Brittleness evaluation of the Lower Silurian marine shale reservoirs: A case study of Longmaxi shale in Fenggang block, southern China

The brittleness of shale determines the fracturability of the shale reservoir and has a great influence on the exploration and development of shale gas. Therefore, prediction of brittleness and evaluation of fracturability of shale are very important in finding favorable areas for shale gas. We have used the mineral composition, rock-mechanics experiment, logging evaluation, 2D seismic data inversion, and fracture development degree to evaluate and analyze the vertical and plane brittleness characteristics and the main controlling factors of Longmaxi shale. In addition, we established the fracturability index of shale based on the brittleness index, critical strain energy release rate, and fracture toughness. The results indicate that the brittle mineral content of Longmaxi shale in the Fenggang block is between 69% and 90%, and shale samples are prone to brittle deformation by microscopic observation. Brittle deformation has a positive effect on the porosity and percolation ability of shale; the fractures in the upper and lower parts of the Longmaxi shale are relatively developed, and the degree of core fracture development is consistent with rock brittleness. The brittleness distribution of 2D seismic data inversion is in good agreement with the brittleness predicted by well logging; shale reservoirs with low fracture toughness, low critical strain energy release rate, and high brittleness index have high fracturability. Therefore, the research on shale brittleness and fracturability of Lower Silurian Longmaxi shale is expected to have important guiding significance for shale-gas exploration and development in southern China.