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2 GRC Transactions, Vol. 34, 2010 EGS Potential in the Northern Midcontinent of North America Will Gosnold, Richard LeFever, Michael Mann, Robert Klenner, and Hossein Salehfar University of North Dakota Keywords Thermal conductivity, EGS, heat flow, hydrodynamics, radioactive heat production Abstract Heat flow in the midcontinent ranges from 20 mw m -2 to 140 mw m -2. Anomalously high heat flow occurs in parts of Nebraska and South Dakota is due to gravity-driven regional groundwater flow. The region of high heat flow coveras an area of about 1,500 km 2 and is a promising target for EGS. High heat flow observed in a mining exploration well drilled into carbonatite body in southeastern Nebraska is due to high radioactive heat production in the carbonatite. Observed heat flow outside the anomalous regions in the midcontinent region averages 60 mw m -2 ± 25 mw m -2. More than 300 new thermal conductivity measurements of Mesozoic and Paleozoic rocks in the Williston Basin can be used to calculate temperature vs. depth profiles in the sedimentary formations of the midcontinent. The depositional environments for the platform sediments and the intracratonic basins in the midcontinent of North America were similar enough that formations correlate by rock type and rock properties. Introduction A key factor in Enhanced Geothermal Systems (EGS) potential for any area is the depth to which one must drill to reach a desired target temperature. In the simplest case, one can predict temperature vs. depth using the geothermal gradient estimated from several sources, e.g., bottom- hole temperatures or shallow heat flow determinations. This approach is best applied in regions having little variation in rock type, i.e., crystalline basement so that thermal conductivity is relatively constant. Understanding temperature vs. depth relationships in platform sediments and deep basins in the midcontinent and eastern US requires a more complex approach. Thermal conductivity and consequently the geothermal gradient can vary by a factor of 4 in a sedimentary column. Heat transport in regional groundwater flow can cause surface heat flow to vary by a factor of 2 or more. In this paper we present new data relevant to the EGS potential of the northern midcontinent and highlight a potential target for development. Heat Flow Surface heat flow is the product of the geothermal gradient and thermal conductivity, q s = λ ΔT Δz, where q s is heat flow (mw m -2 ), λ is thermal conductivity (W m -1 K -1 ), and ΔT is the vertical Δz temperature gradient. Heat flow for the midcontinent, as defined by latitudes 31 N 61 N and longitude 87 W 104 W, can be determined from several sources. The heat flow database of the International Heat Flow Commission (IHFC) contains 4692 heat flow measurements in North America, 286 of which lie within the midcontinent. Mean heat flow in the IHFC database is 60 mw m -2 ± 25 mw m -2. The Southern Methodist University Geothermal Laboratory database yields the same values based on the IHFC data plus 99 additional temperature gradient sites with inferred thermal conductivity values. A theoretical heat flow value can be estimated based on the linear relation between heat flow and radioactive heat production (Birch et al., 1968) and the variation of heat flow with tectonic age (Vitorello and Pollack, 1980). The linear relation between heat flow and heat production, q = q 0 + Ab, where A is heat production in W m -3, and b is a constant with units of length introduces the concept of a background heat flow, q 0, that may be inferred to be the mantle component of heat flow. In an analysis of heat flow east of the Rocky Mountains, Morgan and Gosnold (1989) obtained values of 31 ± 1 mw m -2 for q 0 and 8.1 ± 0.4 km for b. Combining these heat flow values with the average crustal heat production value of 3.0 μw m -3 (Wollenberg and Smith, 1987), yields an estimate of 55 mw m -2 for q. Vitorello and Pollack (1980) proposed a three component heat flow consisting of mantle heat flow, radioactive heat production, and a tectonic component that decreases as the square root of age. They proposed relatively equal heat flows for q 0 (27 mw m -2 ), A (36 to 18 mw m -2 ), and tectonics 355

