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

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 R o 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 (II 1 ). 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 μm 2 (averaging 0.0378 × 10 −3 μm 2 ). 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.

Heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin of China

“…A lot of studies have found that shale interval rich in silicates has a higher porosity within the Lower Silurian Longmaxi Formation in the Sichuan Basin and its adjacent area in China (Tian et al, 2013;Wang et al, 2013a, b;Wu et al, 2014). Higher porosity can provide much more volume for shale gas, that's why silicates have a positive correlation with gas content within the Lower Silurian Longmaxi shale in this study.…”

Composition and pore characteristics of black shales from the Ediacaran Lantian Formation in the Yangtze Block, South China

“…The XRD analysis in this work has indicated a generally high pyrite content, up to 19%, except in one siltstone sample with extremely low TOC content (0.18%, Table 1). Mineralogical studies have shown pyrite content with a range for the majority of samples from 1% to 9% for Neoproterozoic, Lower Cambrian and Lower Silurian marine shales from the Upper Yangtze Block (Cao et al, 2015;Hou et al, 2014;Jiao et al, 2014;Liang et al, 2014;Ma et al, 2015;Tang et al, 2015;Tian et al, 2013Tian et al, , 2015Wang et al, 2013Wang et al, , 2014Wu et al, 2014), and highly abundant pyrite (ca. 12%) was observed in only a few samples (Han et al, 2013;Tan et al, 2014a).…”

Shale reservoir characterization and control factors on gas accumulation of the Lower Cambrian Niutitang shale, Sichuan Basin, South China

“…The effects of tectonic movements, folding and faulting, are well developed in northern Guizhou (Jiu et al, ; Li, Yu, & Guo, ). In general, the fold axes are primarily NE or NNE trending, and ancient fractures are widespread and well developed, resulting in a complex network of fault systems (Wu et al, ). The tectonic setting of the study area is primarily extensional, with a strike‐slip component.…”

“…The black shale is widely distributed in many strata in various basins in China, and previous researches have shown that shale gas resources in China may total some 36 trillion cubic metres (data from EIA), which suggest the great potential for exploration and exploitation of shale gas (Liu, Ma, Huang, Zeng, & Zhang, ; Zou et al, ). The Sichuan Basin is the key area of the shale gas strategy experimental site, with two target intervals, the Lower Cambrian Niutitang Formation shale and the Lower Silurian Longmaxi Formation shale (Liang et al, ; Wu et al, ).…”