Characteristics and controlling factors of shale gas reservoir in the Fuling area, Sichuan Basin

Abstract

Statistical analysis of massive laboratory analytical data shows that for intervals in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Fuling area, Sichuan Basin, the total organic carbon content is 0.55%–5.89% with an average of 2.54%, presenting a gradual increase trend from top to bottom. Based on whole-rock X-ray diffraction data, the content of clay minerals of the shale is 16.6%–62.8% with an average of 40.9%, gradually decreasing from top to bottom. The content of brittle minerals gradually increases from top to bottom, ranging from 37.2% to 83.4% with an average of 59.1%. Porosity is 1.17%–7.98% with an average of 4.61% through the helium injection method. The horizontal permeability is 0.002–335.209 mD with an average of 23.785 mD by the steady-state method. Massive analytical data from the high-pressure mercury injection method show the micropores are well developed in the shale, and mainly provide the primary specific surface area of pores, while the micropores, transitional pores and mesopores primarily contribute to permeability. The pore types were observed using SEM in combination with argon ion polishing technology. From top to bottom of the shale interval, the organic pores increase and the inorganic pores decrease. Gas desorption data show that the total gas content is 0.44–5.19 m3/t with an average of 1.97 m3/t, and gradually increase from top to bottom of the shale interval. The development of marine shale gas reservoir in Well A is controlled by mineral compositions, the development characteristics of organic matter. TOC is not only a major intrinsic factor controlling the lower reservoir interval, but also an important material to provide the reservoir space for the shale gas. In the late diagenetic stage, the clay mineral assemblages changed from montmorillonite to illite, forming new micropores and leading to an increase of the porosity and great influences on the upper reservoir interval. The content of brittle minerals is more than 50%, facilitating formation of fractures, and the permeability performance can be enhanced significantly. In general, the Wufeng Formation and the bottom interval of the Longmaxi Formation are both high-quality reservoirs and primary pay intervals. Keywords: Shale gas reservoir, Controlling factor, Upper Ordovician, Lower Silurian, Fuling area