Dual-pressure pyrolysis apparatus unravelling how fluid and lithostatic pressure matter in hydrocarbon expulsion

Abstract

Improvement in thermal simulation technology will increases the accuracy of predictive geochemistry. A dual-pressure pyrolysis apparatus was developed, to precisely control the fluid pressure and lithostatic pressure during simulated source rock maturation. Two series of pyrolysis experiments were carried out, simulating the same generation process but different expulsion fluid pressure condition. Episodic hydrocarbon expulsion from source rocks under controlled lithostatic pressure and fluid pressure was for the first time observed through thermal simulation experiments. The results were applied to (1) hydrocarbon expulsion efficiency (HEE) analysis, and (2) compositional analysis of oil and gas expelled under different pressures. Results show that HEE is strongly influenced by both fluid and lithostatic pressures. If the oil discharged during depressurization and normal pressure is taken into account, the calculated HEE was 2–31 times the HEE that only considers the high fluid pressure expelled oil. Compared with previous experimental results, HEE under controlled dual pressure is also lower. Therefore, before applying the results of thermal simulation experiments to unconventional and deep oil/gas evaluation, sufficient attention should be paid to the pressure conditions of the experiments. Within the oil window, when the temperature and hydrostatic pressure of source rocks are dropped due to tectonic events such as uplift, they can discharge more oil and gas than before uplift, as a consequence of adjustment of over-pressured fluid to re-equilibrate to the adjacent hydrostatic conditions. In addition, during pressure and temperature reduction, the expelled fluids have a higher gas to oil ratio. This helps to indicate that sedimentary basins that experienced tectonic events still have great exploration potential. The temperature program, pressure setup, expulsion conditions in the two-series experiments is a tentative work tested on a representative source rock from the Triassic Yanchang Formation (Ordos Basin), indicating that control of fluid and lithostatic pressure are essential for improving the accuracy of thermal simulation predictions. The dual-pressure pyrolysis apparatus has high exploration relevance, particularly when quantitative results are integrated with the depositional, tectonic and thermal histories of specific source rocks.