Sedimentological and geochemical characteristics of organic-rich rocks in the Upper Permian in North Sichuan and West Hubei provinces

Key words: 
• Organic-rich rock /
• Sweet-spot interval /
• Wuchiaping Formation /
• Dalong Formation /
• Sequence stratigraphy /
• Elemental geochemistry /
• Sichuan Basin /
• West Hubei Basin

Abstract: [Objective] As the development of non-conventional petroleum theories, the organic matter accumulation mechanism has been becoming the hotspot. One of the most debated issues is the main controlling factors of organic matter enrichment. Previous studies were lack of the depositional process analyses, which is likely the reasons of the debate. [Methods] This study combines the sedimentological and geochemical methods to conduct detailed sequence stratigraphy, facies and elemental analyses, aiming to address the sedimentary organic matter accumulation mechanism. [Results] The Upper Permian Wuchiaping and Dalong formations in the northern Sichuan Basin and the West Hubei Basin are subdivided into First Wuchiaping Member, Second Wuchiaping Member, Third Wuchiaping Member, First Dalong Member and Second Dalong Member in an ascending order. The Upper Permian is subdivided into 5 third-order stratigraphy sequences SQ1 to SQ5. Based on the sequence stratigraphy analysis, the development history of the rifting basin in northern Sichuan Basin and the West Hubei Basin is subdivided into four stages: initial stage, rapid rifting, rifting climax, and shrinking stage. The initial stage mainly occurred during the deposition of SQ2, and consists of dark cherty limestones of slope facies. The rapid rifting stage mainly occurred during the SQ3, and consists of calcareous shales of deepwater shelf facies. The rifting climax stage mainly occurred during the SQ4, and consists of black bedded cherts of basin facies. The shrinking stage mainly occurred during the SQ5, and consists of medium-thin-bedded limestones. The sweet-spot interval develops mainly from the highstand system tract (HST) of SQ3 to the transgressive system tract (TST) of SQ5, during which the nutrient elements Fe, Cu, Ni, Zn, and redox-sensitive element Mo, V, Fe/Al and S were highly enriched. This sweet-spot interval is also associated with highly active hydrothermal activity indicated by Al/(Al+Fe+Mn) ratios < 0.6 and the Al-Fe-Mn diagram. [Conclusion] The development of high primary productivity is earlier than the strong reducing conditions during the deposition of the sweet-spot interval, suggesting that the reducing conditions in the bottom water resulted from the high consuming of oxygen during the organic matter decomposition， and that the organic matter enrichment was mainly controlled by primary productivity. The nutrient material input was related to the hydrothermal activity and volcanism during the formation of rifting basins. The tectonic activity brought episodically a lot of nutrient elements to the rifting sea, enhancing the primary productivity level. Thus, the primary productivity level is the onset and basic conditions for the extraordinarily high organic matter accumulation in the sweet-spot interval, and the strong reducing conditions in bottom water was the key factor for organic matter preservation and burial.