Characterization and Spatial Distribution of Sand Group Architecture and Channel Types in Tight Gas Reservoirs: A Case Study From the Jurassic Shaximiao Formation of the Jinqiu Gas Field in the Central Sichuan Basin of China

Abstract

There is an abundance of tight gas resources in narrow channel sand-bodies from the Jurassic Shaximiao Formation of the Jinqiu gas field in the central Sichuan Basin of China. The architecture of sand group in the study area is undefined, and the spatial distribution of channel sand-bodies is unclear. The complex and inhomogeneous sandstones have a significant impact on the reservoir’s physical properties and the fluid mobility of the reservoir. In this study, data from drilling cores, logs, seismic, and experiment testing were used to investigate the spatial distribution of sand group architecture and the channel types. There are five channel genetic types, including the multiphase superimposed type, deeply incised type, abandoned type, progradational superimposed type, and normal single genetic type. Based on the channel genetic types, the ratio of sandstone and mudstone, the ratio of width to depth, the connectivity ratio of sand-bodies, and the production dynamic characteristics, the channel sand-body connectivity is defined into three types. The connected sand-bodies occur in the multiphase superimposed and deeply incised types of channels, with an average connectivity ratio of 83%, a ratio of sandstone and mudstone larger than 0.9, and a ratio of width and depth larger than 40. Based on the association of sandstone and mudstone and rhythmic structure, the sand group architecture can be divided into three types, including (A) uniform-grain-sequence pure sandstone architecture, (B) positive-grain-sequence thick sandstone and thin mudstone architecture, and (C) positive-grain-sequence thick mudstone and thin sandstone architecture. There is a high content of natural gas in Types A and B of sandstones, with a daily gas production of 29.16 × 10⁴–47.6 × 10⁴ m³/day and pressure coefficients of 0.72–1.08. The sand group architecture of the study area is mainly controlled by the channel sinuosity and the ratio of accommodation and sediment supply, and Types A and B of sand group architectures occur with large channel sinuosity of 1.14–1.36 and a large ratio of accommodation and sediment supply of 0.61–2.92. Based on the connectivity degree of channel sand-bodies, the sand group architectures, and production data, the channels of the study area can be divided into three types. Type I channels mainly occur in Sand Groups 6, 8, and 9, and Type II and Type III channels occur in Sand Groups 6 and 7 in the western and southern parts of the study area. The technology of fine characterization for channel sand-bodies on the basis of human–computer interaction and seismic attributes is proposed, and geological modelling of the spatial distribution of sand group architectures and channel types is established. The research results achieve a theoretical breakthrough in the characterization of the sand-body structure of tight sandstone reservoirs in narrow river channels and assist in the efficient exploration and development of tight sandstone gas.