High-resolution sequence-stratigraphic characteristics and filling evolution model of Lower Cambrian fine-grained sedimentary rocks in southwestern Sichuan

doi:10.13745/j.esf.sf.2023.2.34

Abstract

Abstract:

The Lower Cambrian shale in Upper Yangtze has good shale gas prospects and is of great importance for the exploration and research of deep marine shale gas in China and even in the world. This paper carried out high-resolution sequence-stratigraphic analysis of the fine-grained marine sediment rocks in the Lower Cambrian, southwestern Sichuan to establish a high-precision sequence-stratigraphic framework of the rock formation which can be used, by sequence comparison analysis, to effectively predict the organic-rich intervals and guide unconventional oil and gas exploration in the region. The fine-grained sedimentary rocks of the Lower Cambrian Maidiping-Qiongzhusi Formations in well Jinshi 103 were examined through elaborate core description and high-precision thin section analysis. The rock formations were divided into 11 lithofacies types: phosphatic rocks; weak-laminar siliceous mudstone; siliceous silty mudstone; deformed bedding argillaceous siltstone; massive siliceous siltstone; dark-laminar silty mudstone; lenticular sequence argillaceous siltstone; wavy lamellar argillaceous siltstone; lean laminated silty mudstone; laminar-rich argillaceous siltstone; and calcareous cementation siliceous siltstone. On this basis, five sedimentary facies types were identified: mixed tidal flat; muddy deep-water shelf; sandy argillaceous deep-water shelf; muddy shallow-water shelf; and sandy muddy shallow-water shelf. By fine core observation, three third-order sequence boundaries—Sinian-Cambrian unconformable surface, SB1, and lithological interfaces between contrasted lithologies, SB2, and between lithologic assemblages, SB3, within the studied rock formation—were identified. Based on quantitative statistical analysis of lithofacies superposition styles and sedimentary structures, 35 parasequences and 14 parasequence sets were delineated in the studied rock formation. Four parasequence superposition styles were summarized: “phosphorite grainstone+calcareous phosphatic sandstone+calcareous phosphatic argillaceous siltstone” parasequence unit (HST1) controlled by tidal flow; “cemented calcareous siltstone+massive siltstone+deformed bedded argillaceous siltstone+dark laminated silty mudstone” (HST2) controlled by synergy of gravity and bottom flows; “lenticular graded argillaceous siltstone/wavy-laminated argillaceous siltstone/deformed bedded argillaceous siltstone+dark laminated silty mudstone” (TST3) controlled by storm wave and co-influenced by bottom flow; and “cemented calcareous siltstone+richly-laminated argillaceous siltstone+poorly-laminated silty mudstone” (HST3) influenced by wave and gravity flow. Finally, 6 system tracts were identified and 3 complete third-order sequence units were delineated. Combined with sedimentary facies/sequence characteristics, a “mixed shelf, to deep shelf, to shallow shelf” evolution model for the Maidiping-Qiongzhusi Formations was established.

Key words: western Sichuan depression, Maidiping-Qiongzhusi Formations, fine-grained sediments, lithofacies, high-frequency sequence

CLC Number:

YUAN Yuxuan, LI Yifan, FAN Tailiang, DU Wei, CHEN Gege, ZHANG Tan, KUANG Mingzhi, LIU Wangwei. High-resolution sequence-stratigraphic characteristics and filling evolution model of Lower Cambrian fine-grained sedimentary rocks in southwestern Sichuan[J]. Earth Science Frontiers, 2023, 30(6): 162-180.

Figures/Tables 15

Table 1 Stratigraphic correlation table for the Lower Cambrian, Upper Yangtze. Modified after [15].

Fig.1 Sedimentary facies distribution of Early Cambrian Qiongzhusi period in Upper Yangtze area. Modified after [18].

Fig.2 Characteristics of mixed tidal flat facies markers in well Jingshi 103

Fig.3 Characteristics of muddy deep-water shelf facies markers in well Jinshi 103

Fig.4 Characteristics of sandy-muddy deep-water shelf facies markers in well Jinshi 103

Fig.5 Characteristics of argillaceous shallow-water shelf facies markers in well Jinshi 103

Fig.6 Characteristics of sandy-muddy shallow shelf facies markers in well Jinshi 103

Fig.7 Lithofacies characteristics of sequence boundaries in the Maidiping-Qiongzhusi Formations in well Jinshi 103

Fig.8 Parasequences within lithofacies assemblages in and characteristics of sedimentary rocks of the Maidiping-Qiongzhusi Formations in well Jinshi 103

Fig.9 Stratigraphic column diagram for the Maidiping-Qiongzhusi Formations in well Jingshi 103

Fig.10 Composite diagram showing the structural characteristics of sequence SQ1 of the Maidiping-Qiongzhusi Formations in well Jinshi 103

Fig.11 Structural characteristics of sequence SQ2 of the Maidiping-Qiongzhusi Formations in well Jinshi 103

Fig.12 Structural characteristics of sequence SQ3 of the Maidiping-Qiongzhusi Formations in well Jinshi 103

Fig.13 Stratigraphic correlation diagram for the Maidiping-Qiongzhusi Formations from Ebiangecun site to well Mx9. Modified from [30⇓⇓-33].

Fig.14 Depositional evolution model for the Maidiping-Qiongzhusi Formations in western Sichuan

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