Tectonic setting and characteristics of natural fractures in Mesaverde and Dakota reservoirs of the San Juan Basin John C. Lorenz and Scott P. Cooper, Geophysical Technology Department 6116, MS 0750, Sandia National Laboratories, Albuquerque, NM 87185-5800 Abstract 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. Ver- tical extension fractures, striking north-south to north northeast-south southwest (parallel to the Laramide maximum horizontal com- pressive 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 rela- tively long and irregular vertical extension FIGURE 1—Structure contour map of the inner or central section of the fractures, whereas the underlying quartzitic San Juan Basin. Contours are drawn on the top of the Upper Cretaceous Dakota sandstones contain more numerous, Pictured Cliffs Sandstone, which overlies the Mesaverde Group. The basin shorter, sub-parallel, closely spaced exten- is asymmetric with the northwest trending synclinal hinge near the north- sion fractures. Conjugate shear fractures in ern edge of the basin. several orientations are also present locally in Dakota strata. the entire basin (e.g., Kelley, 1957; Fassett, Location and general geology Introduction 1991; Laubach and Tremain, 1994) are pri- The San Juan Basin of northwestern New marily descriptive rather than attempting Mexico and southwestern Colorado is This paper has two objectives: 1) to pro- to interpret the kinematics of the important asymmetric, and has strata dipping gently vide a brief characterization of natural structures. We have re-examined the basic to the north and northeast toward the off- fractures that control the production of structural building blocks of the basin and center synclinal hinge located near the Col- natural gas from sandstone reservoirs in have constructed an unconventional but orado–New Mexico state line (Fig. 1). Mesaverde and Dakota strata in the San plausible tectonic scenario for the post- Most of the hydrocarbon production Juan Basin, and 2) to reconstruct a plausi- Jurassic structural history of the San Juan from the San Juan Basin comes from an ble tectonic framework for the genesis of Basin. Because the impetus behind this inner zone, approximately delineated by these fractures. The tectonic model pre- study was hydrocarbon reservoir charac- the outcrop belt of the Upper Cretaceous sented here provides a basis for the extrap- terization, our tectonic scenario was con- Pictured Cliffs Sandstone and separated olation of fracture characteristics from out- structed to provide a framework that from the peripheral areas by a relatively crops and core to the subsurface and for would explain the observed outcrop and continuous hogback nearly 400 km (154 the prediction of fracture characteristics in subsurface natural fracture characteristics mi) long, consisting of gently dipping to the subsurface. This paper is a shortened and that would permit a basic level of sub- near-vertical strata. As described below, version of the report containing more of surface fracture prediction. Thus the paper however, despite the apparent continuity the supporting data (Lorenz and Cooper, presented below is an inversion of the nor- of this feature, this hogback does not have 2001). Copies of the longer report can be mal procedure wherein fractures would be a common mechanical origin along its obtained directly from the authors or are used as kinematic indicators and as first- entire length. Lower Cretaceous strata crop available from the U.S. Department of level data in the reconstruction of a tecton- out at the margins of the San Juan Basin Commerce, National Technical Informa- ic history. Nevertheless, fracture evidence and extend into the subsurface to depths as tion Service, Springfield, VA, or online at and tectonic reconstruction are mutually great as 2,000 m (6,562 ft), where they have www.osti.gov/bridge. supporting, and converge on the same con- been penetrated by thousands of natural- Most of the published tectonic recon- clusion that would have resulted from a gas wells. structions of the San Juan Basin area focus more standard approach to the problem. Significant thicknesses of overburden on one or two of the structural elements strata were stripped from the surface of the that bound the basin. Those that examine basin in post-Cretaceous time. As much as February 2003, Volume 25, Number 1 NEW MEXICO GEOLOGY 3 extension. Our work suggests this fracture set is part of the regional, early Laramide fracture system. Kelley (1957) and Kelley and Clinton (1960) note several domains of relatively uniform fracture strikes in the San Juan Basin as part of their broad-scale aerial- photo study of fractures on the Colorado Plateau. However, the scale of their study did not permit detailed examination and assessment of the fractures at ground level, and subsequent investigators have not rec- ognized these domains. Previous subsurface studies infer the presence of fractures in Cretaceous San Juan Basin reservoirs based on several indirect criteria including association with faults recognized on seismic data (e.g., TerBest, 1997; DuChene, 1989) and anom- alous production rates (e.g., Emmendorfer, FIGURE 2—Extension and conjugate shear fractures as observed and created in laboratory compres- 1992; Gorham et al., 1979; London, 1972; sion tests (σ1—maximum principal stress; σ2 — intermediate principal stress; σ3—minimum princi- Ouenes et al., 1998). Fracturing is typically pal stress; modified from Weijermars, 1997). assumed to have been caused, or at least enhanced, by flexures. Actual subsurface fracture data are rare, although Ortega and 2,500 m (8,202 ft) of overburden may have these sections suggest locally significant Marrett (2000) publish subsurface fracture been removed from northern parts of the thrust indentation and overhang at the data for the Mesaverde for three wells in basin (Bond, 1984). The previous deep bur- basin margins. Erslev (2001) and Cather the central part of the basin indicating a ial and resulting high overburden stress (1999) interpret the structure of the dominate north-south-striking fracture set, accounts for horizontal stylolites that are Nacimiento uplift on the eastern margin of and Lorenz et al. (1999) present a prelimi- locally common in Dakota sandstone the San Juan Basin in the context of overall nary synthesis of the tectonic framework cores. Laramide kinematics. of the basin based on wellbore-image logs Previous studies of natural fractures and and measured fracture characteristics in 32 Fracture terminology coal cleats in the San Juan Basin are prima- Dakota cores. The interpretations herein For simplicity, breaks in rock will be rily descriptive and concentrate on out- build on the conclusions of Lorenz et al. described as either extension fractures or crops of Mesaverde strata (e.g., Whitehead, (1999), expanding them to encompass shear fractures within this paper, consis- 1997; Tremain et al., 1991; Laubach, 1992). Mesaverde reservoirs. tent with the terminology used within the These studies suggest a demarcation in petroleum industry. Extension fractures surface fracture domains that is approxi- Tectonic setting (also termed joints, or tensile fractures or mately coincident with the New Mexi- dilation fractures or Mode I fractures: Pol- co–Colorado border. This demarcation was The inner part of the San Juan Basin lard and Aydin, 1988) have displacements not apparent during the course of this appears to be relatively structureless, with perpendicular to the fracture surfaces. study, perhaps because this study concen- low-amplitude flexures and gentle region- Shear fractures have some displacement trated on the oldest fractures present in the al dips. However, the basin is underlain by parallel to the fracture surface (also termed outcrops, those most likely to be present in a Proterozoic crystalline basement with Mode II or Mode III fractures depending the subsurface rather than fractures related many ancient and locally reactivated faults on displacement relative to the fracture to folding and uplift. Whitehead (1997) trending northeast-southwest and north- front: Pollard and Aydin, 1988). Both even suggests that many of the fractures in west-southeast (Taylor and Huffman, extension and shear fractures can be outcrops, particularly on the Chaco slope 1998). These faults may have been impor- formed in laboratory compression tests area, were caused entirely by surficial, val- tant controls on the fracture patterns in the with specific orientations with respect to ley-wall gravitational processes. In addi- overlying strata, but we suggest here that the applied stress. Extension fractures tion, Condon (1997) reports that the aver- the tectonic features surrounding the basin form