Authigenic embrittlement of marine shale in the process of diagenesis

Abstract

Studies on the origin of shale brittleness are of great significance to understanding shale gas accumulation laws. The current studies, however, mostly focus on the improvement of shale brittleness by biological quartz enrichment, but rarely on the recrystallization of quartz in the process of diagenetic evolution. In this paper, a series of researches were carried out on the Lower Silurian Longmaxi Fm marine shale in the Changning block of the Sichuan Basin. Firstly, shale pore, mineral component and bedding characteristics were observed by means of cathodoluminescence (CL), scanning electron microscope (SEM) and so on. Then, the control effects of quartz origin, overpressure environment and diagenetic evolution on shale brittleness were investigated by analyzing the mineral composition and siliceous composition of shale microscopically. Finally, the authigenic embrittlement model of marine shale was put forward and the geological significance of authigenic microcrystalline quartz to the improvement of shale brittleness in the process of diagenesis was illustrated. The following results were obtained. First, during the sedimentation, burial and diagenetic evolution, shale experiences diagenetic changes which help increase quartz content and form concealed fracture networks, thus benefiting the formation of complex flow pathways in later stimulation. Second, organic matters play a positive role in improving shale brittleness in the process of geological evolution. It promotes the formation of authigenic microcrystalline quartz and improves the brittleness of shale. In conclusion, due to the authigenic embrittlement of shale in the process of diagenesis, rock structure is changed, rock strength is enhanced and shale porosity is preserved. Consequently, reservoir space in shale is improved and the enrichment state of shale gas is modified. Therefore, the preservation condition under the control of tectonic movement factors is vital to geological evaluation on shale gas. In addition, marine shale is certainly well brittle after authigenic embrittlement in the process of diagenesis as long as its organic matter content is abundant. Keywords: Marine shale, Quartz, Recrystallization, Authigenic embrittlement, Concealed fracture network, Brittleness improvement, Sichuan Basin, Changning block, Early Silurian, Longmaxi Fm