Abstract: The study analyzed the shale fracture characteristics,controlling factors and differences of fractures developed in marine and continental shales in the second Member of the Lower Cambrian Bianmachong Formation(C₁b²) based on fracture observation and experimental data.The results show that the C₁b² shales show certain features of both marine and continental shales.Its fractures are dominated by fractures at low angle with bedding,interlayer and slip fractures,which are similar to those in continental shales.The fractures in sandstone-shale alternating intervals and sandstone intervals are highly developed,with the high dip-angle structural fractures being dominant.The controlling factors of fracture development are mainly represented as follows:lithology and rock mechanical property control structural fracture development; stratum thickness controls the density and scale of fractures; TOC influences the development of interlayer overpressure fractures; and reservoir heterogeneity controls the development of bedding-parallel fractures.In addition,the TOC,quartz content,brittleness and fracture have different coupling relationships in marine and continental shales.Although the greater brittleness and single layer thickness of marine shale are favorable for reservoir stimulation,cross-layer fractures may form easily under strong deformation,thus are unfavorable for hydrocarbon preservation.However,continental shales have lower single layer thickness,stronger ductility and heterogeneity.They can effectively prevent the development of cross-layer fractures,thus can facilitate the preservation of shale gas.In general,the different lithologic combinations and their corresponding characteristics and controlling factors of fracture development in the C₁b² are of referential value in the aspects of gas preservation,enrichment and reservoir stimulation in marine and continental shale gas.

Key words: heterogeneity, marine shale, continental shale, fracture, shale, Upper Yangtze region