Carbonate burial dissolution related to faults and fractures in the Triassic Daye Formation of the Huandiqiao Section, Huangshi Area, Hubei, China

摘要

Carbonate burial dissolution is generally associated with faults and fractures, accompanied by formation of the fracture-vug system that can provide important reservoir space for oil and gas accumulation. Taking the Lower Triassic Daye Formation in the Huandiqiao Section, Huangshi Area, Middle Yangtze Paraplatform, China, as an example, this study elaborates genetic mechanisms of faults and fractures, features of burial dissolution and cementation, and properties of karst water, accompanied by establishment of a genetic model that illustrates the whole process of burial dissolution and cementation. There are a total of nine reverse faults with dominant northwest dips in the study section. Fractures are selectively developed in hanging walls of the nine faults, with tensile features and dominant north-northeast and north-northwest strikes. Dendritic dissolution and subsequent cementation is prevalent in the hanging wall, while they are rare in the footwall. Most of the cements in the fractures of the hanging walls are calcites, with a fluid inclusion homogenization temperature range of 70 similar to 120 degrees C, overgrowth bands at the pore edge with different luminous intensities, low contents of Na, Ba, K, and Sr, and high contents of Fe, Mn, and Mg. All these characteristics indicate the karst fluid to originate from calcium-rich formation water with hydrothermal genesis. The formation of the dendritic vug and fracture system is ascribed to the upward migration of hydrothermal fluids along the fault planes and fractures that result in selective dissolution. As the water circulation weakens, CaCO3 is gradually supersaturated in the distal zone with reference to the fault as well as the stagnant and weak-circulation zone, eventually forming a complex vug and fracture system.