GEOCHEMICAL CHARACTERISTICS AND SOURCE OF MIDDLE JURASSIC OIL FROM GUANDU STRUCTURE IN THE SOUTHEASTERN SICHUAN BASIN

Based on the systematic analytical data of light hydrocarbon,saturate,aromatic fractions and carbon isotopes,the geochemical characteristics of Middle Jurassic oil from Guandu structure in the southeastern Sichuan Basin was investigated,and the source rock for the oil was determined by oil/ source rock correlation.The studied results reveal the oil shows a dominance of n-alkanes,low amount of branched/cyclic hydrocarbon and a lack of benzene and toluene with a predominance of heptane over methylcyclohexane in C₅-C₈ light hydrocarbons.Its relative low Pr/Ph (1.56),"V" shaped distribution of C₂₇-C₂₉ steranes and enhanced gammacerane indicate a profundal lacustrine source rock with relatively high contribution of aquatic organisms to deposited original organic matter The studied oil exhibits common features for a terrestrial oil,as reflected by its waxy n-alkane distribution and the isotope profile showing a trend toward isotopically light values with increasing n-alkane chain length.According to the correlations based on the parameters of light hydrocarbon and biomarkers,the oil is believed to be not in origin associated with the natural gas and asphalts in Xujiahe stratum (Upper Triassic),and is determined to be derived from Lower Jurassic source rock.