Assessing the geothermal potential of the St. Lawrence Lowlands sedimentary basin in Quebec, Canada Jasmin Raymond, Département de génie mécanique – École de Technologie Supérieure, Montréal, Québec, Canada jasmin.raymond.1@ens.etsmlt.ca Michel Malo, Félix-Antoine Comeau, Karine Bédard, René Lefebvre Centre Eau Terre Environnement – Institut national de la recherche scientifique, Québec, Québec, Canada René Therrien, Département de géologie et de génie géologique, Université Laval, Québec, Québec, Canada ABSTRACT Geothermal power generation from subsurface fluids at moderate temperature can now be considered due to technological developments. Hydraulic fracturing enhances the permeability of a reservoir, which leads to greater groundwater production, and increases heat extraction. Hybrid binary power plants, where the fluid that drives the turbine is heated by warm groundwater and other energy sources, such as bio- or natural gas, can increase the efficiency of power cycles to produce electricity. Sedimentary basins with groundwater at temperatures from 80 to 120 °C are thus becoming targets for geothermal power generation. One such target is the Cambro-Ordovician sedimentary basin of the St. Lawrence Lowlands (SLL) in Quebec, where the geothermal potential is assessed by defining the depth distribution of permeable rock units within this temperature range. A 3D geological model of the SLL basin was built by combining the regional surface geological map, the structural map of the basement top defined by seismic wave travel times and the elevation of 441 geological contacts identified with geophysical well logs from 164 oil and gas exploration wells. Depths of geological units were extracted from the 3D model and mapped in 2D horizontal planes, over which 41 bottom-hole temperature measurements that were uncorrected have been superimposed.

The most permeable rock units with greater geothermal potential are the Cambrian sandstones of the Cairnside and Covey Hill Formations of the Postdam Group that are unconformably lying on the Precambrian basement at the base of the SLL sedimentary sequence. A series of steeply dipping southeastern normal faults, located at the border and within the basin, affect the basement and its sedimentary cover. The Cairnside and Covey Hill Formations are located at depths of less than 1 km to more than 5 km, with an increasing depth toward the southeast. Temperatures between 70 and 100 °C have been measured at depths of 3 to 4 km in these units. The in situ formation temperature can, however, be higher since temperatures were measured shortly after wells were drilled and they may not have reached equilibrium with host rock temperature. Correction of temperature data for this non-equilibrium effect will lead to a more accurate assessment of the geothermal potential of the SLL basin. 1 INTRODUCTION New technologies are currently being developed to produce electricity from geothermal resources of low temperature. One example is heat extraction from an enhanced geothermal system at temperatures between 80 and 120 °C at Cornell University (Tester et al. 2010). Hydraulic fracturing of the Grenvillian basement under the sedimentary sequences of the Appalachians has the potential to increase the permeability of the host rock to create an exploitable geothermal reservoir. Power generation combined with the direct use of the warm water resource for district heating is planned for that project. A second technology with potential to increase the efficiency of heat recovery from reservoirs is the utilization of supercritical CO2 as a heat transmission fluid in deep aquifers, which would also lead to CO2 sequestration. Numerical modeling of heat transfer in permeable formations at 100 °C has shown that CO2 is more efficient than water to extract thermal energy in deep aquifers (Randolph and Saar 2011). Other technologies related to power generation, such as hybrid binary power plants (Borsukiewicz-Gozdur 2010), are

expected to increase the efficiency of power production for low temperature geothermal resources. For example, a hybrid power plant is being constructed in the Rhine Valley, Germany, where electricity will be produced using a Kalina cycle with a working fluid heated by groundwater recovered at 120 °C and excess heat from a biogas plant to increase the system efficiency (Kreuter and Schrage 2010).

Successful development of these technologies will expand capabilities to economically exploit geothermal resources from sedimentary basins that host groundwater at temperatures above 80 °C. Such resources are abundant in Canada and have the potential to provide a large amount of energy with a low carbon footprint. A geological environment that has been extensively studied for that purpose is the Western Canadian sedimentary basin, which contains geothermal resources on the order of 16 to 31 × 1015 MJ (Grasby et al. 2011). The geothermal potential of other sedimentary basins in Canada, where oil and gas exploration was more limited and where less data is available, still has to be defined. One such target is the Cambro-Ordovician sedimentary basin of the St. Lawrence Lowlands (SLL) in Quebec,

