Controlling Effect of Texture on Fracability in Lacustrine Fine-Grained Sedimentary Rocks

Microtextures in lacustrine fine-grained sedimentary rocks generally include the basic features of quantity, size, shape and arrangement of particles, which are the internal factors that affect the mechanical properties of rocks. In this paper, based on mechanical experiments and a rock slice image analysis technique, fracability is systematically evaluated in terms of textures by using the Mesozoic and Cenozoic lacustrine fine-grained sedimentary rocks in eastern China. Analysis show that the fracability of fine-grained sedimentary rocks is improved with increasing particle quantity and size but is reduced with increasing particle sorting, roundness, vertical distribution uniformity and holistic distribution uniformity. Regarding composition, fracability is reduced with increases in clay minerals and total organic carbon (TOC) but is first reduced and then improved with increases in the content of feldspar or carbonate minerals, i.e., when the mineral content exceeds 30%, fracability is positively correlated with it, but it is negatively or weakly correlated for mineral contents below 30%. Fracability is improved with enhanced the diagenesis strength. Regarding structural aspect, the fine-grained sedimentary rocks with strong laminability and continuity have poor fracability. A comparative analysis also shows that the stronger control factors on fracability are vitrinite reflectance (Ro), particle quantity, size, sorting, roundness and holistic distribution uniformity, while the components (e.g., TOC, clay minerals, and felsic minerals) and laminar development degree (e.g., laminar continuity and laminar quantity) are relatively weak factors. Diagenesis plays an important role in fracability for the fine-grained sedimentary rocks in different target layers, but for the fine-grained sedimentary rocks in a target layer, the texture factors are the intrinsic factors controlling the fracability and should be considered as primary considerations in engineering fracturing operations.