Geochemistry and possible origin of crude oils from Bashituo oil field, Tarim Basin

Abstract

The entire range of hydrocarbons in Bashituo oils from Devonian and Carboniferous reservoirs were investigated to determine their origin, as well as their alteration, mixing, and maturity using gas chromatography, gas chromatography–mass spectrometry, and stable carbon isotope analysis. These geochemical studies indicate that the crude oils in the Devonian and Carboniferous reservoirs share similar C15+ molecular marker compositions of marine organic matter origin, whereas their light hydrocarbon compositions show distinct differences. The Carboniferous-reservoired oils are characterized by a relatively greater abundance of cycloalkanes and methylcyclohexane, which indicate a terrestrial organic matter (OM) contribution. Based on biomarker and carbon isotope analysis, Cambrian–Ordovician (J-O) marine rocks are assumed to be the main source for both the Devonian- and Carboniferous-reservoired oils, whereas Carboniferous rocks with terrestrial OM input also contributed to the Carboniferous-reservoired oils. The coexistence of 25-norhopanes, evident humps from unresolved complex mixture, and intact n-alkanes in Devonian-reservoired oils indicate a mixture of early-charged biodegraded oils with late fresh oils, corresponding to at least two oil-generation episodes by the J-O rocks. Light hydrocarbon indicators suggest a relatively high maturity beyond peak oil generation for the J-O–sourced late fresh oil, whereas C15+ molecular marker parameters indicate a maturity equivalent to early peak oil generation for the J-O–sourced, early-charged biodegraded oil. The maturity of the Carboniferous-sourced oil is equivalent to peak oil generation. The application of the entire range of hydrocarbons is essential when assessing a mixed or altered oil system because light hydrocarbons and biomarkers may yield different source-oil–correlations and maturities.