Natural fractures and their contribution to tight gas conglomerate reservoirs: A case study in the northwestern Sichuan Basin, China

Well-developed natural fractures in the tight gas conglomerate reservoirs of the northwestern Sichuan Basin, China, are proved to greatly contribute to the porosity and permeability of such reservoirs. Natural fractures can be storage space for hydrocarbons in the reservoir and also serve as fluid flow conduits. Outcrops, cores, and image logs were assessed in order to understand the natural gas production from the tight conglomerate reservoirs. Based on the type of interaction between fractures and clast grains, here we present three types of fractures in the conglomerate tight gas reservoirs: intergranular fractures, intragranular fractures, and grain-edge fractures. These fractures may be tectonic, diagenetic, or combined tectonic-diagenetic. Factors affecting natural fracture development in conglomerates include composition of grain and interstitial material, grain size, contact behavior of grains, and structural position. Well-developed open fractures make up over 70% of the porosity within the tight conglomerate reservoir, while the local permeability can be 3-5 orders higher than that of the matrix reservoir. Natural fractures can also significantly impact the natural gas productivity from individual wells. Production data suggests that higher natural fracture intensity correlate with higher natural gas productivity from wells. Compared with the intensity of intragranular fractures and grain-edge fractures, the high intensity of intergranular fractures shows a notably stronger correlation with high gas productivity. The impact of fractures on natural gas productivity varies greatly with fracture orientation. East-West fractures are predominant flow channels under the existing East-West stress field, contributing significantly to gas production. Northeast-Southwest-oriented fractures are of secondary importance.