| Citation: |
An-kun Zhao, Dong Wang, Qian Zhang, Zi-hui Lei, Qian Yu, Di Zhang, Ye-xin Zhou, 2024. Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales, southwest Yangtze Plate, China: Insights from geochemical and petrological evidence, China Geology, 7, 747-761. doi: 10.31035/cg2022074
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Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales, southwest Yangtze Plate, China: Insights from geochemical and petrological evidence
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Abstract
Upper Ordovician‒Lower Silurian Wufeng-Longmaxi Formation is the most developed strata of shale gas in southern China. Due to the complex sedimentary environment adjacent to the Kangdian Uplift, the favorable area for organic-rich shale development is still undetermined. The authors, therefore, focus on the mechanism of accumulation of organic matter and the characterization of the sedimentary environment of the Wufeng-Longmaxi Shales to have a more complete understanding and new discovering of organic matter enrichment and favorable area in the marginal region around Sichuan Basin. Multiple methods were applied in this study, including thin section identification, scanning electron microscopy (SEM) observations and X-ray diffraction (XRD), and elemental analysis on outcrop samples. Five lithofacies have been defined according to the mineralogical and petrological analyses, including mudstone, bioclastic limestone, silty shale, dolomitic shale, and carbonaceous siliceous shale. The paleo-environments have been reconstructed and the organic enrichment mechanism has been identified as a reduced environment and high productivity. The Wufeng period is generally a suboxic environment and the early Longmaxi period is a reducing environment based on geochemical characterization. High dolomite content in the study area is accompanied by high TOC, which may potentially indicate the restricted anoxic environment formed by biological flourishing in shallower water. And for the area close to the Kangdian Uplift, the shale gas generation capability is comparatively favorable. The geochemical parameters implied that new favorable areas for shale gas exploration could be targeted, and more shale gas resources in the mountain-basin transitional zone might be identified in the future.
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Keywords:
- Shales /
- Shale gas /
- Sedimentary environment /
- Graptolite /
- Upper Ordovician‒Lower Silurian /
- Organic matter accumulation /
- Radiolarian /
- Geochemical and petrographic evidence /
- Wufeng-Longmaxi Formation /
- Oil-gas exploration engineering /
- Suboxic environment /
- Sichuan Basin /
- Reduced environment /
- Yangtze Plate
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References
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An-kun Zhao, Dong Wang, Qian Zhang, Zi-hui Lei, Qian Yu, Di Zhang, Ye-xin Zhou, 2024. Sedimentary environment and organic matter accumulation of Wufeng-Longmaxi shales, southwest Yangtze Plate, China: Insights from geochemical and petrological evidence, China Geology, 7, 747-761. doi: 10.31035/cg2022074
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Figure 1.
a‒Latest Ordovician paleogeographic maps of the world (base map after Torsvik T and Cocks L, 2016). a‒ paleogeographic map of the Yangtze Plate during the Late Ordovician (after Chen X et al., 2004; Liu ZH et al., 2017).
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Figure 2.
Stratigraphic profile of the Wufeng-Longmaxi succession exposed in the JDP outcrop showing lithology, sample location, and stratigraphic distributions of graptolites (after Zhao AK et. al., 2022).
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Figure 3.
Ternary diagram of the mineralogic composition of five major lithofacies from Wufeng to Longmaxi Formations in the study area (modified from Ma YQ et al., 2016).
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Figure 4.
Stratigraphic profile of the Wufeng-Longmaxi succession exposed in the JDP outcrop showing mineral content. The legend of the lithology is as same as Fig. 2.
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Figure 5.
Outcrop of Wufeng-Longmaxi Formation in the JDP. a‒abundant graptolite fossils; b‒calcareous nodules and bentonite intercalation in the shale; c‒bioclasts in Wufeng Formation; d‒macroscopic characteristics of Wufeng and Longmaxi Formations; e‒pyrite nodules in Wufeng Formation; f‒bio remains in the top of Wufeng Formation; g‒sampling location of siliceous shale.
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Figure 6.
Microscope and SEM images of Wufeng-Longmaxi Formation in the JDP. a‒crustaceans bioclasts were replaced by calcite filling; b‒micrite with micritic structure, calcite about 70%; c‒SEM strawberry pyrite particles; d‒dolomitic mudstone, dolomite d distribution in bands; e‒SEM dolomite particles; f‒bioclasts are mainly radiolarian, circular or elliptical distribution, replaced by carbonate components; g‒radiolarians and sponge spicule; h, i‒dolomite grain and radiolarians.
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Figure 7.
Stratigraphic profile of the Wufeng-Longmaxi succession exposed in the JDP outcrop of paleo-productivity (TOC and Mn data from Zhao AK et. al., 2022).
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Figure 8.
Stratigraphic profile of the Wufeng-Longmaxi succession exposed in the JDP outcrop showing terrigenous influx and redox environment (V/Cr and Al2O3 data from Zhao AK et. al., 2022).
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Figure 9.
Comprehensive profile of the Wufeng-Longmaxi succession exposed in the JDP outcrop showing total organic matter, and geochemical data of redox conditions, productivity, and detrital input.
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Figure 10.
Relationships of TOC to proxies of redox condition and detrital input.
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Figure 11.
Enrichment factors (EFs) of Mo vs. U. The lines exhibit Mo/U ratios equal to seawater (SW). The patterns of U-EF and Mo-EF are compared to the model of Algeo TJ and Tribovillard N et al., (2009).
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Figure 12.
Crossplots of various geochemical components (A-D) of siliceous content.
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Figure 13.
Relationships of TOC to proxies of paleo-productivity.
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Figure 14.
Diagrammatic sketch illustrating the probable environmental evolution of the Upper Yangtze Basin during the Late Ordovician and Early Silurian