The effect of high pressure on hydrocarbon generation from Type-I kerogen source rocks: Implications for petroleum system evolution

Abstract

This study examines the effect of high water pressure (up to 900 bar) on hydrocarbon generation from Type-I kerogen-rich source rocks and compares the results with previously observed effects on Type-II and Type-III kerogens. An immature Type-I oil shale sample from the Duwi Formation, Egypt, was pyrolysed under anhydrous, low-pressure hydrous, and high water-pressure conditions at 320 °C (end of bitumen generation) and 350 °C (oil window) for 6 and 24 h, respectively.

Pyrolysis at 320 °C showed that bitumen generation was promoted in the presence of water under low-pressure hydrous compared to anhydrous conditions but retarded at high water pressures. At 350 °C, oil generation was also retarded by increasing pressure, with maximum oil yield at 500 bar before dropping by 72% at 900 bar. Lower bitumen yields at 500 bar and higher yields at 900 bar confirm more retention of oil and bitumen in the rock at higher pressure. High water pressure systematically decreased hydrocarbon gas yields, with a more prominent effect at 320 °C because of temperature's dominant impact over pressure at 350 °C. Similarly, non-hydrocarbon gas yields decreased as water pressure increased, with maximum yields under anhydrous and low-pressure hydrous conditions. The retardation effect on bitumen generation was less significant than that on oil and gas generation.

This study highlights pressure's impact on petroleum generation, particularly in overpressured basins. Elevated pressures on Type-I kerogen source rocks retard oil expulsion, and the retained oil and bitumen within the rock can be directly cracked to gas, suggesting that under such conditions, oil yields may be lower, while unconventional gas resources are likely to be more abundant.