Organic geochemistry of oil and source rock strata of the Ordos Basin, north-central China

Abstract

Paleozoic and Mesozoic strata and a suite of oil samples from wells in the Ordos Basin were studied to determine which strata are source rocks for oil produced in the basin. Analyses included total organic carbon, Rock-Eval pyrolysis, vitrinite reflectance, and conventional biomarker analyses on source rock extracts.Results reveal that Carboniferous coal and organic-rich fluvial-deltaic mudstone samples appear to be gas prone and mature to overmature. Both Upper Triassic and Middle Jurassic lacustrine mudstone samples contain organic matter of sufficient quantity and good quality to be slightly immature or to have low thermal maturity. Oil-oil correlations result in the establishment of one genetic family that can be divided into subfamilies based on degree of oxicity in the source environment, differences in thermal maturity, and differences in clay versus carbonate content of the source rock. An oil-source rock correlation is established between produced oil and Upper Triassic source rock strata. Vitrinite data indicate that the source rock is more thermally mature in the western part of the basin than in the east. These results should drive future exploration strategies for the basin.A bitumen vein is classified as pre-oil solid bitumen using biomarker data. Age-related biomarkers suggest it is derived from a pre-Jurassic source rock. Similar veins in other basins globally are linked to very rich source rocks.