Toward Predicting Fault-Zone Architecture and Permeability Structure in Clastic Sediments and Sedimentary Rocks

Different fault-zone structures affect fluid flow at different scales. The architectural elements that are the most important for fault seal at all scales are clay-rich fault cores and mixed zones. In contrast, fracture networks are the key structures facilitating fault-parallel flow and conduit behavior. None of these features is present in every fault zone. Thus, understanding where and why clay cores, mixed zones, and fractures form will improve prediction of fault-zone architecture and permeability structure, thereby improving our ability to predict whether or not individual faults will act as seals or conduits.

A variety of factors control the formation of fault-zone structures in clastic sediments and sedimentary rocks at low confining pressures. We focus on factors that can be determined through petrographic study or constrained by borehole geophysical analyses. Chief among these are (1) degree of consolidation and (2) amount and mineralogy of grain-bonding cement. These variables collectively define grain contact strength, which is determined both by contact area (largely the result of consolidation) and the strength of the contact bond (controlled by cement). Where grain contact strength is relatively high, open, transgranular fractures can form in a fault zone. Where grain contact strength is very low, fractures cannot form, but grain-boundary sliding and disaggregation are possible, and mixed zones can be produced by particulate flow. The primary requirement for formation of clay-rich fault cores is the presence of clay beds or shale in the faulted sequence, since clay smear is a key formation process. However, in sediments with low grain contact strength, clay cores may be augmented by the injection of clay-rich clastic dikes. Thus, faults that form when sediments are in a poorly lithified state, with low grain contact strength, are more likely to form seals than faults that initiate in sedimentary rocks with high grain contact strength.