Types and genesis of sweet spots in the tight sandstone gas reservoirs: Insights from the Xujiahe Formation, northern Sichuan Basin, China

Abstract

Through comprehensively applying geological and geophysical data, as well as core and thin section observation, the characteristics of reservoirs and fractures in the second member of the Xujiahe Formation (hereinafter referred to as Xu2 Member) in the Yuanba area, northern Sichuan Basin, were studied. Combined with the analysis of the main controlling factors of production capacity, the types and characteristics of the sweet spots in the tight sandstone gas reservoir were determined. The evaluation standards and geological models of the sweet spots were established. The results are as follows: (1) There are bedding-parallel fracture-, fault-induced fracture-, and pore-dominated sweet spots in the tight sandstone gas reservoirs of the Xu2 Member. (2) The bedding parallel fracture-dominated sweet spots have developed in quartz sandstones with well-developed horizontal fractures and micro-fractures. They are characterized by high permeability and high gas output during production tests. This kind of sweet spots is thin and shows a limited distribution. Their logging responses show extremely low gamma-ray (GR) values and medium-high AC values. Moreover, the bedding parallel fracture-dominated sweet spots can be mapped using seismic methods. (3) The fault-induced fracture-dominated sweet spots have well-developed medium- and high-angle shear fractures. Their logging responses show an increase in peaks of AC values and total hydrocarbon content and a decrease in resistivity. Seismically, the areas with well-developed fault-induced fracture-dominated sweet spots can be effectively mapped using the properties such as seismic entropy and maximum likelihood. (4) The pore-dominated sweet spots are developed in medium-grained feldspathic litharenites with good reservoir properties. They are thick and widely distributed. (5) These three types of sweet spots are mainly determined by sedimentation, diagenesis, and tectonism. The bedding parallel fracture-dominated sweet spots are distributed in beach-bar quartz sandstones on the top of the 1st sand layer group in the Xu2 Member, which develops in a shore-shallow lake environment. The fault-induced fracture-dominated sweet spots mainly occur near faults. They are increasingly developed in areas closer to faults. The pore-dominated sweet spots are primarily distributed in the 2nd and 3rd sand layer groups, which lie in the development areas of distributary channels near provenances at western Yuanba area. Based on the geological and seismic data, a comprehensive evaluation standard for these three types of sweet spots of the tight sandstone reservoirs in the Xu2 Member has been established, which, on the one hand, lays the foundation for the development and evaluation of the gas reservoir, and on the other hand, deepens the understanding of sweet spot in the tight sandstone gas reservoirs.