Original paper

Estimates of shale gas contents in the Posidonia Shale and Wealden of the westcentral Lower Saxony Basin from high-resolution 3D numerical basin modelling

Mohnhoff, Daniel; Littke, Ralf; Sachse, Victoria F.

Abstract

A high-resolution 3D numerical basin model of the west-central part of the Lower Saxony Basin was built to reconstruct the thermal and burial/uplift history. The main objective of this study was to estimate the gas content in two source rock formations, namely the Posidonia Shale and the Wealden Shales. Based on the organic-geochemical data and source rock quality for subunits of these formations the gas storage capacity of these gas shale horizons was analysed in detail. The numerical simulations included the drafting of bulk adsorption capacity and adsorbed gas content maps. The amount of stored gas in the pore space of the source rocks was estimated for the central part of the basin where high thermal maturities are reached, i.e.the gas window. The reconstruction of the gas generation and storage history shows that these parameters depend strongly on the burial/uplift history of the inverted Lower Saxony Basin. While the rapid burial prior to inversion led to an effective generation and storage of methane in these rocks, the subsequent strong uplift had only a small influence on the total amount of adsorbed gas. At present day, the Posidonia Shale and the Wealden Shales are undersaturated in thermally generated methane regarding the bulk adsorption capacity and the amount of gas, which is actually present in an adsorbed phase. For the Posidonia Shale, the reduction in adsorbed gas content was caused by deep burial and high temperatures, leading to a reduction in adsorption capacity and subsequent desorption of gas volumes. For the Wealden Shales, which did not reach burial depths similar to those of the Posidonia Shale, a reduction of adsorption capacity and hence adsorbed gas contents was caused by uplift during the Late Cretaceous basin inversion.

Keywords

numerical basin modellingshale gaslower saxony basinwealdensorptionposidonia shalepetroleum systems