Production from the Lower Silurian Longmaxi formation shale gas reservoir in Fuling, Changning, and Weiyuan fields in the Upper Yangtze area has reached over 10 billion cubic meters. The Wufeng-Gaojiabian formation of the Lower Yangtze area is a new area of shale gas exploration in China. The objective of this study was to evaluate the potential of the shale gas reservoir in this area.
An innovated lithofacies classification method was developed that incorporates total organic carbon (TOC), grain size, matrix mineralogy, and lithology. An integrated workflow with input derived from microscopic observation, thin section analysis, ion-milled backscatter scanning electron microscope (BSE), X-ray diffraction, X-ray fluorescence (XRF) element analysis, gas adsorption test, and other organic geochemical experiments provides significant advantages for lithofacies classification. This paper applies an advanced technology in pore geometry analysis of various lithofacies, which has demonstrable value in guiding the shale gas exploration in new areas such as the Lower Yangtze area.
Reservoir characterization was performed on an exploration well in the Tangshan area of China. The lithofacies of the Wufeng–Gaojiabian formation shale can be classified into four types: organic-rich argillaceous/siliceous shale, organic-rich/clay-rich siliceous shale, organic-rich siliceous shale, and organic-lean micritic dolomitic mudstone. The first three lithofacies types have potential for shale gas accumulation, and the organic-rich siliceous shale has the best potential. Careful BSE analyses were done on different shale samples, and an interactive algorithm was used to determine the porosity of the organic-rich siliceous shale, which ranges from 5% to 7%. The shale shows heterogeneity in pore geometry; intergranular pores and intragranular pores dominate the pore spaces. The pores are well connected, but organic pores are rarely seen under microscope. Nutrition adsorption tests performed on organic-rich siliceous shale samples show dual pore size distribution characteristics; one set ranges from 2 to 60 nm, and the other ranges from 85 to 125 nm. Macropores dominate the pore space and account for 53% of the total porosity. Mesopores account for 28%, and micropores account for 19%. The percentage of various pore size gives insight into the potential shale reservoir.
The comprehensive reservoir characterization of the shale gas reservoir of the Wufeng-Gaojiabian formation in the Lower Yangtze area, which investigated depositional settings, organic geochemical features, lithofacies, and reservoir properties, suggests that the Lower Yangtze area may have potential as a shale gas exploration frontier. The workflow can also be applied to other shale gas plays in China.
