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Overpressure Generation and Evolution in Wufeng-Longmaxi Shales of Jiaoshiba Shale Gas Field, Sichuan Basin, China

Abstract

Shale gas plays have been found in the Lower Paleozoic marine shales of Jiaoshiba region of Sichuan basin, China. The black shales from the Upper Ordovician Wufeng Group (O3w) and the lower part of Lower Silurian Longmaxi Group (S1l) are the gas reservoirs. The gas reservoirs pressure coefficients can reach up to 1.65, showing overpressure characteristics. Gas generation is a commonly hypothesized mechanism for the development of high-magnitude overpressure. However, overpressures developed by gas generation have been rarely measured in situ, with the main evidence for such overpressures coming from source rock microfractures, the physical necessity of overpressures for primary migration, laboratory experiments, and numerical modeling. Multiple evidences from tectonic subsidence rate, shale porosity, sonic velocity–density response to overpressure, sediment burial and hydrocarbon generation history reveal that overpressures in the O3w-S1l shales are primarily generated by gas generation through kerogen catagenesis and petroleum cracking, and are located within the sub-layer 12 and layer 3 gas-bearing shales. The overpressure magnitude by gas generation are estimated by reconstructing temperature, pressure, and fluid-composition conditions of fluid inclusions trapped in fracture cement minerals in the O3w-S1l shales using microthermometry and Raman microspectrometry, which provides convincing in-situ evidence for gas generation overpressuring. Calculated pore-fluid pressures for methane inclusions of approximately 91.8 to 137.1 MPa at the maximum burial of the O3w-S1l shales suggest that gas generation produced a maximum pressure gradient of 23.1 MPa/km and near the lithostatic magnitudes. A PVT thermodynamic model integrating an equation of state to calculate gas densities for the CH4 system over a wide temperature-pressure (T-P) range is employed to predict the fluid pressure changes during the process of tectonic uplift and exhumation of the O3w-S1l shales. Based on the hypothetical initial uplift pressure coefficient of 1.7, the modelling results demonstrate that if the diffusion amount of shale gas reaches approximately 21.9% of the total gas content before tectonic uplift and exhumation, the overpressure by gas generation can be retained by 85 Ma to the present-day pressure coefficient of 1.65. Gas generation alone can provide all of the overpressure observed in the O3w-S1l shale gas reservoirs.