3 (27 mw m -2 ). These numbers give an estimate of 57 mw m -2 for a tectonic age of 1.2 Ga in the midcontinent. Applying these heat flow values to estimate temperature at depth can include or omit a model for the vertical distribution of heat production. The exponential model of Lachenbruch (1970), A = A 0 e _ z b, retains the linear relation between q and A with differential erosion of the surface and is preferred here. This implies that A and q decrease with depth and the general expression for temperature vs. depth where thermal conductivity also changes n Z with depth is T = T 0 + i q i. Basement rocks are heterogeneous i=1 λ i and can vary widely in radioactive heat production laterally and vertically. For an in-depth discussion of the variability of heat production and its effect on lateral heat flow see Tester et al., 2006, Ch Example temperature vs. depth profiles for different radioactive heat production scenarios and for multi-layer thermal conductivity with different background heat flows are shown in Figures 1 and 2. Figure 1. Temperature vs. depth profiles in the crystalline basement with different radioactive heat production values where heat production decreases exponentially with depth. Thermal Conductivity Figure 2. Temperature vs. depth profiles in a sedimentary basin with multiple sedimentary formations having different thermal conductivities and different heat flows. It is essential that thermal properties used to project subsurface temperatures are appropriate for a particular site or region. Thermal conductivity in basement rocks can vary laterally and vertically, but the variations are relatively small, i.e., ± 10 percent. Tester et al., (2006) adopted a standard value of 2.6 W m -1 K -1 which allows for a slight decrease in conductivity with increasing temperature, and we use that value in this study. However, variability of thermal conductivity in sedimentary rocks can be large, for example 0.7 W m -1 K -1 for some shales to 4.7 for some dolomites (Roy, Beck, and Touloukian, 1981). Roy, Beck and Touloukian s (1981) compilation included measurements on rocks from different sedimentary environments and different geological ages. These differences affect characteristics such as compaction, porosity, mineralogy and crystal size that in turn affect thermal properties. Conductivities from Roy, Beck and Touloukian (1981) were used in several subsurface temperature studies of midcontinent basins (Gosnold and Eversoll, 1981; Gosnold, 1991), but there have always been questions about this practice. To address these questions we have undertaken a systematic analysis of lithology, density, porosity and thermal conductivity on Williston Basin core samples from the North Dakota Geological Survey s Wilson M. Laird Core and Sample Library in Grand Forks, ND. There are 54 distinct lithologic formations in the Williston Basin, but fewer than half of the formations have been sampled by coring during exploration drilling. The reason for this sampling pattern is that core recovery is expensive and the targets of interest are in the deeper parts of the basin where kerogen is thermally mature. Consequently, there are few samples of the upper 1 to 2 km of clastic dominated sediments. However, cuttings of most formations are available and will be added to our sampling program during the next two years. We are measuring thermal conductivities with Portable Electronic Divide Bar (PEDB) instruments (Antriasian, 2010). The PEDB can handle samples up to 65 mm in diameter and operates at temperatures from 10 C to 35 C. To achieve accuracy in temperature vs. depth projections, we correct the measured thermal conductivities for in situ temperatures using the expression ( ), where λ 0 and T 0 are laboratory 1n(λ) = 1n(λ 0 ) +1n T (T 0 )M values and M is a constant dependent on lithology (Touloukian et al., 1970). The data we are acquiring can be widely applied to the sedimentary rocks throughout the midcontinent because the formations are laterally continuous with similar sedimentary environments throughout the region. The importance of these measurements is evident in Table 1, which compares measured conductivities with conductivities taken from Roy, Beck, and Touloukian (1981). The differences between the measured and published thermal conductivities for our samples average 22 percent with some measured values higher and some lower. Although the basin contains 54 distinct formations and 32 Table 1. Thermal conductivities measured on Williston Basin core samples using the PEDB are corrected for in situ temperature compared to conductivities taken from earlier literature based on lithology. N is the number of samples measured in this study, and the column labeled Pub. λ gives the conductivities used from Roy, Beck, and Touloukian (1981) based on lithology. Formation System Rock Type Cond. (W/m/K) N Pub. λ Pierre Cretaceous Sh 0.88 ± Madison Mississippian Ls 2.49 ± Duperow Devonian Ls 3.03 ± Souris River Devonian Ls 2.80 ± Dawson Bay Devonian Ls 2.95 ± Winnipegosis Devonian Ls 2.85 ± Ashern Devonian Ls / Do 2.97 ± Interlake Silurian Do / Ls 3.60 ± Stonewall Silurian Do / Ls 3.70 ± Stony Mt. Silurian Do / Ls 3.87 ± Red River Ordovician Ls / Do 3.28 ± Black Island Ordovician Do - Ss 4.16 ± Deadwood Cambrian Do - Ss 3.26 ±

