Critical geological characteristics and gas-bearing controlling factors in Longmaxi shales in southeastern Chongqing, China

Abstract

The southeastern Chongqing area, one of the highest potential shale gas accumulation areas in China, experienced strong tectonic movements. Due to the tectonic uplift in Himalayan period, the target shale formation is characterized by shallow burial depth and abundant fractures. The Lower Silurian Longmaxi shale was deposited under deep and shallow continental shelf environments with abundant pyrites and graptolite fossils. The lithology of the formation includes black carbonaceous shale, calcareous shale, and siliceous shale. The thickness of the shale varies greatly from 40 to 200 m from southeast to northwest with obvious zonation. Total organic carbon (TOC) content ranges from 1.0 to 4.0% and decreases upward in the formation owing to the change of sedimentary facies. The organic matter reaches dry gas zone. Porosity ranges from 0.5 to 7.9%, and the permeability is measured in the microdarcy to nanodarcy range. Abundant fragile minerals exist in the shales which benefit the hydraulic stimulation. Isothermal adsorption tests show that the sorption gas content in place of the Longmaxi shale ranges from 1.0 to 4.5 m 3 /t and the gas content in place reaches the standard of commercial exploitation. Factors impacting gasbearing characteristics were analyzed using a mathematical statistic method, which demonstrates that TOC is the most important factor, especially to the sorption gas content. Meanwhile, clay content, thermal maturity, porosity, and mineral components are also significant factors regarding shale gas-bearing characteristics.

Gas flow mechanisms under the effects of pore structures and permeability characteristics in source rocks of coal measures in Qinshui Basin, China

“…They are commonly considered to be the channels connecting the micropores and fractures, greatly improving the permeability and serving as storage space for free gases. Moreover, these pores and fractures may significantly enhance the exploitation of unconventional gas in coal measures (Jing et al., 2016).…”

Quantitative characterization of micropore structure for organic-rich Lower Silurian shale in the Upper Yangtze Platform, South China: Implications for shale gas adsorption capacity

“…The classification of the nanoscale pore size in the shale mainly accords to the International Union of Pure and Applied Chemistry (IU-PAC) standard, namely the pore width less than 2 nm for micropore, pore width between 2 and 50 nm for mesopore, pore width greater than 50 nm for macropore (Sing, 1985). Pore structure characteristics contribute significantly to the gas storage and adsorption capacity of shales (Ross and Bustin, 2009;Slatt and O'Brien, 2011;Wei et al, 2014;Jing et . Although mesopores and macropores might be present, micropores dominate in gas shale systems (Bu et al, 2015;Yang et al, 2016a).…”

Natural Fracture Development Characteristics and Their Relationship with Gas Contents─A Case Study of Wufeng–Longmaxi Formation in Luzhou Area, Southern Sichuan Basin, China

“…On the one hand, a large amount of biogenic silicon provided by the sedimentation after the death of low-grade siliceous organisms increases the brittleness of the reservoir and facilitates the development of fractures in the later tectonic movement . Based on the actual shale gas exploration in the Sichuan Basin, the concentrated development member of fractures also witnesses high shale gas-bearing properties, with high TOC contents. , Moreover, the brittle mineral content, fracture density, and gas-bearing data at different depths in Wells L205 and L208 were statistically analyzed to study the relationship between the degree of fracture development and the gas-bearing property of shale in the Lu203 well area. On this basis, the correlation of brittle mineral contents, fracture density, and gas-bearing data was established, as shown in Figure .…”