Thermal effects of Zechstein salt and the Early to Middle Jurassic hydrothermal event in the central Polish Basin

Abstract

Deep gas potential in the Polish Basin may factor significantly in European geopolitics, and thermal effects can influence that outcome there and elsewhere. Deep (3 km [9843 ft]) well data from the Kujawy area of the central Polish Basin reveal average geothermal gradient (36C/km), thermal conductivity of Mesozoic strata (k = 2.29 W/m K), and present-day heat flow (Q = 82.4 mW/m2) that is 3% less than that obtained using the entire borehole. The extrapolated surface temperature (–6.2C) is in good agreement with temperatures during the Weichselian glaciation. The thermal conductivity of the Upper Permian Zechstein (4.89 W/m K) is in good agreement with values from the North Sea and northern Germany. Steady-state heat-flow theory (one-dimensional [1-D]) predicts present-day temperature (199C) at the base of Zechstein cap rock at 6-km (19,685-ft) depth in Kujawy. This is reduced just more than 10C by low Zechstein thermal gradients (16.8C/km). Because of thermal refraction, two-dimensional and three-dimensional models of Zechstein salt pillows can significantly negate this cooling effect; however, such effects appear absent in the Kujawy wells studied. A widespread Early to Middle Jurassic (195–175 Ma) hydrothermal event appears to have reached maximum in the Kujawy area. A 455C paleotemperature at 7-km (22,966-ft) depth (Carboniferous) is predicted by 1-D conductive heat transfer; however, geologic evidence does not support this result. The discrepancy is reconciled by convective heat transfer with upward fluid flow (3.3 10–10 m/s [10.8 10–10 ft/s]), resulting in a maximum paleotemperature of 273C at 7-km (22,966-ft) depth, despite a paleoheat flow of 142 mW/m2. The trend of intensity of the hydrothermal event correlates with the present-day heat-flow trend. Hydrothermal event sites are subparallel to the major northwest-southeast structural and regional heat-flow trend, whereas other sites as close as 14 km (45,932 ft) and without hydrothermal event are not. The decay of the hydrothermal event is consistent with localized cylindrical plumes (10-km [32,808-ft] radius) that cool by conduction. Results suggest a long-term (185 m.y.) structural control on heat flow. Linear regression to vitrinite paleotemperatures yields a 185-Ma Jurassic surface temperature of approximately 21.3C that is approximately 13C higher than the present-day temperature for Warsaw, Poland. The duration of maximum reservoir and source rock paleotemperature (50 m.y.) is contrary to the kinetics of nitrogen and CO2-producing wells. Equilibrium thermodynamics predicts approximately 60% methane for present-day Kujawy reservoirs, with considerable uncertainty that should be removed by anticipated new deep drilling.