Characteristics and formation mechanism of natural fractures in the tight gas sandstones of Jiulongshan gas field, China

Abstract

Natural fractures control the migration and accumulation of oil and gas in the tight sandstones. Understanding the characteristics and formation mechanism of natural fractures has important guiding significance to the comprehensive prediction and evaluation of the fracture distribution. Focusing on the tight sandstones of the Xujiahe Formation (T3x) of Jiulongshan gas field, northwest Sichuan Basin, China, the characteristics of natural fractures were characterized quantificationally, and a comprehensive evaluation method of fracture formation mechanism is proposed. Two types of natural fractures were identified in the T3x, namely tectonic fractures and diagenetic fractures. Most of the natural fractures are tectonic shear fractures, which can be subdivided into steep fractures and nearly horizontal fractures according to their dip angles. Three sets of tectonic fractures were identified in the study area, namely: NW-SE trending, NNE-SSW trending and NEE-SWW trending fractures. According to the characteristics of fracture sets, crosscutting relationships, acoustic emission tests and fluid inclusion analysis, and combined with the tectonic evolution history of the study area, the natural fractures were formed in four periods. Among them, the diagenetic fractures formed at the process of diagenesis are the first period fractures. The second to fourth periods are tectonic fractures formed at the end of the Triassic to the Early Jurassic, the Late Cretaceous and the end of Pliocene to the early of Pleistocene, respectively. Horizontal tectonic compression, uplift denudation and overpressure were the main force sources for the formation of the steep tectonic fractures. The formation of nearly horizontal fractures is related with the thrusting or inter-formational sliding caused by tectonic compression.