Variability of lithofacial parameters of Lower Jurassic geothermal aquifer in the Malanów region revealed by interpretation of geophysical well logs and seismic data

The Lower Jurassic aquifer is the main target for exploitation of geothermal waters in the Polish Lowlands as it covers over 80% of the area of Poland. Particularly favorable geothermal conditions dominate in the Mogilno-Łódź Trough (central part of the Polish Lowlands), where that aquifer occurs at greatest depth, which results in high temperatures of geothermal waters. The main Lower Jurassic reservoirs in that area are Sinemurian, Pliensbachian, and Upper Toarcian sandstones. These sediments show high lithofacial variability, which results in local changes of hydrogeothermal parameters. Such local variability can be effectively estimated using the seismic data and the interpretation of well logs. Both methods were used for determination of depths to the top surface of Lower Jurassic formation and of distribution of porosity, and clay content, which enabled the researchers to recognize the spatial arrangement of groundwater horizons. The study area is a local structure within the Mogilno-Łódź Trough, in the vicinity of Malanów village. This is a synclinal structure built of Lower Jurassic strata close to the Turek Salt Pillow. Temperature of groundwater in that structure exceeds 80 °C and favorable reservoir parameters ensure relevant discharge. Sandstone horizons show thicknesses over 50 m, low clay contents and porosity around 0.15–0.20.