Revealing the Natural Fracture System in the LongMaxi Shale Gas Reservoir, Sichuan Basin, China

Abstract

Abstract Shale gas reservoirs in the LongMaxi Formation, Sichuan Basin, are the biggest proven shale gas reservoirs in China. The natural fracture system plays an important role in the shale gas exploration and development. Both high-angle calcite-cemented fractures and low-angle bedding fractures were observed in the core and resistivity image logs but the relationships between these two kinds of fractures is still not clear. Identifying and characterizing the natural fracture system in the shale gas reservoir were the objective of this case study. The borehole resistivity image logs were acquired in several shale gas wells in southeast Sichuan Basin. Weobserved a spiky resistivity log response in some intervals of Longmaxi Formation. Image logs reveal that all these intervals have low-angle conductive features. Based on the crosscutting relationship of bedding and fractures, these features are conductive bedding fractures, which cut the high-angle resistive fractures in the formation. Both conductive and resistive bedding fractures are identified in the image logs and core data. The geochemical spectroscopy logs in this interval show low pyrite volume, which indicates the conductive bedding fractures are not filled by conductive minerals. It could be an effective pathway of fluid flow. The natural fractures can be divided into three types: low-angle conductive bedding fracture, low-angle resistive bedding fracture, and high-angle resistive fracture. The bedding fractures and high-angle resistive fractures are mainly in the brittle zone of the LongMaxi Formation. We observed that the high density of the conductive bedding fractures will appear spiky on the resistivity logs, which will cause high horizontal permeability. Multiwell correlation shows that there will be more conductive bedding fractures in areas where the shale gas reservoir was undergoing tectonic stresses, which could have a negative impact on the shale gas enrichment. The bedding fractures were formed after the high-angle resistive fractures. Some bedding fractures were cemented by calcite, while some are still open. The identification of bedding fractures will help to understand the complex natural fracture system in the tight shale gas reservoir. The conductive bedding fractures will greatly enhance the horizontal permeability and will influence the fluid flow dramatically.