Overpressure generation and evolution in Lower Paleozoic gas shales of the Jiaoshiba region, China: Implications for shale gas accumulation

摘要

Overpressured shale gas plays have been found in the Lower Paleozoic marine shales of Jiaoshiba region in the Sichuan basin, China. The black shales from the Upper Ordovician Wufeng Group (O(3)w) and the lower part of Lower Silurian Longmaxi Group (Sil) are the producing reservoirs. Overpressure is widespread in the O(3)w-S(1)l shales with the pressure coefficient up to 1.74. Fluid inclusion microthermometric and Raman spectroscopic techniques, PVT and basin modeling are used to investigate overpressure generation and evolution in the O(3)w S(1)l shales. The results show that thermocatalytic dry gas generation is the main genetic mechanism for overpressure development in the O(3)w-S1l shales with specific characteristics as follows: (1) the maximum burial depth of overpressured shales is more than 6200 m without anomalously low density and high primary porosity according to wireline log and core data; (2) the over-mature O(3)w-S(1)l shales with the mean TOC values ranging from 3.36 wt% to 4.48 wt% and the vitrinite reflectance values of 2.7%-2.9% Ro determined by kerogen FTIR (Fourier transform infrared spectroscopy) characteristics, has excellent potential to produce dry gas; (3) overpressured reservoirs are characterized by high gas saturations, and the pressure coefficients positively correlate with gas saturation; (4) the abnormally high pressures recorded in the fluid inclusions are coincident with high homogenization temperatures, which is supportive of overpressure occurring during dry gas generation and provides convincing in-situ evidence for gas generation overpressuring. The calculated pore-fluid pressures for methane inclusions within the maximum buried O(3)w-S(1)l shales are approximately 90.3-139.4 MPa, suggesting that gas generation produced a maximum pressure gradient of 23.1 MPa/km, which is equivalent to the litho static magnitudes. The overpressure evolution in the O(3)w-S1l shales is noticed to have evolved through four stages: a rapidly increasing overpressure caused by thermocatalytic gas generation ( > 145-85 Ma), a rapid pressure release (85-61Ma), overpressure preservation (61-20 Ma), and a relatively slow pressure release (20 Ma - present). During post-generation evolution (85 Ma- present) of the O(3)w-S(1)l shales, both the overpressure and pressure coefficients decreased with time due to the intensive tectonic uplifts and denudation, and approximately 19.00/0-28.5% of the initial total gas content was continuously released until the present-day overpressure state. The study of overpressure evolution in the O(3)w-S51 shales provides basic information for the further study of the conversion between adsorbed gas and free gas, and the method and technique used in this study to stress the overpressure generation and evolution may be generally applied to other overpressured or normally pressured shale gas systems.