Origin of abnormal pressure in the Upper Paleozoic shale of the Ordos Basin, China.

Overpressure in the Upper Paleozoic shale of the Ordos Basin (UPSOB) in China occurred mainly in the Early Cretaceous, and is modeled using the PetroMod simulation method in this paper. The origin of paleo-overpressure was analyzed based on the multiple-logging combination method, Bowers' method, and the velocity–density cross-plot method using logging data and shale porosity. The causes of the underpressure and normal pressure, and the history of their evolution were investigated based on a large amount of drilling and pressure data for the basin. The results indicate that paleo-overpressure in the UPSOB produced in the Early Cretaceous was not the cause of disequilibrium compaction, but this occurred owing to increased hydrocarbon generation. Since the Late Cretaceous, during the overall uplift of the Ordos Basin, the decrease in temperature has led to a reduction in pore pressure of 23.93–34.35 MPa, accounting for 56.69%–69.47% of total pressure loss. A combination of the expansion and diffusion of natural gas has led to a decrease in pore pressure of 9.52–22.60 MPa, accounting for 27.63%–41.83% of total pressure loss. The effects of pore rebound on pressure loss were weak. Therefore, temperature decrease alone could not have led to the current underpressure and normal pressure, and the expulsion of gas owing to tectonic uplifting might have been the most important factor. The UPSOB likely had a relatively high gas expulsion efficiency, especially during late-stage tectonic uplifting, which may be a major risk for shale gas exploration. • Paleo-overpressure was caused by the hydrocarbon generation expansion. • Temperature decrease alone can't result in present underpressure and normal pressure. • Gas expulsion caused by tectonic uplifting may be the major reason for pressure loss. • High gas expulsion efficiency may be a major risk for shale gas exploration. [ABSTRACT FROM AUTHOR]