Sedimentary Environment Heterogeneity and Their Controlling Factors of the Wufeng–Longmaxi Formation in the Weiyuan Area, Sichuan Basin

The Wufeng and Longmaxi formations across the Ordovician–Silurian transition in the Weiyuan area of the Sichuan Basin are important marine shale gas source rocks. The complex paleotopography of this area, characterized by a paleogeographic pattern of “subaqueous high – subaqueous slope – subaqueous plain – subaqueous sag”, results in pronounced sedimentary environment heterogeneity in these two formations. Although previous studies have analyzed the sedimentary environment of this area, insufficient temporal resolution and a lack of comparative studies have made it impossible to clearly reveal the vertical and lateral variations of sedimentary environment indicators and their main controlling factors. In this study, biostratigraphic investigations and major- and trace-element analyses were conducted on wells W-1, W-2, and W-3, and a comparative analysis was performed together with previously published data from well W-4. Among these, wells W-2 and W-3 were located on the subaqueous slope, well W-1 on the subaqueous plain, and well W-4 in the subaqueous sag. The results show that: (1) Near the paleo-uplift area, strata corresponding to the Metabolograptus extraordinarius–Cystograptus vesiculosus biozone are absent in well W-2, and well W-3 lacks the Paraorthograptus pacificus–C. vesiculosus biozone; in contrast, wells W-1 and W-4 exhibit continuous deposition of the Wufeng Formation and the lower part of the Longmaxi Formation, extending up to the Spirograptus guerichi biozone. (2) During the deposition of the Wufeng Formation, the Weiyuan area was characterized by suboxic condition and low paleoproductivity in the Dicellograptus complexus–M. extraordinarius biozones. (3) The sedimentary environment shows significant heterogeneity during the deposition of the Longmaxi Formation: well W-4 in the subaqueous sag exhibits euxinic and stagnant water conditions with high paleoproductivity during the M. persculptus–Parakidograptus acuminatusinterval, and anoxic conditions with low paleoproductivity during the C. vesiculosus–Coronograptus cyphusinterval; well W-1 on the subaqueous plain maintains euxinic-anoxic conditions with high paleoproductivity up to the C. cyphus biozone, and then shows a transition from anoxic to suboxic conditions with medium-to-low paleoproductivity during the C. cyphus–S. guerichi interval; wells W-2 and W-3 on the subaqueous slope exhibit anoxic conditions with medium-to-high paleoproductivity during the C. cyphus–Stimulograptus sedgwickii interval, and then shift to suboxic conditions with medium-to-low paleoproductivity during the S. guerichi biozone. (4) This marked heterogeneity in sedimentary environment during the deposition of the Longmaxi Formation was mainly controlled by regional paleotopography and sea-level changes.