Fractures and Structural Heterogeneities within a Deformed Paleozoic Limestone in the Basin and Range Province

Scott P. Cooper and John C. Lorenz
Sandia National Laboratories, Albuquerque, NM

Heterogeneous deformation structures in limestones that influence porosity and permeability can change over the production area of a single well. This is illustrated by the ductile, brittle and dissolution characteristics of a deformed Paleozoic limestone in the Basin and Range province, where superimposed compression, extension, and folding within an outcrop only one square kilometer in extent have produced a complex suite of deformational structures. In addition to numerous natural fractures of various types, larger-scale structural heterogeneities include: 1) thrust faults with meter-scale displacement and local folding of bedding, 2) pods of disrupted bedding, related to thrusting, up to ten meters along strike, 3) normal faults in several orientations, filled locally with calcite mineralization up to several tens of centimeters thick, 4) meter-scale caves along some of the faults, and 5) folding on the scale of tens of meters. Smaller-scale heterogeneities include chert nodules and beds, and pervasive calcite-filled fractures, commonly occurring as en echelon conjugate arrays and as bed-normal extension fractures. Although the cherts are typically less than a meter in scale, they offer significant local mechanical contrasts in bedding, and are pervasively fractured. The strata are composed of alternating layers of dark-gray and light-gray limestone which accommodated strain in significantly different ways reflecting different mechanical properties: darker rock is more brittle and typically more heavily fractured. Many of the structures are large enough to influence subsurface fluid flow. Similar features are present in other examples of deformed limestones throughout the Basin and Range province.