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Irrefutable evidence of fossil remains from Precambrian sediments and proved petroleum reserves in upper Proterozoic (Riphean-Vendian) strata of the Irkutsk basin, USSR, suggest that unmetamorphosed Precambrian sedimentary rocks should be a focus for hydrocarbon exploration.
Since 1965, a dramatic increase in publications which document worldwide occurrences of Precambrian life forms discloses that, by the end of the Proterozoic, organic evolution had produced diversified assemblages of relatively highly developed macroorganisms and microorganisms. Some of these organisms have generated crude oil in the Nonesuch Shale of northern Michigan and kerogen (which yielded hydrocarbons) in stromatolitic carbonate rocks in Africa. Kerogen has been extracted from ~2,300-m.y. old Transvaal (Africa) stromatolitic limestone containing coccoid and complex filamentous cyanophytes (a type of algae). Also, aromatic and aliphatic hydrocarbons have been obtained from the ~2,800-m.y. old Bulawayan stromatolitic limestone of Rhodesia.
Additional evidence indicates that commercial reserves of petroleum from Precambrian strata are possible. An oil discovery in Lower Cambrian rocks in 1962, at Markovo in the Irkutsk basin of the Siberian platform area, led to four noncommercial and eight commercial fields producing from Lower Cambrian and Upper Proterozoic strata. Reserves there may be as much as 80 to 100,000,000 bbl of oil in Lower Cambrian strata and about 2.2 Tcf of dry gas in the upper Proterozoic (Riphean-Vendian). Reserves at Sredne-Butuobin in the extreme northeast part of the basin have been estimated to be on the order of 10 to 25 Tcf of gas. About one-third of the gas is in Lower Cambrian strata and about two-thirds in the Proterozoic. The chemical composition and the acritarch forms of these Cambrian and P ecambrian hydrocarbons are different; therefore, the conclusion that they were generated separately and are indigenous to the containing strata appears to be valid.
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