Fracture studies in Cretaceous and Paleocene strata in and around the Piceance Basin, Colorado; preliminary results and their bearing on a fracture-controlled natural-gas reservoir at the MWX site

Abstract

Fractures in exposed strata of the Mesaverde Group (Late Cretaceous) and overlying Wasatch Formation (Paleocene-early Eocene) were measured and described at 137 sites within five separate areas in and around the Piceance basin. These data supplement information previously gathered at more than 300 sites in younger strata of the basin interior. The data were used to test the feasibility of predicting fracture orientations and other characteristics (dimensions, spacing, aperture) in correlative rocks at depth, as an aid to development of methods to stimulate production of natural gas from low-permeability sandstone reservoirs of the Mesaverde Group. Fractures in exposed strata of the Mesaverde Group along the Grand Hogback monocline are similar in orientation to those documented in core from the nearby MWX site, where equivalent strata are buried at depths of 1200-2500 m. The monoclinal strata, then, may be suitable for study as an exposed analog of the gas-bearing reservoir at depth. However, Mesaverde strata in DeBeque Canyon, also relatively close to the MWX site, are cut by fractures of wholly different orientation than those documented in core, and fractures in younger strata immediately surrounding the MWX site likewise do not match those found at depth. We explain these results as the effect of sampling fractures from two different fracture systems that overlap only partially in space and perhaps not at all in time. The older of the two systems is the structurally lower one, and dominates the buried Mesaverde strata at the MWX site. Equivalent fractures along the Grand Hogback monocline are exposed at the surface only as a fortuitous result of monoclinal folding. West of the monocline, at and near the MWX site, surface exposures are dominated by a structurally shallower and areally extensive system of joints previously documented in the Green River and Uinta Formations (both of Eocene age) of the northern Piceance basin. The transition zone between the two fracture systems is well exposed along the White River west of the monocline. INTRODUCTION The U.S. Department of Energy's Multi-Well Experiment (MWX) in the Piceance basin near Rifle, Colorado (fig. 1), is a research effort coordinated through Sandia Laboratories to investigate means of stimulating production of natural gas from lowpermeability sandstone reservoirs. Reservoir sands at the MWX site are contained within the Mesaverde Group of Late Cretaceous age, and lie at depths of 1200-2500 meters. Correlative rocks are exposed at the surface along the Grand Hogback monocline, 15 km to the northeast at its nearest point, and again in DeBeque Canyon about 60 km westsouthwest of the MWX site. Between these two areas, and surrounding the MWX site at the surface, lie slightly younger (Paleocene-early Eocene) mudstones and subordinate sandstones of the Wasatch Formation. Study of fractures in surface exposures of the Mesaverde Group and Wasatch Formation in 1983 was directed at two principal goals: (1) to explain, as fully as possible, the origin and character of fractures previously documented in core from the buried Mesaverde strata at the MWX site, and (2) to determine to what extent the various properties of those fractures could have been predicted from surface studies before drilling began. Preliminary results are presented in this report. The fracture history of younger rocks chiefly oil shales of the Green River Formation and sandstones of the overlying Uinta Formation, both extensively