The effects of pressure and hydrocarbon expulsion on hydrocarbon generation during hydrous pyrolysis of type-I kerogen in source rock

Abstract Hydrous pyrolysis experiments were conducted on immature petroleum source rocks to define the roles of pressure and hydrocarbon expulsion in deep petroleum formation. This study can contribute to estimating the deep oil and natural gas prospects of lacustrine type-I kerogen and has important implications for deep petroleum exploration. The simulation temperature was 450 °C and the heating duration was 48 h. Water pressure ranged between 50 and 1200 bar, and lithostatic pressure ranged between 125 and 2000 bar. Under semi-closed conditions, increasing water pressure leads to more oil and less gaseous hydrocarbons being generated in the source rocks. The dryness of the hydrocarbon gas primarily confirms that increasing water pressure decreases the cracking magnitude of oil, which may indicate that high water pressures retard oil cracking. The decreasing contents of S 2 , HI and H/C in the solid residue confirm that high water pressure enhances the efficiency of hydrocarbon generation. Oil cracking is enhanced in the 125–625 bar lithostatic pressure range, probably because the pyrolysis conditions gradually approach those of a closed-system. The increasing gaseous hydrocarbon yields also confirm the accelerated cracking of oil within this pressure range. As lithostatic pressure increased from 625 to 2000 bar, the decreasing trends in oil and gaseous hydrocarbon yields indicate that hydrocarbon generation rate was retarded by high pressure within the closed system. The values of S 2 , HI, H/C, R o and T max in solid pyrolysis residue confirm that high lithostatic pressure reduces the efficiency of hydrocarbon generation and maturation. Furthermore, hydrocarbon expulsion greatly impacts hydrocarbon generation within the source rocks. Increasing water pressure increases the effect of hydrocarbon expulsion and causes the pyrolysis conditions to gradually approach an open system; this phenomenon lowers the gas yields but increases oil yields. Increasing lithostatic pressure from 125 to 625 bar decreases the effect of hydrocarbon expulsion and causes the pyrolysis conditions to steadily approach a closed system, thereby lowering the oil yield but increasing gas yield. The results demonstrate that different pressures clearly have different effects on hydrocarbon generation in the source rock, and hydrocarbon expulsion also has a significant influence on hydrocarbon generation in source rock. More importantly, this study shows that the lacustrine type-I kerogen has a good potential to yield deep oil and natural gas.