Introduction
1. Regional geological settings
Fig. 1. Depositional facies map of the Mao-1 Member (a) and stratigraphic composite column of the Maokou Formation (b) in the Sichuan Basin (modified from Reference [1]). GR—Gamma ray; Δt—acoustic time difference; ρ—density; ϕCNL—neutron porosity. |
2. Sample collection and experimental methods
3. Type, paragenetic association and evolution sequence of the authigenic clay minerals of the Middle Permian Mao-1 Member
3.1. Authigenic clay mineral types in the Mao-1 Member
Fig. 2. Outcrop occurrence and SEM characteristics of main clay minerals within the Mao-1 Member in eastern Sichuan Basin. (a) Huayingshan section, Mao-1-b submember, limestone interbedded with clay; (b) Shizhu section, Mao-1-a submember, limestone interbedded with clay; (c) Tieqiaocun section, Mao-1-a submember, eyelid-shaped limestone; (d) Huayingshan section, Mao-1-a submember, argillaceous micritic bioclastic limestone, fibrous sepiolite, SEM; (e) Well XT1, 4 445.56 m, argillaceous bioclastic limestone, flake talc, SEM; (f) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, feather-like stevensite, SEM; (g) Well XT1, 4 415.03 m, argillaceous bioclastic limestone, flake-shaped disordered talc, SEM; (h) Huayingshan section, Mao-1-a submember, argillaceous micritic bioclastic limestone, scaly montmorillonite, SEM; (i) Huayingshan section, Mao-1-c submember, argillaceous micritic bioclastic limestone, scaly montmorillonite, SEM; (j) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, laminar chlorite single crystal precipitates as the pore liner, SEM; (k) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, tiny plate-shaped chlorite aggregates lining the pores, SEM; (l) Huayingshan section, the Mao-1-a submember, argillaceous bioclastic limestone, honeycomb-like illite, SEM. |
3.2. Paragenetic association and evolution sequences of authigenic clay minerals in the Mao-1 Member
3.2.1. The evolution sequence of sepiolite-stevensite-disordered talc-talc
Fig. 3. The characteristics of sepiolite-stevensite-disordered talc-talc paragenetic association within the Mao-1 Member in eastern Sichuan Basin. (a) Well XT1, 4 440.52 m, argillaceous micritic bioclastic limestone, in sepiolite-stevensite paragenetic association, SEM; (b) Well XT1, 4 431.31 m, argillaceous bioclastic limestone, in stevensite-disordered talc paragenetic association, SEM; (c) Well XT1, 4 440.52 m, argillaceous micritic bioclastic limestone, in disordered talc-talc paragenetic association, SEM; (d) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, shrinkage fractures developing within and around flaky talc, argon ion polishing-SEM; (e) Well S6, 4 164.82 m, argillaceous bioclastic limestone, shrinkage fractures developing within and surrounding flaky talc, argon ion polishing-SEM; (f) Well HS4, 2 679.79 m, argillaceous micritic bioclastic limestone, shrinkage fractures developing within and around flaky talc, argon ion polishing-SEM. |
Fig. 4. The distribution frequency of clay mineral types (a) and corresponding Mg/Si ratio (b) in the Mao-1 Member in eastern Sichuan Basin. |
Fig. 5. X-ray diffraction pattern of the sepiolite and talc in the Mao-1 Member in eastern Sichuan Basin. |
3.2.2. The evolution sequence of sepiolite-montmorillonite-chlorite/illite
Fig. 6. The characteristics of sepiolite-montmorillonite-chlorite/illite and sepiolite-talc-chert mineral combinations within the Mao-1 Member in eastern Sichuan Basin. (a) Well XT1, 4 452.63 m, argillaceous bioclastic limestone, sepiolite and montmorillonite, SEM; (b) Huayingshan section, Mao-1-a submember, argillaceous micritic bioclastic limestone, montmorillonite and chlorite, SEM; (c) Huayingshan section, Mao-1-a submember, argillaceous bioclastic limestone, montmorillonite and illite, SEM; (d) Xingwen section, Mao-1-a submember, sepiolite (talc)-rich layers associated with siliceous concretions, SEM; (e) Well XT1, 4 395.10 m, argillaceous bioclastic limestone, assemblage of calcite-talc-dolomite-silica, plane-polarized light; (f) Well S6, 4 170.53 m, bioclast-bearing argillaceous limestone, assemblage of sepiolite-talc-chert, argon ion polishing-SEM. |
3.2.3. The evolution sequence of sepiolite-talc-chert
4. Diagenetic evolution model of the authigenic clay minerals in the Mao-1 Member
Fig. 7. Tectonic-thermal history of eastern Sichuan Basin (a) and the types of authigenic clay minerals assemblages and their diagenetic evolution models of the Mao-1 Member (b). K—Cretaceous; J1l—Lianggaoshan Formation; J1z—Ziliujing Formation; T3x—Xujiahe Formation; T2l—Leikoupo Formation; T1j—Jialingjiang Formation; T1f—Feixianguan Formation; P2c—Changxing Formation; P2l—Longtan Formation; P2m—Maokou Formation; P2q—Qixia Formation. |
4.1. The diagenetic evolution phases from A period of eogenetic stage to A1 period of mesogenetic stage
4.2. The diagenetic evolution phases of A2 period in the mesogenetic stage
4.3. The diagenetic evolution phases of B1 period in the mesogenetic stage
4.4. The diagenetic evolution phases from B2 period of mesogenetic stage to telogenetic stage
5. Hydrocarbon geological significance of the authigenic clay mineral evolution of the Mao-1 Member
5.1. The clay minerals adsorbing organic matter is beneficial for the enhancement of the hydrocarbon generation potential of source rocks
Fig. 8. The influence of the authigenic clay mineral evolution in the Middle Permian on the source and reservoir assemblage and the model diagram (section position of |