Heat-flow data suggest large ground-water fluxes through Fruitland coals of the northern San Juan basin, Colorado-New Mexico John McCord* Geoscience Department, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 Marshall Reiter New Mexico Bureau of Mines and Mineral Resources and Department of Geoscience New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 Fred Phillips Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 ABSTRACT Temperature logs and heat-flow data from wells in the northern San Juan basin, Colora- do-New Mexico, suggest that coals of the Fruitland Formation are an aquifer with appreciable horizontal ground-water movement. Calculated hydraulic conductivities, based on ground- water flow rates estimated from heat-flow data, are several orders of magnitude greater than hydraulic conductivities measured in laboratory or pumping tests. Fractures and other prefer- ential pathways probably create high regional hydraulic conductivity that allows significant heat advection. INTRODUCTION Recent geological and geochemical studies have indicated that a wide variety of phenom- ena, ranging from ore deposition to helium concentration and temperature distribution pat- terns, imply large-scale fluid transport through sedimentary basins (cf. Bethke and Marshak, 1990). Corresponding basin-scale hydrologie models of solute and heat transport have con- firmed that ground-water advection can poten- *Present address: Intera, Inc., 6850 Austin Center Blvd., Suite 300, Austin, Texas 78731. tially supply the necessary transport capability (Garven and Freeze, 1984; Bethke and Mar- shak, 1990). However, such models have re- mained controversial because the necessary advection, in general, requires permeabilities substantially in excess of those measured on cores or by aquifer stress tests using pumping wells (Fowler, 1986). Garven (1986) and Major- owicz (1989) have suggested that regional per- meability is much greater than locally measured values. In situ tests may underestimate actual permeabilities because pressure reduction during tests allows expansion of the matrix to decrease permeability. Regional-scale permeability may be controlled by infrequent high-permeability pathways that are rarely sampled (e.g., fractures or interconnected channel deposits). If either (or both) of these hypotheses are valid, regional- scale permeability must be inferred from means other than hydraulic tests. The application of some regional-scale tracer, such as water temperature or a conservative radionuclide, would be one alternative for estimating regional permeability. In this paper we analyze hydraulic head and temperature distributions in the Fruitland For- mation in the northern San Juan basin, Colorado-New Mexico. The observed tempera- ture distribution is difficult to explain either by thermal conduction or by a combination of con- duction and advection of heat when measured thermal and hydraulic conductivities are used. One possible resolution of the discrepancy is more rapid advection of water than would be calculated from the measured hydraulic conductivities. HYDROGEOLOGIC FRAMEWORK The San Juan basin is a Late Cretaceous- early Tertiary (Laramide) structural depression on the eastern margin of the Colorado Plateau. The basin center contains -4000 m of sedimen- tary rocks ranging in age from older Paleozoic to Eocene. In this paper we focus on the younger Cretaceous Fruitland Formation and adjacent units (Bauer, 1916) in the northern basin. The Fruitland Formation contains interbedded sand- stone, coal, and shale and is typically - 1 0 0 m thick. The sandstone is generally fine grained and clayey and has low matrix permeability (Stone et al., 1983). The Fruitland is overlain by the Kirtland Shale, a low-permeability aquitard, and underlain by the Pictured Cliffs Sandstone, which aquifer pumping tests indicate has a hy- draulic conductivity in the range of 10~ 8 m/s (Stone et al., 1983). Below the Pictured Cliffs Sandstone is the Lewis Shale, an aquitard with very low permeability. The hydraulic head distribution within the Fruitland Formation is shown in Figure 1. The formation is recharged in southern Colorado, Figure 1. Map of study area showing hydraulic head contours, heat-flow contours, heat-flow sites, and line of cross section A-A' (see Fig. 2) (after McCord, 1988). Heat-flow sites: 1— Southern Ute 2-2; 2—Kelly A#3A; 3—Atlantic State; 4—Allison #59; 5—Com G8; 6—Roelofs A#1A. Lines B-B', C-C' show heat-flow profiles. GEOLOGY, v. 20, p. 419-422, May 1992 419