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COOPER, SCOTT, P., and JOHN C. LORENZ, Sandia National Laboratories, Albuquerque, NM; and LAUREL B. GOODWIN, New Mexico Institute of Mining and Technology, Socorro, NM

ABSTRACT: Controls on Fracture Orientation and Type within the San Juan Basin, New Mexico and Colorado

Investigation of natural fractures in outcrops along the northern edge of the San Juan Basin indicates that one of the dominant fracture sets within both the Mesaverde Group and the Dakota Sandstone is composed of bed-normal extension fractures that strike between N5W and N25E when bedding is returned to horizontal. Two pairs of conjugate shear fractures, one with a bed-parallel axis of intersection and the other with a bed-normal axis of intersection, both with a bedparallel bisector to the acute angle, are limited to the Dakota Sandstone. These fracture sets and orientations are also observed within core from the San Juan Basin and are compatible with N-S to NNE-SSW shortening in the area studied.

Basement-involved thrusts are observed on the eastern, northern and western margins of the basin. The southern extent of the basin is bounded by the Chaco slope, which dips gently basinward, away from the basement-cored Zuni Uplift. Local variability of fracture strike around the basin is common and is interpreted to be due to local variations in the maximum shortening direction and to mechanical stratigraphy. Changes in mechanical stratigraphy that control fracture characteristics are attributed to compositional, diagenetic and depositional variability across the basin. Petrophysical controls on mechanical properties are evidenced by the fact that shear and extension fractures are found in the Dakota Sandstone, whereas only extension fractures are observed in the Mesaverde Group.

AAPG Search and Discovery Article #90915©2000 AAPG Rocky Mountain Section, Albuquerque, New Mexico