Will Gosnold

Anomalously low heat flow density in eastern Karelia, Baltic Shield: a possible palaeoclimatic signature

Abstract We report new heat flow density (HFD) values in seven drill holes in the Kamennye Lakes area in eastern Karelia, Russia, approximately at latitude 63°15′N, longitude 36°10′E. The investigated holes are 250–750 m deep and they intersect Archaean ultrabasic serpentinites and talc-carbonate rocks. Measured gradients range from 0.8 to 3.7 mK m−1 and the apparent HFD values from 2.4 to 11.6 mW m−2. The holes are not technically disturbed by fluid flow or any drilling effects. Average heat production of the rocks as analysed in the core samples of the deepest measured hole is 0.25 μ W m−3, but the low heat production is not a critical factor in producing the low HFD values. This is due to refraction of heat as shown with 2-D conductive simulations of heat transfer in a low heat-production formation surrounded by higher heat production. Hydrogeological disturbances can be ruled out by the presence of saline groundwater in the sections deeper than 150–400 m, and low topographic variation in the area, as well as Peclet number estimates, which suggest negligible convective heat transfer in the bedrock. All the temperature profiles are curved indicating recent palaeoclimatic disturbances. Inversion studies with singular value decomposition techniques yielded a climatic warming of about 1.0–1.5 K which started 150–200 years ago and was preceded by a cool period which lasted about 100 years. Nevertheless, recent climatic changes cannot explain the very low apparent HFD values, but long-period effects of the Weichselian glaciation are sufficient to decrease the HFD values to the levels measured. These effects were investigated with forward simulations and suggest that present temperature gradients in the range of 1–4 mK m−1 in the uppermost 1 km can be created by a very cold ground temperature (−10 to −15°C) during the glaciation time (60-11 ka ago).

Integrating geophysical data in GIS for geothermal power prospecting

Geographic information system (GIS)–based resource assessment is an important and relatively inexpensive tool for identifying areas that are of interest for geothermal power production. Of particular interest is the under-exploited industry of co-produced fluids and low-temperature formation waters in oil- and gas-producing basins. Obtaining bottom-hole temperature (BHT) data is now free and easily accessible due to the efforts of the National Geothermal Data System (NGDS). Oil- and gas-producing sedimentary basins in Colorado, Illinois, Michigan, and North Dakota contain formation waters of a temperature that is adequate for geothermal power production (90–150 °C) using existing binary power plant technology. While resource assessment gives a broad picture of the energy available in a basin, the problem remains of knowing where a power plant must go, and if it is economically feasible to do so in any given area. The Denver, Illinois, Michigan, and Williston sedimentary basins were evaluated using a play fairway analysis methodology to identify optimum locations for geothermal power production. These regions have been previously assessed for thermal energy in place, and geothermal gradients from that study, along with gravity anomaly information, magnetic intensity, and digital elevation models (DEMs) for slope analysis were incorporated into a geodatabase for map generation. Raster layers were created and then reclassified into nine classes each, with high geothermal gradient, low magnetic intensity, low Bouger anomaly, and low slope receiving the highest values. The layers were then weighted using a matrix weight assignment similar to that used in the Environmental Protection Agency’s DRASTIC water pollution model, and combined with the “Raster Algebra” tool in ArcGIS. Areas of greatest potential were identified and overlaid on a DEM layer. This shows locations where temperature will be highest at the shallowest depths in regions of soft sediments, refining the map creation process.