4 Figure 3. Temperature vs. depth profiles based on the thermal conductivities given in Table 1. Each point on the curves represents a formation top. The decrease in temperature gradient from the upper section to the deeper section is due to the lower thermal conductivity of the clastic dominated upper section and the higher thermal conductivity of the carbonate dominated lower section. remain to be analyzed for thermal conductivity, a comparison of computed temperature vs. depth plots for using both sets of thermal conductivities indicates that there are significant differences. The temperature vs. depth plots in Figure 3 show a difference of 5 C at a depth of 3500 m, which is due to a 20 to 30 percent lower thermal conductivity of measured values for Devonian formations. Hydrodynamic Disturbances The Geothermal Map of North America (Blackwell and Richards, 2004) shows an 80,000 km 2 area east of the Black Hills in central South Dakota and northern Nebraska where surface heat flows of mw m -2 occur (Figure 4). Data on this anomalous heat flow include hundreds of temperatures measured in flowing water wells (Adolphson and LeRoux, 1968; Schoon and McGregor, 1974), and bottom-hole temperatures obtained in oil exploration. Gosnold (1999) showed that the high heat flow is due to advective heat transport in a gravity-driven, confined aquifer system that formed when the Black Hills were uplifted during the Laramide Orogeny. Ground water enters Paleozoic strata that comprise the aquifer system through steeply dipping outcrops on the east flank of the Black Hills where it flows to depths of 2 km. The downward flow creates a region of low heat flow which is shown by the hatchures in Figure 4. The gravity-driven flow continues up dip eastward in gently westward dipping Paleozoic aquifers (Downey, 1986) and discharges through subcrop contacts into the Dakota Sandstone (Cretaceous). Figure 4. Midcontinent heat flow contour map generated by a minimum curvature process in Surfer. The Burton, Nebraska site designated A in the figure. High heat flow in southeastern Nebraska, site B, was observed in a mining exploration well drilled into a high radioactivity carbonatite body. High heat flow at Valentine, Nebraska, site C, is due to gravitydriven up dip regional groundwater flow on the west side of the Chadron Arch (Downey, 1986). The first insight into the heat flow anomaly came in 1981 when the Nebraska Conservation Division converted a gas exploration well near the NE-SD border to a heat flow hole. The exploration well, designated Burton after the nearest town, was drilled through 400 m of Pierre Shale (Cretaceous) and 350 m of Paleozoic carbonates and bottomed in the crystalline basement. Completion of the well for heat flow involved hanging 750 m of 1 ¼ in iron casing, grouting the annulus, plugging the bottom of the casing and filling it with water. The well was allowed to sit for a year to dissipate the thermal effects of drilling and then temperatures were logged at 1 m intervals. The temperature gradient is 94 K km -1 in the Pierre shale and 37 K km -1 in the carbonates. A measured thermal conductivity of 1.2 W m -1 K -1 for the Pierre Shale in SD, (Gosnold, Todhunter, and Schmidt, 1997 yields a heat flow of 112 and an estimated thermal conductivity of 3.03 W m -1 K -1 for the carbonates. Comparison of the measured temperature profile to a calculated temperature profile based on heat flow of 58 mw m -2 (Figure 5) shows that temperatures at depths greater than 1 km within the heat flow anomaly are nearly 50 C warmer than temperatures at similar depths outside the anomaly. This comparison is possible because the groundwater flow system has existed throughout the Tertiary and the crust is in thermal equilibrium. Summary The midcontinent of North America has promising targets for development of EGS, especially in the anomalously high heat 357

