LORENZ, JOHN C., Sandia National Laboratories, Albuquerque, NM

ABSTRACT: Reservoir Fracture and Permeability Trends Inferred from Reconstructions of Tectonic Stress Orientations: Example from the Green River Basin, Wyoming

Because stresses both create fractures and control subsequent fracture permeability, it is useful to reconstruct the stress orientations and magnitudes present during the tectonic evolution of a basin. Mesozoic and younger strata in the Green River basin of southwestern Wyoming have been subjected to several tectonically derived stresses. The most obvious source of stress is the Sevier fold and thrust belt to the west. However, compressional stress, derived from this belt and propagated eastward intothe basin, was self-limiting. The maximum stress that strata in the basin were subjected to was that force required to break the strata and lift a section of it against gravity to create an overthrust sheet. Although this process occurred repeatedly over the course of 100 million of years, it appears to have imposed only secondary stresses on many nearby strata. The significantly shorter-lived and thicker-skinned Laramide thrusts that impinged on the southern and northern basin margins created significantly more north-south compression, commonly overwhelming the contemporaneous east-west stress derived from the Sevier belt. Moreover, the strata at the western margin of the basin were subjected to east-west extension during subsidence of this narrow foredeep in front of the thrust belt. Strata that were lithified while under north-south compression are predicted to contain a locked-in generally north-south stress despite evidence for current east-west extension in parts of the basin. Thus, the age of deposition of a given set of strata relative to the timing of thrust event is an important part of the problem. Local structures complicate the stress patterns, but can be allowed for on an individual basis.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.