Gravity‐Driven Jointing in Layered Rocks: Mechanical Analysis and Numerical Modeling

Abstract

The origin of open‐mode fractures or joints, common in sedimentary basins (reservoirs), remains incompletely understood. These fractures require effective tensile stress for their formation, yet they usually develop at depths where effective compressive stresses dominate. Through 2‐D finite‐difference elastoplastic modeling, we demonstrate that effective tensile stresses and fracturing can be internally generated in stiff competent layers separated by compliant incompetent layers. Two primary factors drive this phenomenon: the layered structure of sedimentary units with varying mechanical properties of the layers and the lithostatic compression of the unit, leading to its gravity‐driven horizontal widening or stretching. This stretching occurs when the average horizontal stress over the unit’s thickness is compressive. Various mechanisms can accommodate the stretching, including preexisting faults or weak zones, free borders like canyons, and horizontal extension of tectonic or gravitational origin. Tensile horizontal effective stresses σxx occur within the competent layers, while the incompetent layers generally remain under effective compression. The magnitude of σxx increases with the lithostatic stress or burial depth, as does the tensile strength σt of incompletely lithified sediments. Jointing occurs within depth intervals where |σxx| > σt. In our modeling approach, effective tensile stresses σxx are generated due to lithostatic compression σv. In the previous numerical modeling studies, these stresses resulted from extension directly applied to the lateral boundaries of the layers and did not depend on σv. We provide an analysis of the distinctions between these two approaches.