Thermostratigraphy of the Williston Basin

Transcription

1 GRC Transactions, Vol. 36, 2012 ostratigraphy of the Williston Basin William D. Gosnold 1, Mark R. McDonald 1, Robert Klenner 1, and Daniel Merriam 2 1 Department of Geology and Geological Engineering, University of North Dakota, Grand Forks ND 2 Kansas Geological Survey, Lawrence KS Keywords Heat flow, thermal conductivity, thermal gradient, boreholetemperatures, co-produced fluids, sedimentary basins Abstract We present a scheme for determining temperatures of strata in a sedimentary basin using heat flow, formation lithology, thickness and thermal conductivity. We calibrated the scheme on five sites in the Williston Basin where temperature vs. depth profiles enabled an iterative analysis of temperature gradient, thermal conductivity and heat flow. Comparison of the temperature projections to bottom hole temperatures provides insight on determining a reliable correction for BHT data. Large scale application of the scheme using stacked structure contours can provide a complete and accurate assessment of geothermal resources in a basin. The Geothermal Resource in Sedimentary Basins Large-scale adoption of alternatives to fossil fuels has long been delayed for a number of reasons such as non-competitive economics, marginal technology, and the apparent abundance of conventional resources. At present, the economic disincentive and its attendant effects are declining due to record-high crude oil prices. Consequently, opportunities for shifting to alternative energies such as geothermal, nuclear, solar, wind, unconventional gas, hydrogen, and ethanol are growing. A potentially significant, accessible, sustainable and environmentally benign domestic energy resource is geothermal energy in sedimentary basins. The geothermal resource in sedimentary basins includes hot waters that are coproduced with oil and gas, hot waters from permeable formations and the heat energy stored in impermeable formations. The thermal energy in coproduced fluids is estimated to be between 9.44 x J and 4.51 x J (McKenna et al, 2005) and the total thermal energy in permeable sedimentary formations is estimated to be approximately 1 x J (Tester et al., 2006). However, the estimate by Tester et al. (2006) was based on a prior USGS report (Sorey et al., 1983) that considered only one or two aquifers in each basin. The estimate by Sorey et al., (1983) was based on only the principal water-producing formations and generally excluded petroleum-bearing formations. A later analysis (Gosnold, 1999a) indicated that if all formations that have capacity for hot water production are considered, the resource would be significantly greater. Specifically, the USGS estimate of the resource for North Dakota and South Dakota (Sorey et al., 1983) included only the Dakota Group and the Madison aquifer in the Williston and Kennedy basins and totaled approximately 8.2 x J. Analysis of all formations with capacity for fluid production in those basins indicates that the accessible resource base is approximately 6.6 x J (Gosnold, 1999a). If this analysis applies to all basins, the total resource might be eight times larger than the estimate reported by Tester et al. (2006). Perhaps more significant is that the resource base would be yet an order of magnitude larger if the heat contained in impermeable formations could be extracted with EGS technology or down-hole heat exchanger systems (Gosnold et al., 2010). The essential data for assessing the feasibility of producing geothermal power are temperature and depth of the resource and availability of fluid to transport the energy to the surface. Additional considerations include detailed stratigraphy, fluid composition, surface climate, geographic location, and accessibility of the power grid, as well as hydrologic, mechanical, geochemical, and thermal properties of the basin. We present five calibration tests of thermostratigraphy to estimate subsurface temperatures and to iteratively determine heat flow and thermal conductivity. Methodology ostratigraphy has been applied in regional and detailed assessments of geothermal resources in sedimentary basins (Gosnold, 1984, 1991, 1999b; Gosnold et al., 2010; Crowell and Gosnold, 2011; Crowell et al., 2011) and in the Geothermal Map of North America (Blackwell and Richards, 2004). Assuming heat flow, q, is conductive and constant, the temperature gradient, dt dz, varies inversely with thermal conductivity (λ) according to 663

