Synergetic effects of matrix components and diagenetic processes on pore properties in the Lower Cambrian shale in Sichuan Basin, South China

Abstract Synergetic effects of matrix components and diagenetic processes on pore properties were investigated from macroscale to nanoscale. Methods were combined to explore shales selected from well J1 in South China, in terms of elements measurement, organic geochemistry analysis, X-ray diffraction, nuclear magnetic resonance and direct imaging observation (2D wide horizon and 3D reconstruction). Three lithofacies were identified and porosity/permeability decrease in the order: authigenic siliceous shale > detrital clastic-rich shale > carbonate-rich shale. Further, different porous properties among lithofacies can be detected via image stitching technology, to achieve compatibility of large vision and high resolution. In this way, pore parameters are intuitively linked to related matrixes, which facilitates the elaboration of internal relationship between properties of pores and diagenetic mechanisms of matrixes. Authigenic siliceous shale is characterized by small pore size distribution (30–60 nm), high SEM-based surface porosity (6.04%) and good connectivity rate (60.66%), which is attributed to authigenic quartzes providing pre-existing frameworks and abundant OM contributing pore volumes. Detrital clastic-rich shale is characterized by broad pore size distribution (20–600 nm), moderate SEM-based surface porosity (3.09%) and fair connectivity rate (27.84%), since abundant detrital grain and lean OM trigger the domination of inorganic-related pore which are isolated with large diameters and small amounts. Carbonate-rich shale is characterized by small pore size distribution (10–100 nm), low SEM-based surface porosity (0.77%) and poor connectivity rate (12.86%), due to strong compaction, intensive cementation and limited OM migration. Even with similar geological history, diverse pore types and porous properties can be detected among the three lithofacies, which may attribute to different matrix components and their diagenetic processes. At over-mature stage, brittle minerals (quartz and calcite/dolomite) control OM distribution and prevent secondary pores from compaction. Whereas, ductile matrixes (clay and OM) provide the dominant pore volumes. Especially, OM can facilitate connectivity with porous attribute and migrating ability.