Structural-Sedimentary Filling Processes and Its Resource Implications of Ediacaran Intracraton Rift in the Middle-Upper Yangtze Transition Zone

Key words: 
• The Upper-Middle Yangtze Block /
• Sinian /
• Rift basin /
• Filling Processes

Abstract: Abstract: 【objective】The proposal of the Mianyang-Changning intracratonic theory directly led to the discovery of the Anyue super-giant oil and gas field on its eastern side, further affirming the controlling role of tectono-sedimentary differentiation on oil and gas resources. In the transitional zone of the Upper-Middle Yangtze Block, the Sinian period exhibited a structural-sedimentary differentiation pattern akin to that observed in the Mianyang-Changning trough in central Sichuan. The sedimentary characteristics and filling processes provide essential insights for hydrocarbon exploration and the assessment of sedimentary mineral resources in this region. 【Methods】The study focuses on macroscopic sedimentary structures, depositional sequences, and microfacies in thin sections from outcrops and drilling data. Combined with the analysis of relevant tectonic settings and geodynamic causes, a detailed analysis of the development characteristics of sedimentary systems is conducted. On this basis, the vertical evolution and lateral comparison of depositional facies across the rift are dissected to reveal the changes in sedimentary systems and the structural-sedimentary filling processes during the development and evolution of the Sinian period's intra-continental rift basins in the central Upper Yangtze transition zone. 【Results】Influenced by regional transtensional tectonic activity, the rifting activity led to the formation of half-graben-style fault basin structures, with significant sedimentary differentiation in the carbonate gravity flows along the gentle slope margins of the rift and the deep-water continental shelf under a regional shallow marine sedimentation backdrop. During the Doushan Period, a compensatory filling feature with overlapping centers of subsidence and sedimentation developed. In the Dengying Period, structural-sedimentary filling characteristics of thickened shallow-water carbonate sedimentation and under-compensated deep-water sedimentation were observed. The deep-water sedimentation at the subsidence center, which developed during the Doushan Period, possesses favorable hydrocarbon source rock conditions and potential for shale gas exploration; during the Dengying Period, the high-energy beach bars at the edge of the prograding platform on the eastern side of the Upper Yangtze, controlled by faults, and the high-energy beach bars at the edge of the prograding platform on the gentle slope of the Central Yangtze are high-quality facies belts for oil and gas reservoir development, and they form near-source hydrocarbon reservoir configurations with the hydrocarbon source rocks within the rift basin. The synsedimentary faults controlling the basin provide pathways for the upwelling of phosphorus-rich hydrothermal fluids. Concurrently, the western fault and eastern inclined fault basin structure provide favorable sedimentary topographic conditions for the deep-sea phosphorus-rich upwelling currents to enter the shallow water sedimentation area of western Hubei from the deep-water area of the fault basin. 【Conclusion】The Sinian period's tectono-sedimentary filling process is under the control of depositional timing, fault basin activities, and the integrated action of the carbonate factory. The high-energy mound-shoal complexes at the platform margins, controlled by faults, along with the high-quality hydrocarbon source rocks within the rift basins, create favorable source-reservoir combinations, which is a new field deserving attention for exploration of hydrocarbon. Furthermore, the formation and filling processes of the basin create favorable tectonic-sedimentary environments for the source, migration, accumulation, and mineralization of sedimentary-type phosphorite deposits.