where the potential for CO2 sequestration has been recently assessed (Malo and Bédard, In Press). Data gathered for the purpose of CO2 sequestration can also be used to delineate areas with significant geothermal potential. The study reported here describes work conducted to better define the stratigraphic settings and to build a 3D geological model of the autochthonous sedimentary sequence in the SLL basin to assess its geothermal potential. Bottom-hole temperature measurements from geophysical logs have been combined with the 3D geological model to identify the depth of geological formations having greater potential to produce groundwater with sufficiently high temperature for geothermal power generation and for direct use. The method used to develop the 3D geological model is briefly described and followed by maps of the depths of geological units with potentially suitable characteristics for geothermal energy, which are thus being targeted for geothermal exploration.

2 GEOLOGICAL SETTING In the province of Quebec, the SLL basin covers an area of about 20 000 km2 (Figure 1). The autochthonous sedimentary sequence of the SLL was deposited during the Cambrian and the Ordovician and underwent little to moderate deformations. The upper rock units are fine-grained siliciclastic rocks belonging to the Queenston, Loraine, Sainte-Rosalie and Utica groups, which are considered as caprocks of low permeability. Rock units that can be potential reservoirs are limestones of the Trenton, Black River and Chazy Groups, dolomites of the Beekmantown Group and sandstones of the Postdam Group (Comeau et al. 2012). A simplified stratigraphic column of the rock units in the SLL basin is shown in Figure 2. The porosity and permeability of potential reservoir units has been determined from measurements on core samples. Average and median porosities reported by Bédard et al. (2012), which are representative of the

Figure 1. Geological map of the SLL basin modified from Globensky (1987) according to Comeau et al. (2012). Accurate heat flow data available for the region are shown with yellow circles and were taken from Misener et al. (1951), Saull et al. (1962) and Fou (1969). The location of the seismic line shown in Figure 3 is indicated with the A-A’ red dashed line.

whole basin, and median horizontal permeability reported by Tran Ngoc et al. (In Press), which were measured on samples taken in the area southeast of Trois-Rivières (Figure 1), are summarized in Table 1. The rock units that have sufficient porosity and permeability to yield significant groundwater are the Cairnside and Covey Hill Formations of the Postdam Group. Those sandstone units were deposited in a rift environment during the Cambrian and unconformably overly the crystalline basement of the Grenville Province formed during the Precambrian. Steeply southeast dipping normal faults with a southwest-northeast orientation affect the sedimentary sequence, deepening and thickening the units towards the southeast of the SLL basin (Castonguay et al. 2010). Logan’s Line, a major thrust fault zone, delineates the southeastern boundary of the SLL basin, which extends under the Appalachians Province (Figure 1 and 3).

The Earth’s heat flux in the vicinity of the SLL basin, measured in five wells that were 250 to 760 m deep (Misener et al. 1951; Saull et al. 1962; Fou 1969), ranges. from 31 to 52 mW/m2 and geothermal gradients vary from 6 to 27 °C/km (Figure 1). Three of these wells were drilled in the SLL basin. The average thermal conductivity measured on shale samples of the Loraine, Sainte-Rosalie and Utica groups ranges between 1.27 and

Figure 2. Simplified stratigraphic column of the SLL basin sedimentary sequence (Comeau et al. 2012).

2.12 W/mK, which is smaller than the average value of 5.02 W/mK reported for dolomite and sandstone of the Beekmantown and Postdam groups (Saull et al. 1962).

Bottom-hole temperatures recorded in oil and gas exploration wells were collected by Majorowicz and Minea (In Press), who corrected the data to estimate equilibrium formation temperatures. Vertical variations of temperature and surface heat flux were calculated from corrected temperatures, accounting for heat generation due to decay of radioactive elements in the crustal rocks. Based on these corrections, an average surface heat flux of 57 mW/m2 was reported for the area covering the SLL basin and the Appalachian Province (Majorowicz and Minea, In Press). Estimates of temperatures at greater depths were extrapolated to produce temperature distribution maps at various depths (Figure 4). Although estimation of surface heat flow and extended temperatures at greater depths did not account for vertical changes of thermal conductivity according to the variation of rock units, the map shows potential thermal anomalies that still have to be confirmed with an assessment of the stratigraphy down to the basement and the measurement of thermal conductivities of rock units.

