Thermal effects of Zechstein salt and the Early to Middle Jurassic hydrothermal event in the central Polish Basin Gary W. Zielinski, Pawel Poprawa, Jan Szewczyk, Izabella Grotek, Hubert Kiersnowski, and Robyn L. B. Zielinski 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 (36°C/km), thermal con- ductivity of Mesozoic strata (k = 2.29 W/m K), and present- day heat flow (Q = 82.4 mW/m 2 ) that is 3% less than that obtained using the entire borehole. The extrapolated surface temperature (–6.2°C) 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 agree- ment with values from the North Sea and northern Germany. Steady-state heat-flow theory (one-dimensional [1-D]) predicts present-day temperature (199°C) at the base of Zechstein cap rock at 6-km (19,685-ft) depth in Kujawy. This is reduced just more than 10°C by low Zechstein thermal gradients (16.8°C/ 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 455°C paleotemperature at 7-km (22,966-ft) depth (Carboniferous) is predicted by 1-D conductive heat transfer; however, geologic evidence does not support this re- sult. The discrepancy is reconciled by convective heat transfer AUTHORS Gary W. Zielinski  Omegalink International Ltd., P.O. Box 2152, Campton, New Hampshire; omegalink@juno.com Gary Zielinski holds a Ph.D. from Columbia University under M. G. Langseth (Heat Flow Department), Lamont-Doherty Earth Observa- tory. He spent six years at Gulf Research and Development (Harmarville Laboratory), where he studied measurement and modeling of hydrocarbon thermal regimes. He continued this work at Brookhaven National Laboratory, as a visiting research associate at Lamont- Doherty, and since 1989, as a director at Omegalink International Ltd. Pawel Poprawa  Polish Geological Institute, Polish Geological Survey, 4 Rakowiecka Street, Warsaw, Poland; pawel.poprawa@pgi.gov.pl Pawel Poprawa studied geosciences and petro- leum geology at Jagiellonian University, Krakow; AGH Technical University, Krakow; and Uni- versity College Dublin, completing his M.S. thesis on the organic geochemistry of hydrocarbon source rocks. He now heads the Petroleum Prospection Division of the Polish Geological Institute, assessing the shale gas potential of Poland and its neighbors via basin analysis and thermal, tectonic, and burial history reconstructions. Jan Szewczyk  Polish Geological Institute, 4 Rakowiecka Street, Warsaw, Poland; jan.szewczyk@pgi.gov.pl Jan Szewczyk is a graduate in physics of the lithosphere from Warsaw University and holds a Ph.D. in nuclear geophysics from the Polish Geological Institute. His work includes the deter- mination of petrophysical parameters from log analysis, geothermics of Poland, hydrogeo- logic and geothermal conditions in deep bore- holes, and paleoclimatic change based on the subsurface temperature data. He authored a new method of borehole heat-flow determination and coauthored a new heat-flow map of Poland (2009). Izabella Grotek  Polish Geological Institute, Polish Geological Survey, 4 Rakowiecka Street, Warsaw, Poland; hubert.kiersnowski@pgi.gov.pl Izabella Grotek is a graduate in geology (petrol- ogy) from Warsaw University, with a Ph.D. from Copyright ©2012. The American Association of Petroleum Geologists. All rights reserved. Manuscript received September 20, 2011; provisional acceptance November 28, 2011; revised manuscript received January 9, 2012; revised manuscript provisional acceptance February 16, 2012; 2nd revised manuscript received March 12, 2012; final acceptance April 2, 2012. DOI:10.1306/04021211142 AAPG Bulletin, v. 96, no. 10 (October 2012), pp. 1981 – 1996 1981