2 Fourier s law, q = dt dz λ (1) and the temperature at depth can be calculated from, n qz T( z) = T0 + i (2) i=1 λ i where: T (z) is temperature at depth z, T 0 is surface temperature, q is heat flow (mw m -2 ), z i is formation thickness (m), λ i is the formation thermal conductivity ( ) and dt dz (K km-1 ) is the temperature gradient. Hereafter we refer to Eq. 2 as TSTRAT. Heat Flow The most critical element in thermostratigraphy is reliability of the heat flow data. Adequate sampling of continental heat flow would require boreholes spaced on a 10 km X 10 km grid and thermal conductivity measurements on cores extracted from multiple levels in the boreholes. The 10 km X 10 km grid spacing, 1 site per 100 km 2, would allow discrimination of variations in heat flow due to variability of radiogenic sources in the crust and due to different heat flow provinces (Roy et al., 1968). Unfortunately, the actual density of conventional heat flow sites in the Williston Basin is closer to 1 site per 11,000 km 2. Also of concern is that few published heat flow data include evaluations of quality that can guide the user. We address these matters for the Williston Basin by examination of prior heat flow research in the basin and by calibrating projections of TSTRAT with equilibrium temperature vs. depth logs from five boreholes ranging in depth from 940 m to 2,845 m. We then use TSTRAT to calculate temperatures on formation tops to the Precambrian surface at each of the five sites Figure 1. Locations of Williston Basin heat flow sites (black triangles and solid red star), deep wells with equilibrium temperatures (open red stars) and cored well sites with thermal conductivity measurements (open diamonds). Numbers identify wells in Figures 3-7 and Tables 2-6. Solid red star designated Lone Tree is an oil field site where the first heat flow measurement in the basin was made (Blackwell, 1969). and compare the results with BHT data. The first heat flow reported for the Williston Basin was 58.6 mw m -2 in the Lone Tree oil field (Fig. 1) based on a measured temperature gradient of 39.9 K km -1 and an estimated thermal conductivity of 1.5 (Blackwell, 1969). Combs and Simmons (1973) reported heat flow of 92 mw m -2 at two sites, NDGS 3342 and NDGS 3479, (Fig. 1) near the Lone Tree site based on measured temperature gradients of 55 K km -1 and 56 K km -1 and estimated thermal conductivity of 1.7. Scattolini (1978) reported heat flow for 31 sites in North Dakota, but rated only seven of the sites as high quality. These seven sites average 55.3 ± 15.7 mw m -2 based on measured temperature gradients and thermal conductivities measured on chips and fragments as described by Sass et al., (1971). Majorowicz et al., (1986) reported heat flows of 70 to 100 mw m -2 for portions of the Williston Basin in northwestern North Dakota, southwestern Manitoba and southeastern Saskatchewan based temperature gradients calculated from bottom hole temperatures (BHT) and estimated thermal conductivities. Gosnold (1990) reexamined these previous heat flow determinations and found the reported high heat flow was due to thermal conductivity estimates that were too high by as much as 40 percent. All temperature gradients used in heat flow determinations in the Williston Basin were measured in shales, mudstones and s that are the dominant lithology in the upper 1 km to 2 km. In the subsurface these rock types are solid heat conductors, but when extracted by coring they quickly undergo decompaction and dehydration. Consequently measurement of thermal conductivity must occur within hours of core extraction and the samples must be sealed to prevent dehydration