of the kerogen takes place after the peak of the main phase of oil generation, when the residual oil-generating potential is sharply reduced, and further maturity of the organic matter generates only gas.
The level of catagenesis of the organic matter is very important for assessing the oil-gas prospects of sedimentary basins. This determines the beginning and attenuation of processes of generation of oil and gas. Cores of carbonate and clastic rocks from 60 areas of West Siberia were studied. Stage of catagenesis was determined from vitrinite reflectance , and pseudo-vitrinite for carbonates. Large areas with levels of catagenesis favorable for petroluem are indicated, and discoveries have been made in the Paleozoics of several of these areas. Up until now, however, no large pools have been found in these areas.
Taken from Fotin, Melenevskiy, and Danilova, 1995; digested
in Petroleum Geology, vol. 31, no. 4, 1997, one map.
Copyright 2002 James Clarke. You are encouraged to print out
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