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Characterization of the Upper Ordovician Wufeng-Lower Silurian Longmaxi Marine Shale of Well Xiye-1 in Northwestern Guizhou Province, Southwest China: Implication for Shale Gas Potential


A detailed study of the Ordovician Wufeng-Silurian Longmaxi shale of well Xiye-1 in northwestern Guizhou province was conducted based on a systematic analysis of series of experimental measurements for core samples. Trace elements concentrations reveal that dysoxic to anoxic paleo-environmental setting prevailed during the deposition of the Wufeng-Longmaxi shale in northwestern Guizhou Province. The shale has high total organic matter content (TOC) with an average of 2.02% (ranging 0.36%∼6.73%) and high maturity with vitrinite reflectance (Ro) ranging from 2.94% to 3.65% (averaging 3.38%). The organic matter type is sapropelic (I) to humic-sapropelic (II1), which has strong hydrocarbon generation potential. The mineralogy of the shale is mainly composed of 44.7% quartz and 32.6% clay minerals on the 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 μm2 to 0.1098×10-3 μm2 (averaging 0.0378×10-3 μm2). The porosity shows a positive correlation with TOC and brittle minerals (quartz, feldspar, calcite and dolomite) content, but a negative correlation with clay minerals content. The relationship between TOC, clay minerals content and gas adsorption capability shows a positive correlation between TOC and absorbed gas content, which indicates that organic matter can adsorb large amount of gas in the shale besides the free gas in pores, but a negative correlation between clay minerals content and absorbed gas content, which may be caused by the presence of moisture and lower TOC of the clay-rich shale.