Fracture development characteristics and controlling factors for reservoirs in the Lower Silurian Longmaxi Formation marine shale of the Sangzhi block, Hunan Province, China

Abstract

The Lower Paleozoic Silurian Longmaxi Formation shale in the Sichuan Basin of southern China has achieved commercial exploitation in recent years, and has become a rapid growth point in China's oil and gas reserves. However, the Longmaxi Formation on the peripheral margin of the Sichuan Basin has not made substantial progress. Fractures, that as fluid reservoirs and migration channels, play an important role in shale gas exploration and development. Based on data from field outcrop observations, drilling cores, well logging, geochemical tests, microscopic thin section observations, and focused-ion-beam scanning electron microscope (FIB-SEM) combined with energy dispersive spectrometer (EDS) analysis and cathodoluminescence microscope (CLM) technology, the development characteristics of fractures in the Longmaxi Formation SY-5 well in the Sangzhi block of Sangzhi-Shimen synclinorium, NW Hunan Province, are described systematically, and their important roles, primary dominant factors and effectiveness in the shale gas reservoir are discussed. The results showed that tectonic factors are the dominant factors of shale fracture development, followed by nonstructural factors. The fractures were mainly ductile shear fractures, with short extension distances and small apertures, and they had at least two openings in the burial history. The statistical fracture density of drilling cores was no obvious correlated with the contents of total organic carbon (TOC) (0.18%–2.49%) and clay minerals (13.0%–34.0%), and the same with brittle quartz (32%–72%), which indicates that they have controlled the development of fractures together. These results could be used for estimating effective permeability, fracture distribution, numerical simulation, well deployment, hydraulic fracturing and reserves calculation in the processes of exploration and development.