ABSTRACT: Tectonic Indenters, Stresses, and Fracture Arrays: Predictive Concepts for Rocky Mountain Reservoirs

John C. Lorenz

Maximum horizontal stresses oriented normal to both thin- and thick-skinned thrust faults have been documented in numerous geologic provinces. Studies in the Canadian and American Rockies suggest that thrusts may act as indenters and stress sources where they impinge on adjacent strata. The stress trajectories may radiate from a local thrust front or may maintain a perpendicular relationship to a curvilinear front. The stress arrays in many areas are recorded by a sympathetic array of fractures that formed in otherwise undeformed strata, at depth, and parallel to the maximum horizontal compressive stress during thrusting. Stress vectors, however, may be complex. Sources of complexity include (1) superposition of indentation stresses on regional stresses, (2) pre-existing tructural inhomogeneities that alter local stress trajectories (as in the Arabian platform), and (3) zones of weak strata, such as evaporites, that limit the distribution of stresses to certain horizons (as in the Jura region). Therefore, the distribution of stresses and fracturing need be neither vertically linear nor spatially uniform, and stresses at shallow depths (several kilometers) need not reflect regional plate motions. Finally, studies of Mesaverde strata suggest that fracturing will be limited to strata with suitable formation pressures and rock properties. Other studies (Zagros, Ouachitas, Pyrenees) indicate that thrust-related fracturing may occur in platform strata several hundreds of kilometers beyond a thrust front. Thus, subsurface reservoirs in Rocky Mountain basins in ront of the Sevier thrust belt or adjacent to Laramide overthrust blocks are likely to contain fracture patterns that can be predicted by consideration of tectonic indenters, stress arrays, and local heterogeneities.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990