Publications

Tectonic setting and characteristics of natural fractures in Mesaverde and Dakota reservoirs of the San Juan Basin

Lorenz, John C.; Cooper, Scott P.

The Cretaceous strata that fill the San Juan Basin of northwestern New Mexico and southwestern Colorado were shortened in a generally north-south to north northeast-south southwest direction during the Laramide orogeny. This shortening was the result of compression of the strata between southward indentation of the San Juan uplift at the north edge of the basin and northward to northeastward indentation of the Zuni uplift from the south. Right-lateral strike-slip motion was concentrated at the eastern and western margins of the basin to form the Hogback monocline and the Nacimiento uplift at the same time. Small amounts of shear may have occurred along pre-existing basement faults within the basin as well. Vertical extension fractures, striking north-south to north northeast-south southwest (parallel to the Laramide maximum horizontal compressive stress) with local variations, formed in both Mesaverde and Dakota sandstones under this system, and are found in outcrops and in the subsurface. The less-mature Mesaverde sandstones typically contain relatively long and irregular vertical extension fractures, whereas the underlying quartzitic Dakota sandstones contain more numerous, shorter, sub-parallel, closely spaced extension fractures. Conjugate shear fractures in several orientations are also present locally in Dakota strata.