Experimental Study on the Effect of Mineral Composition on Shrinkage Fractures: Implications for the Origin of the Diagenetic Shrinkage Fractures in Marine-Continental Transitional Shales

Natural fractures in shale have been commonly recognized as a vital factor in shale gas production. Diagenetic shrinkage fracture is an important type of natural fracture; however, its formation mechanism and prediction remain largely unexplored. Given the similarity between diagenetic shrinkage fractures and desiccation cracks, the influence of smectite, kaolin, quartz, and feldspar on shrinkage fractures were investigated using the physical simulation experiment of desiccation in this study. Here, water evaporation, surface cracks initiation and propagation processes were monitored during the whole desiccation. The morphological characteristics of cracks patterns were quantitatively described. Diagenetic shrinkage fractures of transitional shale with different minerals were observed using SEM. The results show that the development and morphology of cracks are affected by the mineral composition, and the sediments with higher clay content tend to form more complex fracture networks. We further propose a morphological prediction model for cracks and compare this model with diagenetic shrinkage fractures under SEM. We found that the effects of mineral composition on both were similar, with more well-developed diagenetic shrinkage fractures in those clay-rich samples. Taken together, this study implies the feasibility of using physical simulation experiment of desiccation cracks to invert diagenetic shrinkage fractures and provides new insights into the mechanism and development regularity of diagenetic shrinkage fractures.