Statistical analysis of geological factors controlling bed-bounded fracture density in heterolithic shale reservoirs: The example of the Woodford Shale Formation (Oklahoma, USA)

The geological controlling factors of natural fracture distribution and density in the Woodford Shale Formation are quantitatively investigated in this study. The Upper Woodford Shale member in southern Oklahoma, is exceptionally brittle due to the abundance of recrystallized radiolaria chert beds that alternate with siliceous shale beds. Bed-bounded compressional fractures are well developed in the structural-active region. Two areas were characterized and compared to represent low and high confining pressure conditions respectively. A statistical analysis workflow consisting of Principal Component Analysis (PCA) and Partial Least Square Regression (PLS) was proposed to quantify the controlling factors' relationship and their Variable Importance in Projection (VIP) which reveals the contribution of each controlling factor to the fracture density. The result indicated that natural fractures' occurrence in shale reservoirs are impacted by three controlling factors: rock hardness, hard bed ratio (thickness percentage of hard beds within a stratigraphic unit interval) and bed frequency (number of beds within a unit interval). The contributions from these individual controlling factors are different under different reservoir conditions. Bed frequency (layering anisotropy) is the dominant controlling factor that contributes on average three times more than the hard bed ratio and hardness to fracture density. Hardness's contribution to fracture density in the subsurface scenario is lower by 22% than the surface scenario due to the increase in confining pressure. The analytical results highlight the importance of the layering effect on fracture density which has often been overlooked in geomechanical modeling workflow. A new concept of Fracture Density Index (FDI) is defined and can be further applied to predict fracture density in other unconventional reservoirs.