Sedimentation And Sequence Development Of Zhangjiatan Shale

Mud shale is one of the most common and widespread rock types on Earth.However,the study of mudstone sedimentology has been lagging behind compared with other common sedimentary rock types.In recent years,it has been found that the origin of mud shale can be the product of multiple sedimentation processes,and the morphology of its final sediments mainly depends on the interaction of physical,biological and chemical actions in the sedimentation process.Based on the new progress and new methods of shale sedimentology in foreign countries,the terrestrial mudstone sedimentology research in the Yanchang Formation in the southeastern Ordos Basin is carried out to meet the requirements of shale gas exploration.Yanhe section,as a normal stratotype section,is characterized by continuous outgrowth of Chang 7 member with clear top-to-bottom boundaries.The shale of Chang 7 member is a typical deep lacustrine shale with complex genesis.Based on the study of Chang 7 shale sedimentology,the detailed work flow of continental mudstone sedimentology anatomy that meets the requirements of shale gas exploration is summarized,that is,different types of mudstone lithofacies can be classified in gross scale by combining the hierarchical framework classification method with the mudstone microfacies.The microstructure and microfabric characteristics of different types of mudstones were revealed by further dissection,section identification,scanning electron microscope and energy spectrum scanning.It is determined that there are eight types of shale mainly developed in Chang 7member of the study area,which are black hillock cross-bedded siliceous mudstone,black len-intercalated silty laminated siliceous mudstone,gray massive clayey mudstone,thin layer tuffaceous mudstone,gray massive silty mudstone,black massive clayey mudstone,black shale and hybrid mudstone.The causes of the formation are storm deposit,ultra-dense flow deposit,suspended deposit and biodisturbance,suspended deposit,turbidity current deposit,collapse and sandstone puncture.The top and bottom boundaries of Chang 7 in Yanhe section are clear.The Chang 8 section is mainly fluvial facies,which is adjacent to Chang 7 with the maximum flood surface as the boundary.The Chang 7 section is mainly delta facies.In the eastern part of the basin,the upper part of Chang 7 suffered extensive erosion,and the lower part of Chang 6developed waterway erosion surface on the outcrop.The middle-upper Triassic boundary represented by the boundaries of Chang 7 and Chang 6 oil formations is a third-order sequence interface with several meters to tens of meters downward and continuously traceable laterally upward.The Chang 7 deposition represents a complete sequence evolution process,namely,the transgressive system tract at the bottom,the high water level system tract at the middle and lower part,and the low water level system tract at the middle and upper part.