Advective cooling within sedimentary rift basins—application to the Upper Rhinegraben (Germany)

Abstract

Measurements of dispersed vitrinite along several exploration wells within the northern Rhinegraben are indicative of a thermal graben history that is influenced by a combination of basal conductive and groundwater-flow related convective heat transfer. To determine the conductive/convective components of heat transfer within the rift today, a series of 2D numerical groundwater flow and heat models are developed along a cross-sectional transect across the northern Rhinegraben. Fault zone permeability is varied in the simulations of these models to determine the possible fluid pathways and the effects of circulating groundwater on the graben temperature field. Depending on the fault permeability, negative thermal anomalies always develop in areas of cold recharging groundwater along the graben flanks regardless of fault permeability, whereas hot discharging groundwater near the topographic low of the graben only results in positive thermal anomalies under the assumption of high fault permeability. Simulation results suggest that the modern groundwater flow system has an overall net cooling effect on the temperature field of the rift.Without convective cooling by groundwater, vitrinite reflectance levels in wells would be expected to be much higher on average than are observed. Although relatively high heat flow densities (100mW/m2) are documented in the Rhinegraben, an average of only 65mW/m2 would be sufficient to produce the observed vitrinite reflectance levels. Thus, a long-lived (>10My) cooling convective fluid flow in combination with a high basal heat flow seems to be active.