Pyrite Origins,Sulfur Isotope,Sedimentary Environment Analysis Of Organic-rich Black Shale Of The Lower Cambrian In Yichang Area,Middle Yangtze

As a new type of unconventional oil and gas resources,shale gas occupies an important proportion in the field of oil and gas exploration and development in China.The Paleozoic marine shale layer in southern China has excellent conditions,large reservoir thickness,good organic matter type and high maturity.At present,shale gas from the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation has been exploited on a large scale.Compared with the Wufeng-Longmaxi Formation,the shale formation of the Lower Cambrian Shuijingtuo Formation is thicker and TOC cotent is higher,and the shale gas generation conditions should be more favorable.However,with the exception of industrial shale gas production breakthroughs in Yichang,Hubei and Weiyuan,Sichuan,no breakthroughs have been achieved in other regions.The main reasons remain unknown.In this paper,we selected more than 200 core samples in EYY1 Well in Yichang,Hubei Province,TOC content was measured,65 samples were selected for total sulfur content and sulfur isotope measurement,and a several samples for SEM imaging analysis.Combined with the existing sulfur isotope profiles of Lower Cambrian profiles in other areas of the Yangtze platform,in-depth analysis of the depositional environment of the black shale and the origins of pyrite in the Lower Cambrian Shuijingtuo Formation are investigated,and the key findings are as follow:?1?The forms and particle sizes of pyrite formed in different deposition environments are distinct.The pyrite formed in sulfide-anoxic environment are all strawberry-like pyrite monomers or aggregates,and the particle size is less than 5?m and the distribution range is narrow?in the lower and middle black shale of the Shuijingtuo Formation?;while the pyrite formed in the pore water in the oxidizing environment has a larger particle size,basically ranging from 6?m to10?m,and the distribution range is wider?such as the Yanjiahe Formation,Upper part of Shuijingtuo Formation?.?2?During the deposition of the Lower Cambrian Shuijingtuo Formation,the sulfur isotope values of the black shale had a positive deviation and a negative deviation,and the?³⁴S value changed from small to large and then to small.The depositional environment of organic-rich shale at the bottom was oxygen-deficient environment.Sulfate bacteria in seawater have a strong reduction effect,and sulfur isotope fractionation between sulfate and pyrite is small.In contrast,the sedimentary environment in the upper part of the Shuijingtuo Formation evolves into an oxidizing environment,and sulfate bacteria reduction occurs in the pore water below the seawater-sediment interface.There is an input of sulfate radicals rich in light sulfur isotopes,which causes a negative drift of the sulfur isotope composition of pyrite.The information of the redox recovery of the depositional environment and the sulfur isotope inferred by the pyrite morphology and particle size in the black shale are consistent with each other.?3?The difference in sulfur isotope values of different sedimentary facies may be mainly due to the different supply reservoirs of sulfate,resulting in different initial sulfur sulfate isotope compositions.As a result,the pyrites formed by the reduction of sulfate bacteria have distinct sulfur isotope compositions.?4?The sulfur isotope changes in the organic-rich black shale of the Cambrian Shuijingtuo Formation may correspond to the changes in the oxygen content of the atmosphere-ocean system during its deposition period,which may be closely related to geological events such as sea level rise,tectonic activity,and volcanic activity.