Characterization of the Upper Ordovician and Lower Silurian Marine Shale in Northwestern Guizhou Province of the Upper Yangtze Block, South China: Implication for Shale Gas Potential

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Abstract

A detailed study of the upper Ordovician Wufeng shale and lower Silurian Longmaxi shale in northwestern Guizhou province of the upper Yangtze block was conducted on the basis of systematic analyses of a series of experimental measurements on core samples. Trace element ratios V/Cr, U/Th, and V/(V + Ni) reveal that variable paleoredox conditions existed during the deposition of the shale and the degree of anoxia decreased upward. The shale has high thermal maturity with an average Ro value of 3.38% (ranging from 2.94 to 3.65%) and high amounts of organic matter with an average total organic carbon (TOC) content of 2.02% (ranging from 0.36 to 6.73%), the type of which is sapropelic (I) to humic–sapropelic (II1). This indicates that the shale is good gas-producing shale in terms of geochemistry. The mineralogy of the shale is composed of 44.7% quartz and 32.6% clay minerals on average. The shale is characterized by low porosity ranging from 0.6 to 4.4% (averaging 1.8%) and low permeability varying from 0.0066 × 10–3 to 0.1098 × 10–3 μm2 (averaging 0.0378 × 10–3 μm2). The correlations of porosity with the TOC content, quartz content, and clay mineral content demonstrate that the shale interval rich in quartz and organic matter has higher porosity than the clay-rich interval. The gas adsorption capacity increases with the increasing of the TOC content, indicating that organic matter is responsible for adsorbing gas in this shale. Comprehensive analyses through experimental data show that the studied Wufeng–Longmaxi shale succession, especially the lower section, holds the potential for shale gas exploration and development because of TOC enrichment, high thermal maturity, good fracability, and potentially large gas content.

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