--> The Trapping Temperature and Pressure of High Density Methane Inclusions in the Jiaoshiba Shale Gas Field, Sichuan Basin, Central China

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The Trapping Temperature and Pressure of High Density Methane Inclusions in the Jiaoshiba Shale Gas Field, Sichuan Basin, Central China

Abstract

A debate has been ongoing among respected geologists concerning the cause of overpressure in shale gas deposits. Some think the present-day overpressures are actually preserved from a normal pressure gradient established in the geologic past, the extremely low permeability (more precisely, the extremely high capillary pressure) of the shale permits the normal pressure preserve to overpressures during the subsequent uplift and erode. The purpose of this article is to use the trapping pressures of methane inclusions in Jiaoshiba shale gas reservoirs to present the direct in-situ evidence to demonstrate the overpressure generation mechanism in shale gas deposits. High density pure methane inclusions are discovered in the samples of quartz veins from Wufeng-Longmaxi Formation of Jiaoshiba shale gas field through the microthermometric and Raman spectral analysis of fluid inclusions. The Raman scatter peak v1 of methane inclusions are applied to calculate the density of methane inclusions, the Raman scatter peak v1 is in the range from 2910.57 cm-1 to 2911.27 cm-1, and the corresponding density values range from 0.254 to 0.288 g/cm3, which signifies methane inclusions of high density. Taking the minimum homogenization temperatures (216.3 C) of two-phase fluid inclusions paragenetic with high density pure methane inclusions as the trapping temperature of the latter, the equation of state for the CH4 system is employed to calculate the trapping pressure for high density pure methane inclusions in geologic history, and the trapping pressure is found to be as high as 102.6-137.3MPa, and the corresponding pressure coefficients is 1.63-2.18, indicating medium-to-high overpressure characteristics. The abnormal high pressure and relatively high temperature during the formation of the high density methane inclusions may represent the temperature and overpressure of bearing-gas shale at the early stage of the Yanshanian uplifting. The discovery of high density methane inclusions can provide a key evidence for studies on the formation and evolution of overpressure in Wufeng-Longmaxi Formation of Jiaoshiba shale gas field.