Quantitative Analysis of the Formation and Evolution of Underpressure in Tight Sandstone Reservoirs in the Craton Basin: The Chang 7 Member in the A Area of the Ordos Basin.

The Ordos Basin is a typical craton basin. Underpressure exists in the tight sandstone reservoirs of the Chang 7 member of the Upper Triassic Yanchang Formation in the Yishan slope of the basin. However, overpressure was verified in these reservoirs during the Early Cretaceous period, and the evolution process from overpressure to underpressure is still unclear. In this paper, the overpressure evolution process of various lithologies in the Chang 7 member in the study area was reconstructed by using basin simulation. Then, the evolution process from overpressure to underpressure in these lithologies was quantitatively analyzed. These analyses were performed by considering the effects of temperature reduction, elastic rebound, and brittle rupture of overpressured mudstone during tectonic uplift. We also summarized the effect of the pore pressure evolution on oil migration and accumulation. The results show that the overpressure genesis in the Chang 7 member was mainly hydrocarbon generation and disequilibrium compaction, which began to form in the Early Early Cretaceous and peaked at the end of the Early Cretaceous. Since the Late Cretaceous, elastic rebound and temperature reduction due to tectonic uplift have led to current underpressure in the sandstone of the Chang 7 member. Their contributions to pressure reduction of the Chang 7 member range from 53.2% to 60.5% and 39.5% to 46.8%, respectively. In addition to these influences, the formation of nearly normal pressure in the dark mudstone of the Chang 7 member and two sets of black mudstone at the top and bottom of the Chang 7 member is also influenced by the brittle rupture of overpressured mudstone. This effect leads to the release of overpressure in these mudstones and simultaneously causes an increase in the pore pressure of the adjacent tight sandstones. During the entire hydrocarbon accumulation period, the excess pressure difference between the source rock and reservoir rock of the Chang 7 member was the main force driving hydrocarbon migration. This force was 1.5-3.5 MPa and 5-11 MPa during the Early Cretaceous and Late Cretaceous and Paleogene, respectively. The excess pressure difference between the dark mudstone caprock and the reservoir rock can reach 3 MPa, which strengthens the physical sealing ability of the caprocks. This study provides new concepts for the quantitative evaluation of the evolution of underpressure and its relationship with the tight oil distribution.