Structure, kinematics and composition of fluid-controlled brittle faults and veins in Lower Cretaceous claystones (Lower Saxony Basin, Northern Germany): Constraints from petrographic studies, microfabrics, stable isotopes and biomarker analyses

Abstract

Investigation of fault rocks is crucial for the evaluation of sealing properties of rock formations considered as oil and gas storage or for the disposal of heat-generating radioactive waste. Even in tight rock formations, fluid flow may concentrate along brittle faults. The present study focuses on the sealing properties of faulted Lower Cretaceous clay- and siltstones encountered in drill cores from the central and eastern Lower Saxony Basin (Germany). Our investigations are based on a multidisciplinary approach including microstructural, mineralogical-geochemical, stable isotope and biomarker analyses.The new data suggest that preexisting discontinuities and/or sedimentary heterogeneities played a significant role for the increase in fluid pressure and formation of mode I veins within low permeability claystones. High-strain domains of fault cores consist of fine-grained fault gouge. A scaly fabric of varying deformation intensity is present around larger faults. Calcite-precipitation in faults and complex mineralized veins, and the differences in trace element concentrations of various veins, reflect several phases of palaeofluid activity. Calcite precipitation in the fault rocks led to elevated amounts of Mn, Ba and/or Sr. Isotope signatures indicate a local carbonate source for the origin of thin calcite coatings of fault planes, but a more complex palaeofluid interaction for the larger faults and for complex veins, consistent with the microfabrics. Biomarker signatures reveal the presence of mature hydrocarbons pointing to hydrocarbon migration in sections where steep, only weakly mineralized faults occur.The new results suggest that faulted claystones represent critical zones with a potentially reduced barrier capability consistent with published data.