Geochemical Characteristics of Oils and Dispersed Organic Matter from the Rocks of the Central Volga-Ural Basin:Hydrocarbon Biomarker Data

摘要

Using the compositions of hydrocarbon biomarkers(normal alkanes,isoprenoids,steranes,and ter-panes)from the Paleozoic sediments of the central Volga-Ural basin within Samara and Orenburg oblasts and Tatarstan,two genetic types of oils were distinguished:post-Domanik and pre-Domanik.The oils of the former type occur in the upper Frasnian,Famennian,Carbonaceous,and Permian sequences and show low concentrations of diasteranes,the presence of C_(30)steranes,medium values of sterane and terpane maturity parameters(K~1,K~2,and Ts/Tm),high C_(29)adiantan/C_(30)hopane ratio,and low values of the CPI index of high-molecular-weight normal alkanes.Since such geochemical signatures were observed only in the organic matter from the siliceous carbonate sequence of the Domanik Horizon of the middle Frasnian substage,the Domanik can be regarded as the major source for the oil fields of the central regions of the Volga-Ural province.Oils of the second genetic type occur mainly in the Middle Devonian terrigenous deposits and show high values of diaster-anes/regular sterane ratio,high sterane and terpane maturity parameters(K~1,K~2,and Ts/Tm),and high ratios of C_(29)neohopane to C_(29)hopane and tricyclic terpanes to pentacyclic terpanes.Their sources were probably the Middle Devonian rocks from the most deeply buried geostructural zones of the Buzuluk depression.