3 CREATION OF THE 3D GEOLOGICAL MODEL AND ASSOCIATED 2D MAPS

A three dimensional geological model of the SLL basin was built at a regional scale using the GOCAD software (Mallet, 1992). The geological map shown in Figure 1 was used to constrain the location of rock units at surface. A structural map of the basement top in two-way travel time units derived from seismic data was used to constrain geometry and depth of the Precambrian basement. Seismic wave two-way travel time was converted using Precambrian depths from well data, since there is no unique algorithm available for the area to convert two-way travel time to depth. Geophysical logs from 164 oil and Table 1. Porosity and permeability of reservoir rocks in the SLL basin.

Group (Formation)

Trenton, Black River, Chazy

Beekmantown (Beauharnois-

Theresa)

Postdam (Cairnside-Covey Hill)

Porosity1

Average (%) 0.88 1.25-1.49 3.77-6.09

25 percentile (%) 0.40 0.13-0.10 2.10-3.50

Median (%) 0.55 0.75-0.4 3.50-5.70

75 percentile (%) 1.15 1.85-1.70 5.00-8.30

Permeability2

25 percentile (mD*) 0.01 0.02-0.02 0.03-0.10

Median (mD*) 0.01 0.07-0.05 0.12-0.24

75 percentile (mD*) 0.04 0.50-0.20 0.30-0.60

1: Bédard et al. (2012), 2: Tran Ngoc et al. (In Press), * mD: miliDarcy.

Figure 3. Geological cross-section based on the interpretation of the seismic profile M2001 (Castonguay et al. 2010) showing the location of the SLL basin rock units relative to the Precambrian basement and the Appalachians. Location of the seismic line is given in Figure 1.

Figure 4. Temperature estimated at 4 km depth based on the extension of corrected bottom-hole temperatures from geophysical logs and drill stem tests (Majorowicz and Minea, In Press). gas exploration wells were used to determine the elevations of formation contacts. Among these well logs, 89 were re-interpreted to better define the contacts. The revised contacts were integrated to those of the other wells to provide an extensive dataset and built the 3D geological model. A total of 441 contacts were positioned at depth; 62 for the top of the Cairnside Formation, 30 for

the top of the Covey Hill Formation and 31 for the top of the Precambrian basement. Locations of major faults were interpreted according to the map of the Precambrian basement depth and differences in elevation between the formation contacts in the wells.

The depths of the top of formations of interest for geothermal energy, which are the Cairnside and Covey

Hill Formations as well as the Precambrian basement, were plotted in two dimensions to better visualize their geometry (Figures 5, 6 and 7). The surface location of Logan’s Line and the interpreted seismic profiles available in the area were indicated on the maps. Bottom-hole temperatures recorded with geophysical logs that intersected these units were superimposed on the 2D maps to evaluate temperature at depth. The temperatures shown on the maps are uncorrected measurements that were recorded shortly after the wells were drilled and are not in equilibrium with undisturbed formation temperatures. A total of 41 temperature measurements taken from 35 wells were plotted on the maps; 14 measurements were recorded in the Cairnside Formation, 11 in the Covey Hill Formation and 16 in the basement. The temperature data were not interpolated to create contour maps because the available points are spread over a large area, which can induce significant uncertainty. The temperature value at each well is shown on the maps with a circle that increases in size with increasing temperature.

4 GEOTHERMAL POTENTIAL ASSESMENT

Heat transfer in sedimentary rocks with low groundwater flow mostly occurs by conduction. In these conditions, the Earth’s heat flux, q (W/m2), is given by:

z

Tkq

∆

∆×= [1]

where k (W/mK) is the thermal conductivity of the host rock and ∆T/∆z (K/m) is the geothermal gradient. The Earth’s heat flux is typically constant over a relatively wide area, so that a decrease of the host rock thermal conductivity results in an increase of the geothermal gradient. The greatest geothermal potential in a sedimentary basin is therefore expected where permeable formations, which can yield groundwater, are overlain by thick caprock with low thermal conductivity.

On the southeast side of SLL basin, the Cairnside and the Covey Hill formations are at depths that range from 3 km to more than 5 km (Figures 5, 6 and 7). Wells with uncorrected bottom-hole temperatures in the range of 70 to 100 °C intersect these formations between the depths of 3 and 4 km. These wells are located along Logan’s Line, where the thickness of the caprock is on the order of 2 to 4 km.

