Modelling the Impact of Pleistocene Glacial Periods on Permafrost, Gas Hydrates, and Pt-Effects in the Reservoir of the Mittelplate Oil Field, Northern Germany

S. Grassmann¹, B. Cramer², G. Delisle², T. Hantschel³, J. Messner⁴, and J. Winsemann^5
1Aabar Petroleum Investments Company PJSC, P.O. Box 107888, Abu Dhabi, UAE
email: [email protected]
²Federal Institute for Geosciences and Natural Resources (BGR), Germany
³IES GmbH – Integrated Exploration Systems, Germany
⁴State Authority of Mining, Energy and Geology (LBEG), Germany
⁵Gottfried Wilhelm Leibniz University, Hannover, Germany

The Mittelplate oil field is located in northern Germany and constitutes the largest oil accumulation in this country containing estimated initially recoverable reserves of more than 60 million tons. At Mittelplate petroleum is trapped at a depth of about 2000 to 3000 m in Middle Jurassic Dogger sandstones at the flank of the Buesum salt diapir. The structure was charged from deeply buried mature parts of the Lower Jurassic Posidonia Shale deposited in the rim syncline.
2D basin modelling was carried out along a section crossing the Mittelplate field and four adjacent salt structures in order to reconstruct basin dynamics and the evolution of the petroleum system, and to investigate the impact of Quaternary climate on deeply buried sediment strata. During the past 2 million years low surface temperatures as well as thick glaciers which episodically advanced from the northeast acted on the pT-regime in the subsurface of the Mittelplate area. To account for these effects mean annual ground temperatures with a resolution of 1000 years were reconstructed for the entire Pleistocene. This temperature trend was generated by applying oxygen isotope records from ODP-site 659 calibrated to the climate of Northern German on the basis of previously reconstructed surface temperatures for the past 120 k-years. Ice sheets covered the area several times during the Saalian and Elsterian glaciations. According to our ice sheet model used to reconstruct ice sheet advances and retreats over the section, maximum ice thicknesses of up to 1700 m were reached.
By integrating these parameters into 2D basin modelling, up to 20 periods of permafrost were reconstructed for the past 1.25 million years. With a calibrated 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 duration of cold periods also provided the stability conditions for onshore gas hydrates at Mittelplate during the Pleistocene. Basin modelling reveals five phases of gas hydrate stability at depths down to 750 m. The last permafrost and gas hydrate event ceased about 20 k-years ago.
Low surface temperatures and hydraulic systems of the ice sheets also significantly influenced the pT-conditions in the oil reservoir of Mittelplate. The effect of the glaciers on pore pressure in the subsurface strongly depends on the hydraulic boundary conditions at the glaciers base (e.g. permafrost vs. unfrozen ground) which are not well known. Nevertheless, excess pore pressure in the reservoir of more than 10 MPa during glacial overburden is possible. Temperature effects of the Pleistocene cooling on the Mittelplate reservoir is much better constrained with an effect of 3 to 7 °C. Even today the temperature field in the reservoir is still lowered by about 4 °C in comparison to the early Pleistocene. Significant influence of these effects on petroleum generation, however, can be ruled out at Mittelplate.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands