Origin, migration pathways, and prediction of high carbon dioxide accumulations in the Lower Saxony Basin (northwestern Germany): Part II

Abstract

Predrill risk assessment for the presence of nonhydrocarbon gases in oil and methane reservoirs is challenging and perhaps commonly an underestimated task. Although the same assessment principles apply for source and migration scenarios, the interplay of both play elements in basin history is different because of diverse origins and oftentimes unusual occurrences of nonhydrocarbon gases. The southern upper Permian Zechstein-2-Carbonate (Ca2) fairway in northwestern Germany represents such a setting, where CO2 contents >60 vol. % have been encountered in an otherwise methane-dominated area. The CO2 is well constrained to be the product from thermal decomposition of ultra–deeply buried Devonian carbonates (part I published in this issue). In this study (part II), CO2-rich tectonic veins sampled for part I have been directionally reoriented in drill cores, showing a dominant northeast-southwest strike. Only very few faults can be identified in seismic data, connecting the deep CO2 source through the approximately 3-km-thick Carboniferous section with the Ca2 reservoir. Fluid inclusion analyses of hydrothermal cements from the northeast-striking veins reveal up to 100 mol % CO2, formation temperatures up to 315°C, and near-lithostatic pressure conditions. The northeast strike of the faults is parallel to the main contraction direction during Late Cretaceous basin inversion and therefore marks the most likely timing for CO2 migration. The high CO2 content wells are located within the hanging wall of seismically defined northeast-striking faults, enabling lateral injection of CO2 into downthrown Ca2 fault blocks. This pattern enhances predictability of nonhydrocarbon gases for neighboring trap structures and in similar petroliferous basins.