and compressed to prevent decompaction. Unfortunately none of the measurements by Scattolini (1978) met the necessary requirements for handling and all other thermal conductivities used in the heat flow determinations were estimated. The heat flow estimates by Combs and Simmons (1973); Scattolini (1978); Majorowicz et al., (1986) and Blackwell (1969) referenced thermal conductivities of 1.7 reported by Benfield (1947) and Garland and Lenox (1962), and we infer that those values influenced estimates for the shales in the Williston Basin. The question of thermal conductivity of shales and similar rocks was best answered by Blackwell and Steele (1989) who used temperature gradients in a shale sandwich between Paleozoic s in boreholes in Kansas. al conductivities of the s were measured and heat flow in the s was used with the temperature gradients to determine that the thermal conductivity of the shales is of the order of 1.1. Gosnold et al., (1997) used a half-space needle probe technique to measure thermal conductivities of 1.1 and 1.2 W m -1 on fresh cores from the Pierre shale in southern Manitoba and South Dakota and the Eagleford shale in Texas. Recently, we were fortunate to obtain 23 fresh cores from a 1-km deep scientific borehole drilled for methane tests in the Pierre shale in southern Manitoba. al conductivity measurements made with a portable electronic divided bar (Antriasian, 2010) average 0.9 K - ± 0.26 and show an exponential decrease with depth described by λ = e -6E- 4z (3) which we infer is due to increasing methane content. 664

3 within formations (Figure 2), thus selecting a single value for a specific formation is questionable. However the range of thermal conductivity variation is useful in fitting calculated temperatures to observed equilibrium profiles. In this analysis we use five temperature vs. depth profiles that were measured in boreholes at thermal equilibrium. Four of the profiles are entirely in the shale section, but one profile, NDGS 6840, reached a depth of 2845 m and extends through the Madison Group carbonates. The temperature gradient in the Madison between 2640 and 2845 m averages 16.9 ± 2.4 K km -1 but we do not have thermal conductivity measurements on Madison cores from those depths at nearby wells. However, the average temperature gradient in the shale section of NDGS 6840 is 46.9 ± 11.6 K km -1 Figure 2. al conductivities vs. depth measured with a divided bar on 15 formations in the Williston Basin. lithologies are given in Tables 1-5. Figure 3. Temperature vs. depth (smooth blue line) and gradient vs. depth (jagged multi-colored line) in NDGS 6840, a 2,850 m deep well on the North Dakota Montana border. The temperature gradient in the shale units (green section) is 46.9 ± 11.6 K km -1 and the gradient in the carbonates (purple section) averages 21.4 ±9.2 K km -1. TSTRAT Tests The Williston Basin has a bimodal composition with a 1 to 2 km thick layer of Cenozoic and Mesozoic strata consisting principally of shales overlying 2 to 3 km of Paleozoic s and dolomites. There are 54 distinct formations within the Williston Basin and thermal conductivity values have been measured on only fourteen of the Paleozoic formations and one of the Mesozoic formations (Gosnold et al., 2010). Interestingly, thermal conductivity was found to vary significantly Table 1. TSTRAT calculations for NDGS Heat flow used for calculations is 51 mw m-2 except in upper 1 km where post-glacial warming (Gosnold et al., 2011; Majorowicz et al., 2012) has reduced the temperature gradient by about 10 percent. NDGS 6840 Depth meters Temp. C W m-1 K-1 Murphy et al., 2009 Tertiary Cannonball sandstone, siltstone, Ludlow sandstone, siltstone, Cretaceous Hell Creek sandstone, siltstone, Fox hills mudstone, siltsone, sandstone Pierre shale Niobrara shale Carlille shale Greenhorn shale Belle fourche shale Mowry shale Newcastle sandstone Skull Creek shale Inyan Kara sandstone, shale Jurassic Swift shale Rierdon shale, Piper shale, gypsum, Triassic Spearfish siltstone, sandstone, mudstone Minnekahta Opeche shale to mudstone Broom Creek sandstone, dolomite, anhydrite Permian Amsden dolostone Tyler shale to mudstone Pennnsylvanian Otter shale to mudstone Kibbey sandstone Mississippian Charles Mission Canyon Lodgepole Ordovician Bakken shale Three Forks dolostone and Bird Bear Duperow Souris R Dawson Bay and dolostone Prairie evaporites Winnepegosis and dolostone Silurian Interlake dolostone and Stonewall dolostone and Stony Mountain dolostone and Red River Winnipeg carbonaceous sandstone Cambrian Deadwood , sandstone, shale 665

