Abstract
To better understand oil and gas generation and expulsion mechanisms and their controlling factors, two-stage heating program (20 and 5 °C/d) at 11 target temperatures (250-580 °C) have been performed in a semi-open reactor on nine immature lacustrine shale samples from the Triassic Yanchang Formation in Ordos Basin, NW China, with total organic carbon (TOC) contents ranging from 0.5% to 30.0%. The cumulative expelled oil and gas were quantified and correlated with the measured vitrinite reflectance (%R (o)) and residual TOC. The amount of expelled oil increases substantially with increasing maturity in the R (o) range of 0.5-1.25% and ends at R (o) of >1.45%, while the volume of expelled gas increases markedly with maturity when R (o) is >1.0%. Organic richness exerts primary control on the expulsion yields, which increase linearly with increasing original TOC (TOC(o)) per unit weight of rock, whereas the increment decreases with TOC(o) per unit weight of TOC, once the TOC(o) content is above 5%. Marked TOC reduction occurs in the R (o) of 0.5-1.1% due to oil generation and expulsion, but the trend is reversed in the higher maturity range possibly caused by the simultaneous decomposition of minerals. Numerical correlations among heating temperatures, %R (o), TOC content, and expelled oil and gas yields have been constructed, and the minimum TOC contents for effective oil and gas source rocks have been inferred. The lowest TOC contents of 0.5% and 0.48% are required for oil and gas expulsion in the oil and gas generation window, corresponding to the TOC(o) of 0.91% and 0.76%, respectively. The minimum TOC content for effective gas source rocks decreases slightly with increasing maturity; however, a much higher TOC cutoff is required for lower maturity level source rocks. Wide range of TOC content variation in our studied samples provides well constraint of organic richness on oil and gas generation and expulsion behaviors and their evolution trajectory during thermal evolution, which will fascinate source rock quality and exploration potential assessment in other source rock systems.