Numerical modelling of petroleum expulsion in two areas of the Lower Saxony Basin, Northern Germany

Abstract

Abstract The process of petroleum expulsion is mainly determined by the initial quality of the source rock and its sedimentary facies and by the temperature and pressure histories during burial. A numerical model of the complex processes of expulsion was developed based on observations of expulsion phenomena, detailed petrophysical and organic geochemical investigations and data provided by basin modelling. In this model petroleum expulsion is treated as a pressure driven transport in a separate phase using the existing pore system and newly formed microfractures as migration pathways. Microfracturing predominantly parallel to the bedding plane, as it is observed in oil generating source rocks, is initiated when the pore pressure build-up exceeds the rock strength. Major sources of overpressure in source rocks are considered to be volume expansion of organic matter during petroleum generation and compaction disequilibrium. Experimental data were obtained from detailed investigations of the Posidonia Shale source rock of the Lower Saxony Basin. An application of expulsion modelling is presented for two regions of the Lower Saxony Basin, where timing and quantity of petroleum generation and expulsion from Posidonia shale was studied with respect to different heating rates. The efficiency of transport mechanisms and migration pathways through both the pore network and newly-formed fractures could be determined by this quantitative treatment. The modelling results were in good agreement with geological observations and a mass balance based on organic geochemical analysis.