Effects of a Cretaceous structural inversion and a postulated high heat flow event on petroleum system of the western Lower Saxony Basin and the charge history of the Apeldorn gas field

Thermal history and evolution of the Palaeozoic petroleum system of the western Lower Saxony Basin as well as charge history of the Apeldorn gas field was reconstructed using 2-D forward basin modelling software. The Apeldorn gas field is located on an inverted western rim of the Lower Saxony Basin (LSB) and belongs with its relatively anomalous nitrogen content of 73.9 vol. % to one of the most unique gas accumulations in North Germany. Based on thermal calibration studies utilising both, vitrinite reflectance and corrected bottom hole temperatures as calibration parameters, a shallow burial model and an anomalous event of Coniacian high heat flow of 80 to 120 mW/m2 was derived. As result, Lower Triassic and younger successions became subjected to slight changes in thermal maturity as opposed to Carboniferous and Permian successions, which show no assessable impact of the high heat flow event on the coalification pattern. The deep burial model in contrary to the shallow burial model is not supported by the structural reconstruction and backstripping in this more marginal setting. According to the modelling results, the key charge of the present Apeldorn gas field began in Tithonian (late Upper Jurassic) during the major phase of rifting in the Lower Saxony Basin. The present Westphalian coal-derived gas accumulations of the Lower Triassic Buntsandstein reservoir were sourced directly from modelled methane pools at top Rotliegend level. The hydrocarbon potential of the Westphalian source rocks became exhausted in Oxfordian (early Upper Jurassic). Reduction of the hydrostatic pressure during the Coniacian high heat flow event together with uplift during the Coniacian-Santonian inversion led to an extensive free gas exsolution. The resulting gas mixture between the exsolved free gas and the Westphalian coal-derived gas reached and saturated Buntsandstein reservoir. The structural trap became destroyed in course of the inversion leading to a sharp decrease of methane and nitrogen saturation.