Characteristics and development model of karst reservoirs in the fourth member of Sinian Dengying Formation in central Sichuan Basin, SW China

The reservoir space, types and distribution characteristics of karst carbonate gas reservoirs in the fourth member of Sinian Dengying Formation (Deng 4 Member) in central Sichuan Basin are analyzed based on the drilling, logging and seismic data. A development model of karst reservoirs is constructed to support the high-efficiency development of gas pools. The research shows that the reservoirs in Deng 4 Member have mainly small-scale karst vugs and fractures as storage space, and can be divided into three types, fracture-vug, pore-vug and pore types. The development patterns of the karst reservoirs are determined. On the plane, the karst layers increase from 65 m to 170 m in thickness from the karst platform to the karst slope, and the high-quality reservoirs increase from 25.0 m to 42.2 m in thickness; vertically, the reservoirs at the top of Deng 4 Member appear in multiple layers, and show along-bedding and along fracture dissolution characteristics. The reservoirs at the bottom are characterized by the dissolution parallel to the water level during the karstification period, and have 3–5 large-scale fracture-cave systems. Based on the reservoir development characteristics and the genetic mechanism, three types of reservoir development models of karst reservoir are established, i.e., bed-dissolved body, fracture- dissolved body and paleohorizon-dissolved body. The construction of karst reservoir development models and seismic response characteristics of the three types of reservoirs can provide parameter for well placement and trajectory design, and substantially improve productivity and development indices of individual wells and gas reservoirs. The designed production capacity of the gas reservoir has enhanced from the initial 3.6 billion to 6 billion cubic meters, making the profit of the reservoir development increase noticeably.