Shale gas reservoir characteristics of Ordovician-Silurian formations in the central Yangtze area, China

Abstract

The characteristics of a shale gas reservoir and the potential of a shale gas resource of Ordovician–Silurian age in the north of the central Yangtze area were determined. Core samples from three wells in the study area were subjected to thin-section examination, scanning electron microscopy, nuclear magnetic resonance testing, X-ray diffraction mineral analysis, total organic carbon (TOC) testing, maturity testing, gas-bearing analysis, and gas component and isothermal adsorption experiments. A favorable segment of the gas shale reservoir was found in both the Wufeng Formation and the lower part of the Longmaxi Formation; these formations were formed from the late Katian to early Rhuddanian. The high–quality shale layers in wells J1, J2, and J3 featured thicknesses of 54.88 m, 48.49 m, and 52.00 m, respectively, and mainly comprised carbonaceous and siliceous shales. Clay and brittle minerals showed average contents of 37.5% and 62.5% (48.9% quartz), respectively. The shale exhibited type II₁ kerogens with a vitrinite reflectance ranging from 1.94% to 3.51%. TOC contents of 0.22%–6.05% (average, 2.39%) were also observed. The reservoir spaces mainly included micropores and microfractures and were characterized by low porosity and permeability. Well J3 showed generally high gas contents, i.e., 1.12–3.16 m³/t (average 2.15 m³/t), and its gas was primarily methane. The relatively thick black shale reservoir featured high TOC content, high organic material maturity, high brittle mineral content, high gas content, low porosity, and low permeability. Shale gas adsorption was positively correlated with TOC content and organic maturity, weakly positive correlated with quartz content, and weakly negatively correlated with clay content. Therefore, the Wufeng and Longmaxi formations in the north of the central Yangtze area have a good potential for shale gas exploration.

Keywords

reservoir characteristic / shale gas / Upper Ordovician / Lower Silurian / central Yangtze area

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Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant No. 41302123), the Doctoral Program of Higher Education (Specialized Research Fund) of China (Grant No. 20125121130001), and the Science Foundation of Education Department of Sichuan Province (Grant No. 13ZB0190).

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