Post mortem 2D-simulation study of the hydrocarbon generation and migration of a N2-rich gas field on the inverted south-western rim of the Lower Saxony Basin

摘要

Palaeozoic petroleum system of gas field A was reconstructed using a 2-D forward basin modelling software. The 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.% and its cumulative production of 4,27 G.m~3, to one of the most unique gas accumulations in North Germany. According to the modelling results, the key charge of the present gas field began in the Tithonian (late Upper Jurassic) during the major rifting-phase in the Lower Saxony Basin. The present Westphalian coal-derived gas accumulations of the Lower Triassic Buntsandstein reservoir were sourced directly from methane pools at the top Rotliegend level. The hydrocarbon potential of the Westphalian source rocks became exhausted in the Oxfordian (early Upper Jurassic). Based on thermal calibration studies utilising both vitrinite reflectance data and corrected bottom hole temperatures as calibration parameters, an anomalous event of elevated heat flow of 80 to 120 mW/m~2 during the Coniacian was modelled. A high temperature increase during the Coniacian high heat flow event, together with an 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 the Buntsandstein reservoir. The structural trap was destroyed in course of the inversion, leading to a sharp decrease of methane and nitrogen saturation (Fig. 3). Tertiary northeasterly directed tilting did not affect the hydrocarbon accumulation in gas field A.