Study on stratigraphic dip window in accumulation period based on the principle of minimum energy dissipation rate of oil and gas migration: A case study of the Upper Paleozoic tight gas reservoirs in the Ordos Basin, China

Scholars have primarily focused on statistical analysis of exploration practices and simple physical simulation experiments when investigating the relationship between stratigraphic dip and hydrocarbon resources during the hydrocarbon accumulation period. However, there is a notable lack of research on the theoretical relationship between stratigraphic dip and hydrocarbon resources during this critical period. This study addresses this gap by exploring the principle of minimum energy dissipation rate governing oil and gas migration. Through this principle, the existence of a stratigraphic dip window for hydrocarbon migration and accumulation system is strictly proved during the hydrocarbon accumulation period. It is established that when the stratigraphic dip window coincides with the hydrocarbon accumulation period, the effective driving power for hydrocarbon migration is at its weakest, resulting in the lowest energy dissipation rate within the hydrocarbon system. Consequently, the hydrocarbon migration and accumulation yields the highest efficiency, leading to the greatest reserves of hydrocarbon resources. This study resolves the puzzle of why the amount of hydrocarbon resources in 44 natural gas-effective zones and 49 oil-effective zones, that have been put into commercial development in China, as reported by Hou et al. (2021), exhibit the statistical characteristic of “downward parabola of opening” in their average stratigraphic dip during the main hydrocarbon accumulation period. Furthermore, it explains the influence of the stratigraphic dip size during the hydrocarbon accumulation period on oil-gas reservoir formation. Moreover, the theory is used to investigate the evolutionary changes in the stratigraphic dip window of tight gas reservoirs in the Upper Paleozoic strata of the Ordos Basin. The study traces the transition of Upper Paleozoic high-pressure tight paleo-gas reservoirs, where the sum of net buoyancy gradient, excess pressure gradient, and discharge pressure gradient serve as the effective driving force of hydrocarbon migration, to modern low-pressure tight gas reservoirs, where the sum of net buoyancy gradient and discharge pressure gradient prevail. Correspondingly, during the hydrocarbon accumulation period, the stratigraphic dip window of the high-pressure tight paleo-gas reservoirs is relatively small (0.2°–0.3°), gradually evolving into the comparatively larger stratigraphic dip window (0.35°–0.45°) characteristic of the current low-pressure tight gas reservoirs.