Source analysis of quartz from the Upper Ordovician and Lower Silurian black shale and its effects on shale gas reservoir in the southern Sichuan Basin and its periphery, China

Abstract

A major breakthrough in shale gas exploration and development has been made concerning the black shale of the Upper Ordovician and Lower Silurian. In recent years, significant progresses have been made regarding the exploration and development of shale gas in the black shale of the Upper Ordovician and Lower Silurian. Accordingly, we investigated the petrological and geochemical characteristics of the black shale from WF2‐LM5 graptolite biozones and performed a source analysis of quartz as well as evaluated its effect on shale gas reservoirs. The black shale mainly contains clay, carbonate, and quartz with the average TOC of 2.21%. Three quartz types, Q1, Q2, and Q3, were identified via petrological and cathodoluminescence (CL) analysis. Q1 of detrital origin as suggested by its typical morphology with monocrystal structure, large grain‐size, subrounded to angular outlines, and clear edges. This is further supported by the occurrence of medium luminescence under cathodoluminescence and medium intensity of P1 and P2 and high intensity P3 according to CL images. Q2 accounts for more than 90% of quartz and exhibits cryptocrystalline texture, an irregular shape, blurring edges, and low luminescence, and its CL images show low intensity P1, low intensity P2, and high intensity P3. Q3 have the morphologies of siliceous organisms, for example, sponge spicules, radiolaria, and foraminifera. Q2 and Q3 belong to authigenic quartz derived from siliceous organisms. Th/U and V/Cr indicate black shale from LM1–3 mainly deposited in the dysoxic‐anoxic environment while black shale from LM4–5 deposited in the oxic environment. Authigenic quartz‐rich shales from LM1–3 were speculated to be the best shale gas reservoirs characterized by high TOC content, high gas content, and with negligible amount of detrital quartz. Compared with shale from LM1–3, black shale from LM4–5 usually have relatively high detrital quartz content and low TOC content and gas content. Considering the abundant interparticle pore space, the detrital quartz‐rich shale interbedded within the black shale from LM4–5, could also become good shale gas reservoirs if sufficient gas supply is guaranteed.

Differential Characteristics of the Upper Ordovician‐Lower Silurian Wufeng‐Longmaxi Shale Reservoir and its Implications for Exploration and Development of Shale Gas in/around the Sichuan Basin

“…In addition, the siliceous shale at the bottom of the Wufeng-Longmaxi shale has a high content of bioquartz. Bioquartz commonly appears in the form of cryptocrystalline, microcrystalline and microcrystalline aggregates, and its size is between 1 and 5 μm; bioquartz forms during the syndiagenetic-early diagenetic stage Jin et al, 2018;Han et al, 2019). Although microcrystalline bioquartz aggregates filled in the primary pore space and destroyed the interparticle porosity during the early diagenetic stage, the bioquartz aggregates can restrain compaction and preserve the internal pore structure as a rigid framework, which is significant for the preservation of primary pores, the filling of oil in the oil window and the preservation of later OM-hosted pores ( Fig.…”

Micropore Structural Heterogeneity of Siliceous Shale Reservoir of the Longmaxi Formation in the Southern Sichuan Basin, China

“…The study stratum is the Longmaxi Formation of the Lower Silurian in the southern Sichuan Basin [35,36]. It is mainly a set of shallow-deep water shelf facies in the Longmaxi Formation, and stratum thickness is mainly 0-600 m. The lithology is mainly siliceous shale, silty shale and organic (carbonaceous) shale (Figure 1c).…”

Geochemistry, Zircon U–Pb Geochronology, and Lu–Hf Isotopes of the Chishan Alkaline Complex, Western Shandong, China

“…Tectonically, this complex belongs to the Luxi Block at the southeastern margin of the North China Craton and the west side of the Tanlu fault zone (Figure 1a) [28,29]. The Luxi Block is bounded by the Tanlu fault zone to the east, the Liaocheng-Lankao fault zone to the west, the south of the Qihe-Guangrao fault to the north, and the Fengpei fault to the south (Figure 1b) [36][37][38][39][40][41][42][43][44][45]. The exposed formations in the study area include the Neoarchean Taishan rock group, Paleozoic Cambrian to Permian carbonate and clastic rocks, Mesozoic Triassic to Cretaceous clastic and volcanic rocks, and Cenozoic clastic rocks (Figure 1b), with the Neoarchean granodiorite constituting the crystalline basement.…”