Abstract
The southern Sichuan Basin is the core area of China’s efficient development of deep shale gas (burial depth greater than 3,500 m). Reservoir geological characteristics determine whether shale gas can be preserved, enriched, and produced. Taking the Long 11 sub-member of the Wufeng Formation of the Upper Ordovician and the Longmaxi Formation of the Lower Silurian in the East Weiyuan–North Rongchang area as an example, we used the core, logging, production test, and other data, combining X-ray diffraction analysis, LECO Total Organic Carbon (TOC)-S analysis, optical microscopy, and argon ion polishing field-emission scanning electron microscopy, to study the shale mineral composition, geochemistry, reservoir space, pore structure characteristics, and reservoir physical properties. The following results were obtained: 1) The brittle mineral content, organic matter maturity, and TOC content are high, gradually increase from top to bottom, and reach their maxima at small layer 1 of Long 11. 2) Organic pores, inorganic pores, and fractures are important reservoir spaces, among which organic pores and fractures are important seepage channels for shale gas. 3) The shale pore structure revealed by electron microscopy shows that the pore structure in target layers can be divided into four types: unimodal type (mainly organic pores), bimodal type (both organic and inorganic pores), monoclinic type I (mainly organic pores), and monoclinic type II (mainly inorganic pores). The pore morphology is complex, and circular and oval shapes predominate. 4) Sedimentary facies are the main factor controlling the enrichment of shale gas, and the development of fractures is the key to obtaining high yields of shale gas. 5) The class I favorable target area is mainly distributed in wells W206, W206H1, R234H, and R233H and areas to its south, and some areas in the east of the study area.
Highlights
The marine shale gas resources in China are abundant
The XRD analysis of the first segment of the Wufeng Formation–Longmaxi Formation in the study area shows that minerals are mainly quartz, feldspar, calcite, dolomite, and clay minerals, and pyrite and marcasite contents are scarce
Taking the Long 11 sub-member of the Wufeng Formation of the Upper Ordovician and the Longmaxi Formation of the Lower Silurian in the East Weiyuan–North Rongchang area as an example, this paper comprehensively studies the geological characteristics and controlling factors of the enrichment and high-yield of deep shale gas by XRD analysis, LECO Total organic carbon (TOC)-S analysis, optical microscopy, and AIP-FESEM and identified the favorable target areas for shale gas development in the study area
Summary
The marine shale gas resources in China are abundant. Reservoir geological characteristics have always been a hot topic in shale gas exploration and development in China and abroad since they play an important role in controlling the enrichment and production of shale gas (Fan et al, 2018; Li et al, 2019; Fan et al, 2020a; Fan et al, 2020b; He et al, 2021a). The favorable areas for marine shale gas are mainly located in Sichuan Basin, China. Shale gas areas have been found successively, such as Luzhou, Changning, Weiyuan, Fuling, and Dingshan, and breakthroughs in exploration and development have been made. The proven geological reserves in China are approximately 2.0 × 1012 m3. Their output is approximately 154 × 108 m3, making China the second-largest shale gas producer in the world (Nie and Jin, 2016; Nie et al, 2018; Zhang F. et al, 2020; He et al, 2021b)
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