5 Blackwell, D.D., and Richards, M.C., 2004, Geothermal map of North America: Am. Assoc. Petroleum Geologists (Tulsa, Oklahoma), map, scale: 1:6,000,000. Downey, J. S., 1986, Geohydrology of bedrock aquifers in the northern Great Plains in parts of Montana, North Dakota, South Dakota, and Wyoming: U.S. geol. Surv. Prof. paper 1402E, 87p. Gosnold, W., Eversoll, D., 1981,Usefulness of Heat Flow Data in Regional Assessment of Low Temperature Geothermal Resources with Special Reference to Nebraska, Geothermal Resources Council, Vol. 5, Gosnold, W.D., Jr., 1991, Subsurface temperatures in the northern Great Plains, in Slemmons, D.B., Engdahl, E. R., Zoback, M.D., and Blackwell, D.D., eds, Neotectonics of North America: Geol. Soc. America Decade map vol. 1. P Gosnold, W. D., Jr., Todhunter, P. E., and Schmidt, W., 1997, The borehole temperature record of climate warming in the mid-continent of north America: Global and Planetary Change, v. 15, no. 1-2, p Gosnold, W.D., Jr., 1999, Stratabound geothermal resources in North Dakota and South Dakota: Natural Resources Research, v. 8, no. 3, p Gosnold, W.D., Jr., 1999, Basin-Scale groundwater flow and advective heat flow: An example from the northern Great Plains, in Geothermics in Basin Analysis, Forster and Merriam, Eds., Kluwer Academic/Plenum, p Figure 5. Temperature vs. depth plot for the Burton site with a calculated extension to 10 km compared to a normal temperature profile based on a heat flow of 58 mw m -2. The depth to the 150 C isotherm in the heat flow anomaly is 2.9 km less than it is outside the anomaly. flow area on the NE-SD border. Heat transport in deep, regional groundwater flow causes temperatures above 150 C to occur at depths of 5.5 km in a region of about 1500 km 2. Ongoing research on thermal conductivity, radioactive heat production, and temperature vs. depth logging may establish new areas of target for EGS development. The thermal conductivity data on the Williston Basin presented in Table 1 can be used for rocks of equivalent age and lithology for most of the midcontinent region. This data will improve accuracy and help develop resource estimates by calculating temperature vs. depth plots for other intracratonic basins in the midcontinent of North America. References Adolphson, D.G., and LeRoux, E.F., 1968, Temperature variations of deep flowing wells in South Dakota: U.S. Geol. Survey prof. paper 600-D, p Antriasian, A. M., 2010, The Portable Electronic Divided Bar (PEDB): A tool for measuring thermal conductivity of rock samples, Proceedings Wrold Geothermal Congress, Bali, Indonesia, Birch, F., Roy, R. F., and Decker, E. R., 1968, Heat flow and thermal history of New York and New England, in Zen et al., eds., Studies of Appalachian geology; Northern and Maritime: NY. Interscience, p Lachenbruch, A. H., 1970, Crustal temperature and heat production: Implications of the linear heat-flow relation: J. Geophys. Res., v. 75, p Morgan, P., and Gosnold, W. D., Jr., 1989, Heat flow and thermal regimes in the continental United States, in Packiser, L.C., and Mooney, W. D., eds., Geophysical Framework of the Continental United States: Geol. Soc. America Mem. 172, p Roy, R. F., Beck, A.E., and Touloukian, Y. S., 1970, Thermophysical Progerties of Rocks, in, Physical Properties of Rocks and Minerals, Touloukian, Y. S., Judd, W. R., and Roy, R. F., eds. McGraw Hill/CIDAS data series on material properties vol. II-2. Schoon and McGregor, 1974, Geothermal potentials in South Dakota: South Dakota Geol. Survey, Rept. Invest pp. Tester, J. W., Anderson, B., Batchelor, A., Blackwell, D., DiPippo, R., Drake, E., Garnish, J., Livesay, B., Moore, M.C., Nichols, K., Petty, S., Toksoz, N., Veatch, R., Augustine, C., Baria, R., Murphy, E., Negraru, P., Richards, M The future of geothermal energy: Impact of enhanced geothermal systems (EGS) on the United States in the 21 st century. Massachusetts Institute of Technology, DOE Contract DE-AC07-05ID14517 Final Report, 374 p. Touloukian, Y.S., Powell, R.W., Ho, C.Y., and Klemens, P.G., (1970, Thermal Properties of Matter, vol. 2, Thermal Conductivity, Non-Metallic Solids. New York and Washington: IFI/Plenum. Vitorello, I., and Polack, H.N., 1980, On the variation of continental heat flow with age and the thermal evolution of continents: J. Geophys. Res., v. 85, p Wollenberg, H. A., and Smith, A.R., 1987, Radiogenic heat production of crustal rocks; an assessment based on geochemical data, Geophys. Res. Lett., v. 14, p