Molecular and carbon isotope correlation of free,adsorbed and inclusion oils from the Carboniferous sandstone in the Tazhong-4 oilfield

Substantial variations have been observed in the distribution patterns of terpanes and steranes and δ13C values of individual n-alkanes between two crude oils and oil fractions obtained by sequential extraction from eight oil sandstones collected from the Carboniferous oil columns in wells TZ421 and TZ402.The result demonstrates that these crude oils and oil fractions have different origins.All the free oil,adsorbed oil and inclusion oil obtained from the three oil sandstones from the oil columns within the Upper Carboniferous strata in well TZ421 are characterized by high relative amounts of gammaceranes and C28 steranes,and high δ13C values of individual n-alkanes.These oil fractions are classified as type I oils,which were derived from Cambrian-Lower Ordovician source rocks.In contrast,the two crude oils from wells TZ421 and TZ402,respectively,have low relative amounts of gammaceranes and C28 steranes,and low δ13C values of individual n-alkanes.They are classified as type II oils,the source rocks of which are unknown so far.The oil fractions obtained from the five oil sandstones from the oil columns of the Upper,Middle and Lower Carboniferous strata in well TZ402 are mixtures of types I and II oils.From the free oil,the adsorbed oil,to the inclusion oil of these five oil sandstones,theproportion of type I oil increases while the proportion of type II oil decreases.In addition,from the free oil,the adsorbed oil,to the inclusion oil of all the eight oil sandstones from both wells TZ421 and TZ402,the ratios of C23tricyclic terpane/(C23tricyclic terpane+C30hopane) and C21/(C21+ΣC29) steranes decrease,suggesting that the maturity of charging oil increased during filling process.The molecular and carbon isotope compositions of various hydrocarbon fractions by sequential extraction from oil sandstones demonstrate that hydrocarbons of Tazhong 4 oilfield were derived from multiple source rocks by long charging process,including the source rocks within Cambrian-Lower Ordovician strata.