Sedimentary Characteristics and Environment of the Middle Ordovician Black Fine-Grained Rock Series in Central Hunan and Its Surrounding Areas
[Objective]The Middle Ordovician Yanxi Formation is a recently identified set of marine organic rich black fine-grained rock series in central Hunan and its surrounding areas,with good shale gas exploration potential. The research on the sedimentary characteristics of this black fine-grained rock series urgently needs to be strength-ened.[Methods]The lithology and lithofacies combination types of the Yanxi Formation were identified through field exploration and slice identification,and the sedimentary environment was determined through element testing. Basin analysis technology,combined with regional tectonic movement history,was used to identify the nature and morphol-ogy of the prototype basin,determining favorable sedimentary facies zones,spatiotemporal distribution,and sedi-mentary patterns of the black fine-grained rock series.[Results]The lithology of the Yanxi Formation is primarily sili-ceous rock and carbonaceous shale,followed by silty shale and siltstone. Siliceous rocks are mainly biogenic in shal-low marine environments,and carbonaceous shale is formed in a quiet and anoxic deep water detention environment. Four types of lithofacies combinations can be identified within the Yanxi Formation:thick pure carbon shale,carbon shale and siliceous rock combination,silty shale and siliceous rock combination,and silty fine sandstone with thin layer shale combination. Based on different rock facies combinations,the Yanxi Formation can be vertically divided into three sections:upper,middle,and lower;the distribution thickness of the three sections in the region is relative-ly stable and has good comparability. The thick layer of carbonaceous shale in the middle section is the most impor-tant source rock series. The Yanxi Formation is characterized by a thick southeast and thin northwest distribution in the study area,and the types of sedimentary facies include deep water basins,deep water continental shelves,shal-low water continental shelves,and turbidite fans. The distribution positions of each sedimentary facies zone varied in different periods of the Ordovician,with clear zoning and continuous changes.[Conclusions]The central and sur-rounding areas of Hunan were in a stagnant hypoxic water environment during the Middle Ordovician and were a sta-ble depression basin within the block. The deep-water continental shelf facies and deep-water basin facies are favor-able facies belts for the development of organic shale in the Yanxi Formation.
central HunanYanxi Formationblack fine-grained rock seriessedimentary characteristicssedimentary environmentprototype basin
万方数据
维普
