Molecular marker and carbon isotope geochemistry of ordovician oils in the Tuofutai Bulge, northern Tarim Basin: Implications for oil maturity, source characteristics and filling directions

The Tuofutai Bulge in the Tabei Uplift of Tarim Basin, western China, has become a hot spot for petroleum exploration in recent years. Fifteen Ordovician oil samples from 15 wells in the Tuofutai Bulge were analyzed for biomarkers and carbon isotopes using gas chromatography-mass spectrometry and stable carbon isotope analysis to constrain its depositional environment, thermal maturity, source rocks and the possible petroleum charging directions. The samples were characterized by abundant terpenoids, relatively high contents of C-29 regular steranes and dibenzothiophene, relatively low ratios for Pr/Ph and C-22/C-21 tricyclic terpanes, and high ratios of C-35/C-34 homohopanes. Results suggest that the source rocks for the oils were deposited in a reducing marine carbonate and marl environment. Cambrian and Ordovician source rocks contribute to the oils, and the Middle to Upper Ordovician source rocks exert a greater contribution. n-alkanes and abundant 25-norhopanes in the saturated fractions suggest that the oil is a mixture of early biodegraded oil and late normal crude oil charged at later time. Data from methyldiamantane parameters suggest that the crude oil is in a high maturity stage. The tracing of oil charging suggests that preferential migration directions are primarily from south to north, connected with oil migration along the Shenbei No.1 Fault from the Shuntuoguole Lower Uplift to the south of Tabei Uplift.