Sedimentary rocks of the Lorraine, Sainte-Rosalie and Utica groups are mostly shales and siltstones that are expected to have a low thermal conductivity (Clauser 2006). These units can thus provide efficient thermal insulation that could maintain greater temperatures at depth. Rocks of the Appalachian Province overlying those of the SLL basin can have more variable thermal conductivities because of Cambro-Ordovician tectonic activities that displaced sedimentary rocks of various

Figure 5. Depth of the top of the Cairnside Formation and uncorrected bottom-hole temperature measurements that intersected this unit.

Figure 6. Depth of the top of the Covey Hill Formation and uncorrected bottom-hole temperature measurements that intersected this unit.

Figure 7. Depth of the top of the Precambrian basement and uncorrected bottom-hole temperature measurements that intersected this unit.

origins. The areas with the most promising geothermal potential are consequently located along Logan’s Line and to the southeast of this lineament, where the rock units of the Postdam Group have a depth of more than 3 km (Figure 2). This hypothesis is supported by a well located in this zone that has a bottom-hole temperature of 99 °C at 3.1 km depth to the northeast of the SLL basin (Figure 5). Two other wells located to the southeast of Logan’s Line, which are not shown on the maps of Figures 5 to 7 because they did not reached formations of the Postdam Group, had temperatures of 86 and 87 °C at 3,8 and 3,5 km depth, respectively. The increase of drilling depth to reach the formations of interest southeasterly is however a factor that can limit geothermal development due to increasing drilling cost. On the northwest side of Logan’s Line, the thickness of the caprock is generally less than 3 km, which can reduce the rate of temperature increase with depth.

Among caprock of low thermal conductivity, elevated temperature in formations of the Postdam Group could also be explained by groundwater up flow driven by changes in water density at different temperatures, creating convective movements with rising warmer groundwater. The normal faults that offset the geological units at depth may be preferential paths for groundwater movement. Heat generation due to the decay of radioactive elements concentrated in intrusive rocks that potentially cross-cut the Precambrian basement may also create higher temperatures. However, these conjectures are to be taken with a lot of caution, as there is a lack of temperature and thermal properties measurements in wells of the SLL basin, both in terms of number of measurements and quality of data. The actual validity of thermal anomalies and explanation for their presence still have to be verified with further scientific studies in the SLL basin.

5 CONCLUSIONS

Maps presented in this manuscript (Figures 5, 6 and 7), which were generated from a 3D geological model of the SLL basin, indicate that the Cairnside and the Covey Hill Formations, at depths of more than 3 km, are potential targets for low temperature geothermal systems, since temperatures in these units can potentially be above 80 °C. Below these formations, the Precambrian basement could be a target for enhanced geothermal systems. Temperature can be higher at greater depth in the basement and is expected to remain maximal under the zone where the shaly caprock units are thicker along Logan’s Line and to the southeast.

Bottom-hole temperatures from geophysical logs plotted on the maps were recorded after the wells were drilled and may not have been in equilibrium with formation temperatures. Further work to combine corrected bottom-hole temperatures provided by Majorowicz and Minea (In Press) to the 3D geological model of Bédard et al. (2012) will provide a better assessment of the geothermal potential of the SLL basin. There is also a need to collect more heat flow data in the SLL basin to better define the spatial distribution of

underground temperatures. This task may be difficult to achieve in deep wells since most of the wells that reach the target zone at 3 to 4 km depths have been drilled in the late 1970’s. More measurements of host rocks thermal conductivities and of geothermal gradients could however be feasible in shallower wells that have been recently drilled for oil and gas exploration. Imaging the deep basement structures and lithologies with gravimetric and magnetic surveys could additionally provide insights on the locations of major granitic intrusions that could help explain thermal anomalies, once their presence has been verified. Temperatures at depth could then be predicted with numerical simulations of heat transfer constrained by field observations collected during the recommended work.

Economic exploitation of geothermal resources of the SLL basin will depend on the efficiency of the new technologies that are being developed to produce electricity from groundwater of moderate temperature. The work of Borsukiewicz-Gozdur (2010) indicates that the thermal efficiency of a hybrid power plant operating with geothermal fluids at 100 °C is at least twice that of conventional Organic Rankine Cycle power plants. Hybridization of power plant could be an asset for an economic exploitation of the geothermal resources in the SLL basin and will require further thermodynamic simulations of power cycles to demonstrate their potential economic efficiency. Such exploitation of geothermal resources can be classified as a direct use because groundwater in a hybrid plant is used to preheat the working fluid that drives the turbines. The geothermal assessment carried out by the Geological Survey of Canada indicated potential for direct utilization of geothermal resources in sedimentary basins (Grasby et al. 2011), which has been confirmed for the SLL basin in this manuscript according to preliminary observations.

