Enrichment Mechanism Of Organic Matter And Geochemical Characteristics Of Fe-S-C In Wufeng Formation-Longmaxi Formation In Northeast Chongqing

The Wufeng Formation and Longmaxi Formation of Late Ordovician-Early Silurian in Northeast Chongqing are mainly black shales,which have wide distribution area,deep burial,large thickness,high abundance and maturity of organic matter,and can become source rocks.Therefore,Longmaxi Formation and Wufeng Formation have good geological conditions for shale gas accumulation,and are hot areas for shale gas exploration and development in China.In this paper,the organic carbon and elemental geochemistry of organic-rich shale from Wufeng Formation to Longmaxi Formation in northeastern Chongqing are tested by WC1 well in Chengkou area.The characteristics and geochemical characteristics of organic carbon in Wufeng Formation and Longmaxi Formation are mainly discussed.The sedimentary evolution environment,enrichment pattern of organic matter and Fe-S-C geochemical characteristics are further analyzed.The following conclusions are drawn:?1?Organic matter characteristics and sedimentary evolution environment in the study area:organic carbon content in Wufeng Formation is low,reached the highest in the early stage of Longmaxi Formation,and then decreased in the middle and late stages of Longmaxi Formation;organic carbon content and redox state have the same vertical change.When organic matter was low,sedimentary environment was oxidation-oxygen-poor environment.;When organic matter was high,sedimentary environment was anoxic.The redox index was positively correlated with the correlation diagram of organic carbon content,which further proved that the redox condition was the main controlling factor of organic matter enrichment.During the sedimentary period of Wufeng Formation and Longmaxi Formation,the paleoproductivity of Longmaxi Formation is higher than that of Wufeng Formation.?2?The mechanism of organic matter enrichment in the study area:in the early and middle stages of the Wufeng Formation,the rising ocean currents in the ocean transported a large amount of oxygen to the ocean,resulting in the oxidation-oxygen-poor environment in the early stage of the Wufeng Formation,and the content of organic matter was low.In the late Wufeng Formation,although the sea level declined at the beginning of the Hernandt Glaciation,a semi-limited basin was formed due to tectonic uplift,and its reducibility was enhanced,which turned into an anoxic environment with an increase in organic carbon content.In the early stage of Longmaxi,the glacial period ended,the sea level rose,the water body became deeper and hypoxic,and the increase of organic matter consumed oxygen in the water.The two cycles resulted in the lowest oxygen content and the highest organic matter content in the early stage of Longmaxi Formation.In the middle and late period of Longmaxi Formation,because of the tectonic uplift,the sea level decreased,and affected by the terrigenous clastic supply,the ocean oxygen content increased and the organic matter content decreased.?3?Geochemical characteristics of Fe-S-C in the study area:in the early stage of Wufeng Formation,the lower value of delta ³⁴S was due to the continuous supply of SO4^2-to pore water by overlying seawater;the lower content of Fe_py in Wufeng Formation was due to the main limitation of organic matter.In the early stage of Longmaxi Formation,sulphide water was formed,and the exchange between overlying seawater and pore water was stagnant,with a high value of delta ³⁴S.In the early stage of Longmaxi Formation,organic matter was abundant,and the lack of active iron limited the content of Fe_py.In the middle and late period of Longmaxi Formation,the overlying seawater can exchange with pore water,and the value of delta ³⁴S was low.During this period,active iron was abundant and the content of organic matter was low.The dissimilation reduction of iron limited the content of Fe_py.