4 Table 2. TSTRAT calculations for NDGS Heat flow used for calculations is 51 mw m-2 except in upper 1 km where post-glacial warming (Gosnold et al., 2011; Majorowicz et al., 2012) has reduced the temperature gradient by about 10 percent. NDGS 5086 Depth meters Temp. C Murphy et al., 2009 Tertiary Golden Valley , sandstone, siltstone Sentinal Butte sandstone, siltstone, Slope sandstone, siltstone, Cannonball sandstone, siltstone, Ludlow sandstone, siltstone, Cretaceous Hell Creek sandstone, siltstone, Fox hills mudstone, siltsone, sandstone Pierre shale Niobrara shale Carlille shale Greenhorn shale Belle Fourche shale Mowry shale Newcastle sandstone Skull Creek shale Inyan Kara sandstone, shale Jurassic Swift shale Rierdon shale, Piper shale, gypsum, Triassic Spearfish siltstone, sandstone, mudstone Minnekahta Opeche shale to mudstone Broom Creek sandstone, dolomite, anhydrite Permian Amsden dolostone Tyler shale to mudstone Pennnsylvanian Otter shale to mudstone Kibbey sandstone Mississippian Charles Mission Canyon Lodgepole Ordovician Bakken shale Three Forks dolostone and Bird Bear Duperow Souris R Dawson Bay and dolostone Prairie evaporites Winnepegosis and dolostone Silurian Interlake dolostone and Stonewall dolostone and Stony Mountain dolostone and Red River Winnipeg carbonaceous sandstone Cambrian Deadwood , sandstone, shale Table 3. TSTRAT calculations for NDGS Heat flow used for calculations is 51 mw m-2 except in upper 1 km where post-glacial warming (Gosnold et al., 2011; Majorowicz et al., 2012) has reduced the temperature gradient by about 10 percent. NDGS 2894 Depth meters Temp. C Murphy et al., 2009 Tertiary Golden Valley , sandstone, siltstone Sentinal Butte sandstone, siltstone, Slope sandstone, siltstone, Cretaceous Hell Creek sandstone, siltstone, Fox hills mudstone, siltsone, sandstone Pierre shale Niobrara shale Carlille shale Greenhorn shale Belle Fourche shale Mowry shale Newcastle sandstone Skull Creek shale Inyan Kara sandstone, shale Jurassic Swift shale Rierdon shale, Piper shale, gypsum, Triassic Spearfish siltstone, sandstone, mudstone Minnekahta Opeche shale to mudstone Broom Creek sandstone, dolomite, anhydrite Permian Amsden dolostone Tyler shale to mudstone Pennnsylvanian Otter shale to mudstone Kibbey sandstone Mississippian Charles Mission Canyon Lodgepole Ordovician Bakken shale Three Forks dolostone and Bird Bear Duperow Souris R Dawson Bay and dolostone Prairie evaporites Winnepegosis and dolostone Silurian Interlake dolostone and Stonewall dolostone and Stony Mountain dolostone and Red River Winnipeg carbonaceous sandstone Cambrian Deadwood , sandstone, shale 666

5 Figure 4. Plot of equilibrium temperatures vs. depth (solid line), computed temperatures (triangles) and bottom-hole temperatures (solid red circles) near NDGS Temperature vs. depth data are from Blackwell, pers. comm., Bottom-hole temperatures are from oil exploration wells that lie within 10 km of NDGS Computed temperatures are from Eq. 2 and are shown with thermal conductivities and depths to formation tops in Table 1. Figure 5. Plot of equilibrium temperatures vs. depth (solid line), computed temperatures (triangles) and bottom-hole temperatures (solid red circles) near NDGS Temperature vs. depth data are from Scattolini (1978). Bottomhole temperatures are from oil exploration wells that lie within 10 km of NDGS Computed temperatures are from Eq. 2 and are shown with thermal conductivities and depths to formation tops in Table 2. Figure 6. Plot of equilibrium temperatures vs. depth (solid