Sedimentological and Geochemical Characteristics of Late Permian Organic-Rich Rocks in North Sichuan and West Hubei Provinces
[Objective]The current focus of non-conventional petroleum theories is the accumulation mechanism of organic matter.One of the most commonly debated issues regards the main influences on organic matter enrichment previous studies lack analysis of the depositional process,which is the likely reason for the debate.[Methods]This study combines sedimentological and geochemical methods to conduct detailed sequence stratigraphy,and facies and elemental analyses,aiming to address the accumulation mechanism of sedimentary organic matter.[Results]The Late Permian Wuchiaping Formation and Dalong Formation in the northern Sichuan Basin and the West Hubei Basin are subdivided in ascending order into the First,Second and Third members of the Wuchiaping Formation,and the First and Second members of the Dalong Formation.The Late Permian is subdivided into five third-order stratigraphy sequences,SQ1 to SQ5.Analysis of the sequence stratigraphy revealed the four-stage developmental history of the rift basin in the northern Sichuan Basin and the West Hubei Basin:the initial stage followed by rapid rifting,rifting cli-max and a shrinking stage.The initial stage mainly coincided with the deposition of SQ2,and consists of dark cherty limestones of slope facies.The rapid rifting stage mainly occurred during SQ3,and comprises calcareous shales of deepwater shelf facies.The rifting climax stage mainly took place during SQ4,consisting of black bedded cherts of ba-sin facies.The shrinking stage mainly occurred during SQ5,and consists of medium-or thin-bedded limestones.The sweet-spot interval developed mainly between the highstand system tract(HST)of SQ3 and the transgressive system tract(TST)of SQ5,during which the nutrient elements Fe,Cu,Ni,Zn and redox-sensitive elements Mo,V,Fe/Al and S were highly enriched.This sweet-spot interval is also associated with highly active hydrothermal activity indicat-ed by Al/(Al+Fe+Mn)ratios<0.6 and the Al-Fe-Mn diagram.[Conclusions]The development of high primary pro-ductivity occurred prior to the strongly reducing conditions during the deposition of the sweet-spot interval.This sug-gests that the reducing conditions in the bottom water resulted from high consumption of oxygen during the decomposi-tion of organic matter,and that organic matter enrichment was mainly the result of primary productivity.The nutrient material input was related to hydrothermal activity and volcanism during the formation of the rifting basins.Episodic tectonic activity brought abundant nutrient elements to the rifting sea,enhancing the primary productivity level;the primary productivity level was the onset and basic condition for the extraordinarily high organic matter accumulation in the sweet-spot interval,and the strongly reducing conditions in the bottom water was the key factor for organic mat-ter preservation and burial.
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