Shale Facies Characteristics and Sedimentation of the Wufeng Formation-Longmaxi Formation in the Southern Sichuan Basin
[Objective and Methods]The black shales of the Upper Ordovician and Lower Silurian in the southern Sichuan Basin are the main target of shale gas exploration,having attracted extensive attention among domestic and foreign scholars. To provide the theoretical basis for shale exploration and development in the future,the lithofacies division,sedimentary environment,and depositional processes of shale are explored. Through core observation and analysis using optical and scanning electron microscopes,the lithofacies are divided based on mineral composition and content,sedimentary structural characteristics,and comprehensive consideration of biological process and dia-genesis. The paleoenvironment is analyzed by oxidation-reduction (U/Th,V/Cr,V/(V+Ni),Ni/Co),sea level change (Ce*),hydrographic restriction (Mo/TOC),paleo-climate (Sr/Cu),paleo-salinity (Sr/Ba),and paleo-productivity (Cu,biogenic Ba) indices. Depositional processes are identified by core and thin section observation,X-ray fluores-cence scanning,and mineral X-ray diffraction whole rock analysis.[Results]Six lithofacies are identified from the shales of the Wufeng Formation-Longmaxi Formation:biological siliceous,clayey,(felsic-calcareous) silty,calcare-ous silty,calcareous,and felsic silty shale. The shale sedimentary environment of the Wufeng Formation-Longmaxi Formation underwent five stages of bottom-up changes. The paleo-climate shifted from warm and humid at the bottom of the Wufeng Formation to dry and hot at the middle to dry and cold at the top,and then from dry and hot at the low-er part of the Longmaxi Formation to warm and humid at the middle and upper sections. In the lower parts of the Wufeng and Longmaxi formations,the sea level had risen owing to two large-scale transgressions. When the sea level is high,the extent of basin retention is weak,strong in reduction,low in salinity,and high in paleo-productivity. The glacier event at the top of Wufeng Formation caused the sea level to drop. When the sea level is low,the extent of basin retention is strong,weak in reduction,high in salinity,and low in paleo-productivity. The shale of the Wufeng Formation-Longmaxi Formation was mainly formed by suspended sedimentation under low energy conditions. Upwell-ing sedimentation developed in the middle Wufeng Formation and the lower Longmaxi Formation. A small amount of storm surge sedimentation developed at the top of the Wufeng Formation. In the upper Longmaxi Formation,gravity slumping,debris flows,and turbidity currents developed because of the shallowing of water and the increase of terrige-nous input. Therefore,the sedimentary model of the Wufeng Formation-Longmaxi Formation shale was established by integrating the lithofacies,depositional processes type,tectonic evolution,and sedimentary environment evolution.[Conclusions]The frequent changes of sedimentary environment and the diversity of depositional processes control the types and characteristics of lithofacies,further leading to significant differences in organic matter content,porosity,gas content,and other reservoir quality factors of different lithofacies. Further research on shale lithofacies and reser-voir quality is required to provide theoretical basis for shale gas exploration and development.
Wufeng Formation-Longmaxi Formationlithofacies characteristicspaleo-environmentdepositional processesdepositional model
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