pT-effects of Pleistocene glacial periods on permafrost, gas hydrate stability zones and reservoir of the Mittelplate oil field, northern Germany

During the past two million years low surface temperatures as well as episodically advancing ice sheets from Scandinavia acted on the subsurface pT-regime of northern Germany. Their likely effects on the petroleum system of Schleswig-Holstein were investigated. For the entire Quaternary mean annual ground temperature (MAGT) was reconstructed at a resolution of 1000 years by calibrating oxygen isotope records from ODP-site 659 to the climate of northern Germany of the past 120 kyr. The resulting MAGT trend served as input to an ice sheet model and a permafrost model along a 2D section crossing the petroleum bearing south-western part of Schleswig-Holstein. Here advances and retreats of the Scandinavian ice sheet during Saalian and Elsterian glaciation Stages were reconstructed. Maximum ice thicknesses of up to 1700 m and up to 20 periods of regional permafrost in northern Germany were reconstructed for the past 1.25 million years. Based on a basal heat flow of 50 mW/m2 permafrost thicknesses exceeded 100 m during most of these periods, temporarily extending down to depths of more than 300 m. Favourable surface temperatures and long durations of cold periods provided favourable conditions for onshore gas hydrate stability zones at Mittelplate. Implementing these glacial dynamics into 2D basin modelling (PetroMod, IES, Aachen, Germany) of the Mittelplate oil field reveals five phases of gas hydrate stability at depths down to 750 m. The latest of these events occurred during the Weichselian about 20 kyr ago. The effect of the ice sheets on pore pressure in the subsurface strongly depends on the hydraulic boundary conditions at the ice base (e.g. frozen vs. temperate ice sheet base). Excess pore pressure in the reservoir of more than 10 MPa during ice overriding is possible and probable. The calculated temperature effect of the Pleistocene cooling on the Mittelplate reservoir is in the range of 3–7 °C. Even today temperature in the reservoir is still lowered by about 4 °C in comparison to pre-Pleistocene times. Despite the fact that a significant influence of glacial effects on petroleum generation can be ruled out at Mittelplate, we state that pT-effects in reservoirs related to glacial processes in formerly glaciated areas have been underestimated in the past.