line), computed temperatures (triangles) and bottom-hole temperatures near NDGS Temperature vs. depth data are from Scattolini (1978). Bottom-hole temperatures are from oil exploration wells that lie within 10 km of NDGS Computed temperatures are from Eq. 1 and are shown with thermal conductivities and depths to formation tops in Table 3. Figure 7. Plot of equilibrium temperatures vs. depth (solid line), computed temperatures (triangles) and bottom-hole temperatures (solid red circles) near NDGS Temperature vs. depth data are from Combs (1970). Bottom-hole temperatures are from oil exploration wells that lie within 10 km of NDGS Computed temperatures are from Eq. 2 and are shown with thermal conductivities and depths to formation tops in Table 4. Figure 8. Plot of equilibrium temperatures vs. depth (solid line), computed temperatures (triangles) and bottom-hole temperatures near NDGS Temperature vs. depth data are from Combs (1970). Bottom-hole temperatures are from oil exploration wells that lie within 10 km of NDGS Computed temperatures are from Eq. 1 and are shown with thermal conductivities and depths to formation tops in Table

6 Table 4. TSTRAT calculations for NDGS Heat flow used for calculations is 51 mw m-2 except in upper 1 km where post-glacial warming (Gosnold et al., 2011; Majorowicz et al., 2012) has reduced the temperature gradient by about 10 percent. NDGS 3479 Depth Temp. meters C Murphy et al., 2009 Tertiary Cannonball sandstone, siltstone, Ludlow sandstone, siltstone, Cretaceous Hell Creek sandstone, siltstone, Fox hills mudstone, siltsone, sandstone Pierre shale Niobrara shale Carlille shale Greenhorn shale Belle fourche shale Mowry shale Newcastle sandstone Skull Creek shale Inyan Kara sandstone, shale Jurassic Swift shale Rierdon shale, Piper shale, gypsum, Triassic Spearfish siltstone, sandstone, mudstone Minnekahta Opeche shale to mudstone Broom Creek sandstone, dolomite, anhydrite Permian Amsden dolostone Tyler shale to mudstone Pennnsylvanian Otter shale to mudstone Kibbey sandstone Mississippian Charles Mission Canyon Lodgepole Ordovician Bakken shale Three Forks dolostone and Bird Bear Duperow Souris R Dawson Bay and dolostone Prairie evaporites Winnepegosis and dolostone Silurian Interlake dolostone and Stonewall dolostone and Stony Mountain dolostone and Red River Winnipeg carbonaceous sandstone Cambrian Deadwood , sandstone, shale Table 5. TSTRAT calculations for NDGS Heat flow used for calculations is 49 mw m-2 except in upper 1 km where post-glacial warming (Gosnold et al., 2011; Majorowicz et al., 2012) has reduced the temperature gradient by about 10 percent. NDGS 3342 Depth meters Temp. C Cretaceous Pierre shale Niobrara shale Carlille shale Greenhorn shale Belle Fourche shale Mowry shale Newcastle sandstone Skull Creek shale Murphy et al., 2009 Inyan Kara sandstone, shale Jurassic Swift shale Rierdon shale, Piper shale, gypsum, Triassic Spearfish siltstone, sandstone, mudstone Minnekahta Opeche shale to mudstone Broom Creek sandstone, dolomite, anhydrite Permian Amsden dolostone Tyler shale to mudstone Pennnsylvanian Otter shale to mudstone Kibbey sandstone Mississippian Charles Mission Canyon Lodgepole Ordovician Bakken shale Three Forks dolostone and Bird Bear Duperow Souris R Dawson Bay and dolostone Prairie evaporites Winnepegosis and dolostone Silurian Interlake dolostone and Stonewall dolostone and Stony Mountain dolostone and Red River Winnipeg carbonaceous sandstone 668

