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 (%Ro) and residual TOC. The amount of expelled oil increases substantially with increasing maturity in the Ro range of 0.5–1.25% and ends at Ro of >1.45%, while the volume of expelled gas increases markedly with maturity when Ro is >1.0%. Organic richness exerts primary control on the expulsion yields, which increase linearly with increasing original TOC (TOCo) per unit weight of rock, whereas the increment decreases with TOCo per unit weight of TOC, once the TOCo content is above 5%. Marked TOC reduction occurs in the Ro 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, %Ro, 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 TOCo 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.
Highlights
Petroleum expulsion from source rocks is one of the most important but poorly understood subsurface processes
Natural case histories and laboratory experiments are two main ways to investigate petroleum expulsion. The former relies on empirical observations to quantify the differences in product yields between immature and mature source rocks,[3,5,8−10] while the latter depends on experimental methods to quantitatively estimate the generation potential and expulsion efficiency of petroleum from source rocks.[11−13]
The total organic carbon (TOCo) contents vary from 0.51% to 25.99% in nine selected samples and HIo values vary in the range of 388.2−541.5 mgHC/g TOC, showing typical type II kerogen characteristics (Table 1)
Summary
Petroleum expulsion from source rocks is one of the most important but poorly understood subsurface processes. H/C atomic reducing the ratio initial total organic carbon (TOCo) content of the source rock, the bitumen production in the source rock will reduce dramatically, which in turn leads to a decrease in the expulsion efficiency.[3−5] the expulsion efficiency is related to the ability of kerogen to retain the generated petroleum, pore size distribution, permeability, and microfracture system in the source rocks.[6,7] Natural case histories and laboratory experiments are two main ways to investigate petroleum expulsion The former relies on empirical observations to quantify the differences in product yields between immature and mature source rocks,[3,5,8−10] while the latter depends on experimental methods (hydrous pyrolysis, closed anhydrous pyrolysis, and open anhydrous pyrolysis) to quantitatively estimate the generation potential and expulsion efficiency of petroleum from source rocks.[11−13].
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