Abstract:

In the Mosuowan area of the hinterland of the Junggar Basin， the Jurassic low-saturation tight oil reservoirs display significant spatial variations in overpressure intensity. Investigating the genetic mechanisms and distribution patterns of overpressure under the differential denudation setting in the area holds great significance for understanding the tight oil distribution. In this study， we establish an overpressure prediction model by revising Bowers’ method to incorporate the differential denudation effect. By integrating geophysical data analysis， experimental results， and basin numerical simulations， we establish a quantitative reconstruction method for pressure throughout the whole process from chemical compaction to pressure transfer and then to later-stage tectonic uplift. The results indicate that the new model overcomes the limitations of traditional models， which tend to overestimate the formation pressure in denudation areas， yielding an average prediction error of about 5%. The overpressure origins vary from north to south. In the Mosuowan Uplift， overpressure is primarily attributed to chemical compaction and pressure transfer. In contrast， in the Mobei Uplift， pressure transfer is the dominant mechanism， while in the Shixi Uplift， plastic deformation makes a minor contribution to overpressure. The qualification of the contributions from different origins reveals that in the Mosuowan Uplift and the central and southern Mobei Uplift， where overpressure arises primarily from mixed origins， the contribution ratios of elastic to plastic deformation primarily range from 5∶2 to 2∶1. In contrast， in the Shixi Uplift， plastic deformation contributes far less to overpressure， which is predominantly governed by elastic deformation. Basin simulations reveal that the degree of match between the timing of chemical compaction-induced pressurization and hydrocarbon charging stages directly determines the threshold pressure gradient （TPG） for reservoir fluid flowing. Specifically， chemical compaction-induced pressurization occurred in stages， and the time difference between its onset and hydrocarbon charging stages plays a direct role in determining the TPG. Although the late-stage tectonic uplift inhibited chemical compaction， fault activation facilitated the vertical migration of deep fluids， resulting in a dynamic equilibrium between pressurization and pressure transfer. The spatiotemporal coupling characteristics of overpressure origins reveal the presence of three distinct hydrocarbon accumulation patterns in the Mosuowan area： （1） gas reservoirs predominating with subordinate oil reservoirs in the Mosuowan Uplift， attributed to chemical compaction-induced continuous pressurization； （2） the coexistence of oil and gas reservoirs in the Mobei Uplift， associated with chemical compaction-induced slow pressurization； and （3） oil reservoirs predominating with subordinate gas reservoirs in the Shixi Uplift， corresponding to low-temperature-related weak chemical compaction.

Key words: differential denudation, chemical compaction, pressure transfer, overpressure prediction, tight oil reservoir, Junggar Basin