Siliceous Fossils From The Wufeng And Lungmachi Formations In The Hunan-Hubei-Chongqing Region,South China And Their Significance For Evaluation Of Shale Gas

The black shale of the Wufeng and Lungmachi formations of the Ordovician-Silurian transition has provided one of the largest shale gas layers in South China for nearly a decade.Graptolite biostratigraphy of the black shale has resulted in the recognition of 13 precisely defined biozones,which have been widely used by petroleum industries as a standard subdivision.However,except brachiopods from the specific narrow Kuanyinqiao Bed,few fossil have been well studied from this time interval.To enhancce the range of biostratigraphic tools available,the paleontological potential of co-occurring microfossils need to be further explored—particularly of siliceous organisms from the black shale and chert.In this paper,133 samples and 468 thin sections from the Wufeng and Lungmachi formations from Qiliao section(Shizhu County,Chongqing Municipality),Daheba section(Yongshun County,Hunan Province)and Yihuang-1 drillcore(Yichang City,Hubei Province)were collected and processed for paleontological and geochemical studies.Siliceous fossils contain rich assemblages of radiolarians,sponge spicules and some unknown fossils.A systematic paleontological and taphonomic investigation of the radiolarians from the two formations has identified six preservation types,categorized as body fossil(?),as well as mold and cast fossil(?,?2,?3,?4 and ?5).A taphonomic study has also been conducted on the sponges.Elemental analysis demonstrates that they are preserved as isolated pyritized spicules,without remains of soft tissues,while the taphonomic analysis distinguishes four stages of their preservation:(1)burial stage,(2)mortality and decay stage,(3)bacterial sulfate reduction stage,and(4)pyritization stage.In addition,some cone-shaped siliceous fossils are obtained from the chert in the top of the Wufeng Formation in well Yihuang-1.To identify the affinity of these unknown siliceous fossils,Focused Ion Beam Scanning Electron Microscopy(FIB-SEM was used.The results show they are infilled or recrystallized as silica,but without any obvious shell wall preservation or spicule-like microstructure,making it hard to determine whether they are radiolarians.These conical fossils are found only in the upper part of the Wufeng Formation to the lowermost part of the Lungmachi Formation,and can therefore be used as an alternative fossil to determine the approximate age of the black shale in drill cores where graptolites are rare or unavailable.In this thesis,an elemental geochemical analysis is conducted on shale samples from the Wufeng Formation and Lungmachi Formation to investigate the depositional environment.The results of major and trace element analysis show that the silica in the black shales of Wufeng and Lungmachi formations in the Qiliao and Daheba sections is mainly of biogenic rather than hydrothermal origin.Moreover,the apperance of bentonite horizons,radiolarian-rich layers and the beds with TOC>2%are consistent with elevated primary productivity as reflected by proxies like Baxs and P/Al from the black shale.Overall,we propose that the origin of silica enrichment in the black shale is as follows:the deposition of volcanic ash introduced abundant nutrients into the ocean,and promoted primary marine productivity.And as a result,radiolarians,siliceous sponges and other siliceous microfossils flourished,providing as the fundamental source of the biogenic silicon that accumulated in the black shale.