DOES SEDIMENTARY STRATIGRAPHY CONTROL FRACTURE FLOW PATHS?

Sedimentary strata often show repetitive facies patterns reflecting temporal changes in depositional environments. These stratigraphic patterns can be expressed in terms of mechanical stratigraphy. Mechanical units, and the interfaces between units, control fracturing and thus fluid flow within low-matrix permeability rocks. A field study in Door County, WI, demonstrates that carbonate cycle boundaries commonly, but not always, act as mechanical interfaces, which terminate bed-perpendicular fractures. Additional factors such as intense lithification of the cycle caps, organic-rich partings and thick mud/shale cycle bases also influence whether cycle boundaries arrest fracture propagation suggesting that the strength of stratigraphic horizons controls fracture termination. Within the Austin Chalk, TX, the thickness of unfractured marl layers controls fracture termination within the interbedded fractured carbonate layers. Modeling studies support these observations and demonstrate that localized opening along mechanical interfaces (e.g. bedding planes) and distributed shear within mechanical layers (e.g. within clay-rich layers) can act to terminate propagating fractures. Numerical experiments with distributed shear within mechanical units suggest that marl layers within the Austin Chalk as thin as 1 cm can act as mechanical interfaces. Similarly, numerical experiments on fractures approaching mechanical interfaces indicate that low strength interfaces such as organic partings can terminate fractures. The combination of empirically determined relationships from field observations and results of numerical experiments can guide prediction of fracture network from the sedimentary stratigraphy. The key to predicting the fracture network lies in understanding the mechanical controls, which vary with different types of sedimentary facies cycles.