Sedimentary characteristics and environment of the Middle Ordovician black fine-grained rock series in the central Hunan and its surrounding areas

Key words: 
• Central Hunan region /
• Yanxi Formation /
• Black fine-grained rock series /
• Sedimentary characteristics /
• Sedimentary environment /
• Prototype basin

Abstract: The Yanxi Formation is a set of marine organic rich black fine-grained rock series newly identified in recent years in central Hunan, which has good shale gas exploration potential, the research on the sedimentary characteristics and evolution of this black fine-grained rock series urgently needs to be strengthened. The lithology of Yanxi Formation is mainly siliceous rock and carbonaceous shale, followed by silty shale, siltstone; Its lithofacies assemblage includes: thick pure carbonaceous shale, interbedded carbonaceous shale and siliceous rock, and siliceous rock with thin layers of carbonaceous shale;The Yanxi Formation is generally characterized by a thick southeast and thin northwest distribution in the study area;Through field exploration, lithofacies combination analysis, thin section identification and elements testing analysis, four sedimentary facies types can be identified: deep water basins, deep water continental shelves, shallow water continental shelves, and turbidite fans;The Yanxi Formation is characterized by significant trichotomy vertically. The lower member is interbedded with siliceous rock and carbonaceous shale of unequal thickness. The middle black carbonaceous shale is the most important Source rock of the Yanxi Formation. The upper member is siliceous rock with thin carbonaceous shale. The three members are relatively stable in regional distribution thickness and have good contrast;The tectonic evolution history of the ancient land of South China show that in the early Paleozoic the prototype basin type of the Middle Ordovician in the central Hunan region is a rift type intracontinental sea basin; The analysis of major and rare earth elements indicates that the siliceous rocks in the central Hunan region are mostly biogenic, and the study area was in a passive continental margin environment during the Middle Ordovician. The analysis of trace elements indicate that the central Hunan region was in an anaerobic water retention environment during the Middle Ordovician;Large scale marine invasion occurred in the central Hunan region during the Early Middle Ordovician, the distribution area of deep-water basin facies reached its maximum, while the sedimentary range of carbonaceous shale and siliceous rock reached its maximum simultaneously; During late Ordovician sea level rapidly decreased, and most of the study area was dominated by deep-water continental shelf sedimentation; The prototype basin pattern of Hunan region during the Middle Ordovician is as follows: the northwestern Hunan region is the carbonate platform of the Upper Yangtze region, the central Hunan region is the Jiangnan slope zone (deep water continental shelf), the central southern Hunan is a deep water basin supporting black carbon shale sedimentation, and the southeastern Hunan is the turbidite fan facies area on the northern margin of the Huaxia ancient land.