6 REFERENCES

Bédard, K., Comeau, F.-A. and Malo, M. 2012. Capacité effective de stockage géologique du CO2 dans le bassin des Basses-Terres du Saint-Laurent. Institut national de la recherche scientifique - Centre Eau Terre Environnement, INRSCO2-2012-V3.1, Québec, 21 pp, Available on the Internet: http://chaireco2.ete.inrs.ca/en/publica-tions_en.

Borsukiewicz-Gozdur, A. 2010. Dual-fluid-hybrid power plant co-powered by low-temperature geothermal water. Geothermics 39 (2): 170–176.

Castonguay, S., Lavoie, D., Dietrich, J. and Laliberté, J.-Y. 2010. Structure and petroleum plays of the St. Lawrence Platform and Appalachians in southern Quebec: insights from interpretation of MRNQ seismic reflection data. Bulletin of Canadian Petroleum Geology 58 (3): 219–234.

Clauser, C. 2006. Geothermal energy. In: Heinloth, K. (ed.), Landolt-Börnstein, Group VII: Advanced Materials and Technologies, Vol. 3: Energy Technologies, Subvol. C: Renewable Energies, Sprigner Verlag, Heidelberg-Berlin, p 493–604.

Comeau, F.-A., Bédard, K. and Malo, M. 2012. Les régions de Nicolet et de Villeroy: état des connaissances pour le stockage géologique du CO2. Institut national de la recherche scientifique - Centre Eau Terre Environnement, INRSCO2-2012-V1.3 Québec, 52 pp, Available on the Internet: http://chaireco2.ete.inrs.ca/en/publica-tions_en.

Fou, J. T. K. 1969. Thermal conductivity and heat flow at St. Jerôme, Quebec. M.Sc. Thesis, McGill University, Montreal, 79 pp.

Grasby, S. E., Allen, D. M., Chen, Z., Ferguson, G., Jessop, A. M., Kelman, M., Ko, M., Majorowicz, J., Moore, M., Raymond, J. and Therrien, R. 2011. Geothermal energy resource potential of Canada. Geological Survey of Canada, Open file 6914, Calgary, 322 pp.

Kreuter, H. and Schrage, C. 2010. The hybrid power plant Neuried, Germany. Proceedings of the World Geothermal Congress, Bali, 5 pp.

Majorowicz, J. and Minea, V. In Press. Geothermal energy potential in the St-Lawrence River area, Québec. Geothermics.

Mallet, J.L. 1992. GOCAD: a computer aided design program for geological applications. In: Turner, A.K. (ed.), Three-dimensional modeling with geoscientific information systems, Kluwer Academic Publisher, Dordrectht, 123–141.

Malo, M. and Bédard, K. In Press. Basin-scale assessment for CO2 storage prospectivity in the Province of Québec, Canada. Energy Procedia.

Misener, A. D., Thompson, L. G. D. and Uffen, R. J. 1951. Terrestrial heat flow in Ontario and Quebec. American Geophysical Union Transactions 32: 729–738.

Randolph, J. B. and Saar, M. O. 2011. Combining geothermal energy capture with geologic carbon dioxide sequestration. Geophysical Research Letters 38, doi: 10.1029/2011gl047265.

Saull, V. A., Clark, T. H., Doig, R. P. and Butler, R. B. 1962. Terrestrial Heat Flow in the St. Lawrence Lowland of Québec. Canadian Mining and Metallurgical Bulletin 65: 63–66.

Tester, J. W., Joyce, W. S., Brown, L., Bland, B., Clark, A., Jordan, T., Andronicos, C., Alllmendinger, R., Beyers, S., Blackwell, D., Richards, M., Frone, Z. and Anderson, B. 2010. Co-generation opportunities for lower grade geothermal resources in the Northeast - a case study of the Cornell Site in Ithaca, NY. GRC Transactions 34: 475–483.

Tran Ngoc, T. D., Konstantinovskaya, E., Lefebvre, R. and Malo, M. In Press. Caractérisation hydrogéologique et pétrophysique des aquifères salins profonds de la région de Bécancour pour leur potentiel de séquestration géologique du CO2. Institut national de la recherche scientifique - Centre Eau Terre Environnement, INRSCO2-2011-V2.6, Québec.