7 Well Name and it is reasonable to accept a thermal conductivity of 1.1 for the shales. This yields a heat flow of 51.6 mw m -2 for that site. Assuming constant heat flow in the borehole, we calculated thermal conductivity of the Madison as Using heat flow of 51 mw m-2 and adjusting thermal conductivities of each formation penetrated by the borehole, we used TSTRAT to fit a calculated temperature profile to the observed profile. We then calculated temperatures on all formation tops from the bottom of the observed temperature data to the Precambrian basement. (Figure 4, Table 1). thicknesses were taken from NDGS 6840 well log that was downloaded from the North Dakota Industrial Commission (NDIC) website oilgas/. The borehole did not reach basement but we were able to estimate formation thickness from other wells and from the North Dakota Stratigraphic Column (Murphy et al., 2009). We applied the same analysis to each of the other four wells and added the bottom hole temperature data from all boreholes within a 10 km of the well (Figures 5-8, Tables 1-6). A small but persistent misfit between the calculated temperature vs. depth profile and the observed profiles occurs in the upper km of each of the five boreholes. We attribute this misfit to a transient disturbance of the temperature gradient in the upper 1 km from the effects of post-glacial warming (Gosnold et al., 2011; Majorowicz et al., 2012). We adjusted the calculations by using a lower heat flow in the upper sections of the profiles. Discussion NDGS No. Gradient Interval C km -1 m Heat Flow mw m -2 Gradient C km -1 The critical elements necessary to apply TSTRAT in sedimentary basin are a reliable estimate of heat flow and accurate thermal conductivity data. Stratigraphic data are essential, but heat flow and thermal conductivity control the geothermal gradient. To emphasize how critical accurate heat flow data are, we compare the heat flow determinations we made in this analysis for six wells in the Williston Basin with previously published heat flow data (Table 6). The differences between our heat flow calculations and the previously published heat flow values range from 31 percent to 91 percent and are entirely due to lack of accurate thermal conductivities on shales. That is no fault of the earlier researchers who made 669 the best possible estimates for shale conductivities. Ref. However, such data exist in the literature and on databases and can lead to serious missteps in estimating subsurface temperatures. For example, the Geothermal Map of North America (Blackwell and Richards, 2004) shows heat flow of 60 mw m-1 to 75 mw m-2 in the regions in the Williston Basin that include the wells in Table 6. The comparison of calculated temperatures with bottom hole temperatures gives an expected result, i.e., BHTs underestimate subsurface temperatures. The comparison also leads us to suggest that an improved correction scheme for BHTs can be developed by this analysis. Calculation of a general temperature vs. depth profile for a basin based on heat flow, lithostratigraphy and thermal conductivity can be compared to a temperature vs. depth plot of BHTs. Fitting reasonable curves, i.e., linear or 2 nd order polynomials, to each data set and determining the difference between them yields a curve that could be used to correct the average value of a group of BHTs. The principal application of TSTRAT in assessing geothermal resources would be to generate temperature contour maps for aquifers in a basin. We are beginning to apply this concept to a number of basins in the mid-continent region and anticipate an improved assessment of geothermal resources will result. Heat Flow mw m -2 Carrie Hovland a E.L. K. 1 Nelson a Shell USA * * * b b Lone Tree? c Table 6. Heat flow determined is this analysis in shown in on the left in black text and previously published heat flow determinations for the same sites is shown on the right in red text. References: a = Combs and Simmons, 1973; b = Scattolini, 1978; c = Blackwell, References Antriasian, A. M., 2010, The portable Electronic Divided Bar (PEDB): A tool for measuring thermal conductivity of rock samples, Proc. World Geothermal Congress 2010, 6 pp. Benfield, A. E., (1947), A heat flow value for a well in California, Am. J. Sci. 245, Blackwell, D. D., 1969, Heat flow determinations in the northwestern United States, J. Geophys. Res., 74, Blackwell, D.D., and Steele, J. A., 1989, Heat flow and geothermal potential of Kansas, Kan. Geol. Surv. Bull., 226, Geophysics in Kansas, PP Blackwell, D.D., and M. Richards, Geothermal Map of North America, U.S. Subset. Amer. Assoc. 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