This paper is the result of oil and gas exploration engineering.

Objective The brittleness evaluation of shale reservoir is of great significance for determining the favorable stratigraphic zone in shale gas exploration and development.

Methods Based on the basic data of mineral composition, geophysical logging, rock mechanics experiment and fracture development characteristics of Lower Cambrian Niutitang Formation in Xuefeng region, the brittleness characteristics of Niutitang Formation shale are comprehensively analyzed and evaluated by mineral composition and rock mechanics parameter based on geophysical logging, and the influencing factors of shale brittleness are discussed.

Results The results indicate that the brittle mineral composition based on the contents of quartz, feldspar, pyrite and carbonate is the most suitable method to evaluate the brittleness of Niutitang Formation shale in this region. Taking Well XZD−1 in this area as an example, the brittleness index of Niutitang Formation shale is 59.2%−93.8%, with an average value of 74.4%, which is influenced by sedimentary environment, diagenetic evolution, burial depth, tectonism and other factors.

Conclusion The stable deep−water shelf basin facies sedimentary environment and late diagenetic evolution stage determine that the overall brittleness of Niutitang Formation shale in Xuefeng region is relatively high. In this sedimentary environment, biogenic brittle minerals such as siliceous and pyrite are more abundant. The content of brittle minerals is affected by the change in water depth during sedimentation, showing a trend of increasing first and then decreasing from bottom to top, gradually decreasing overall. This leads to a similar vertical variation law in shale brittleness and the development of natural fractures, and the three complement each other. Meanwhile, such brittle minerals have a good positive correlation with organic matter content, and their contribution to shale brittleness is much greater than that of organic matter to shale plasticity. In addition, the correlation between rock mechanics parameters and brittleness index indicates that the Young's modulus plays a more important role in the evaluation of Niutitang Formation shale brittleness in this area than the Poisson's ratio.