Factors Controlling Brittleness of the Wufeng–Longmaxi Shale in the Yangtze Platform, South China: Insights from Geochemistry and Shale Composition

Abstract

Brittleness index is an important parameter used to select favorable fracturing intervals during shale gas exploitation. Mineral compositions exert a significant influence on rock brittleness, and various grain assemblages can be deposited under different paleo-environmental conditions. The Wufeng–Longmaxi marine shale is a hot spot of shale gas exploitation in China. However, for the Wufeng–Longmaxi shale, the relationship between sedimentary environments, grain assemblages, and shale brittleness are rarely discussed. To address these problems, a series of experiments including petrographic observations, mineral compositions, rock mechanics, and major (trace) element analyses were carried out on shale samples from the Wufeng–Longmaxi formations. The Wufeng–Longmaxi formations are divided into upper Longmaxi formation (ULMX), lower Longmaxi formation (LLMX), and Wufeng formation (WF). The results show that WF shale samples deposited under a sedimentary environment with low terrigenous detrital influx, suboxic depositional water, and high paleoproductivity are characterized by the highest energy-based BI, compared to ULMX and LLMX shale samples. Dolomite in the samples grew in situ during early diagenesis. Redox conditions, terrigenous detrital flux control authigenic carbonate contents, and the oxic depositional water of the ULMX formation restrain carbonate mineral formation. In contrast, the suboxic sedimentary environment of the LLMX and WF allowed the generation of carbonate minerals. Compared with the WF, the larger terrigenous detrital flux of LLMX formation do favor more authigenic carbonate formation. The paleoproductivity affects biogenic quartz generation, and abundant biogenic quartz, primarily occurring in the form of micron-size microcrystalline quartz in the shale matrix, and can connect the silt-size brittle minerals to form a mechanical framework, which makes a strong positive contribution to shale brittleness. Terrigenous detrital influx decreases brittleness because the weakening effect of terrigenous clay minerals on shale brittleness is greater than the strengthening effect of terrigenous brittle minerals on shale brittleness.

Pore Structure Heterogeneity of Deep Marine Shale in Complex Tectonic Zones and Its Implications for Shale Gas Exploration

“…HSA-shale and BGS-shale samples are mainly composed of quartz and clay minerals, with the average contents of 51.45% and 24.48%, which are similar to the shallow and middle-shallow shale in other blocks. , Feldspar, calcite, dolomite, and pyrite are relatively rare, with average contents of 4.38%, 9.67%, 5.61%, and 4.41%, respectively. Quartz, feldspar, calcite, and dolomite are common brittle minerals in shale, and they have an important impact on the brittleness index . The mineral composition indicates that both HSA-shale and BGS-shale have similar brittle minerals content and brittleness index, which is beneficial for reservoir fracturing …”

“…Quartz, feldspar, calcite, and dolomite are common brittle minerals in shale, and they have an important impact on the brittleness index. 43 The mineral composition indicates that both HSA-shale and BGS-shale have similar brittle minerals content and brittleness index, which is beneficial for reservoir fracturing. 44 The TOC of HSA-shale and BGS-shale varies greatly, ranging from 0.18% to 4.41%, with an average of 2.94%.…”

Analysis of geological factors affecting propagation behavior of fracture during hydraulic fracturing shale formation

“…X-ray diffraction analysis revealed that shale is mainly composed of minerals such as quartz, feldspar, carbonate, clay minerals, and pyrite (Fig. 1) (Sun et al 2021;Gong et al 2021;Ye et al 2022), and different mineral compositions have different properties, which affect the physical, mechanical and fracture toughness properties of the shale, and are determinative of its brittleness (Kang et al 2020;Zhang et al 2013). Brittleness (Meng et al 2021;Dong et al 2021;Liu et al 2020) is an important characteristic of shale, mainly referring to the content of brittle minerals in shale, and is commonly characterized by the brittleness index.…”

Relative Sea-Level Fluctuations during Rhuddanian–Aeronian Transition and Its Implication for Shale Gas Sweet Spot Forming: A Case Study of Luzhou Area in the Southern Sichuan Basin, SW China

“…Hence, sliding joints are likely to occur between quartz aggregates and organic matter under external force, which can result in the development of effective seepage channels during fracturing, promoting high and stable shale gas production. Therefore, high TOC and high microcrystalline quartz content in the TST stage were key to the development of type I sweet spots, where pore networks were supported by authigenic microcrystalline quartz, and a large number of organic pores and methane were generated from abundant organic matter [88][89][90].…”