Analysis of conductive and convective heat transfer in a sedimentary basin, demonstrated for the Rheingraben

Abstract

The identification and quantification of conductive and convective components in the heat transfer of a sedimentary basin is demonstrated for the Rheingraben. Three different methods of varying complexity as well as three independent data sets are employed: (1) energy budget considerations based on hydraulically perturbed thermal data from shallow boreholes (< 500 m), (2) 1-D vertical Peclet number analysis of thermal data from 22 deep boreholes (> 1000 m), and (3) 2-D finite difference modelling of the fully coupled fluid flow and heat transport equations on a vertical cross-section of the entire Rheingraben. Energy budget considerations yield a conductive basal heat flow density of 84 + 40/-10 mW m-2, and in good agreement with this Peclet number analysis, gives median values in the range 90 ± 35 mW m-2. In the first case, the basement is formed by low permeable, tertiary sediments at about 500 m depth, and in the second by the transition from the sedimentary graben fill to the crystalline basement at depths of between 2000 and 4000 m. It is shown how results from numerical modelling support the flow field assumptions made by methods (1) and (2), as well as the value of 80 ± 10 mW m-2 for average basal heat flow density entering the graben from below. Conversely, the Peclet number range Pe ≤ 1.2 inferred from method (2) can be applied for a (at least partial) calibration of the fully coupled hydrothermal model calculations. This technique is suggested as a potentially interesting thermal method for constraining regional-scale permeability. An interpretation of heat transport is presented that satisfies the experimentally established patterns of both temperature and heat flow density in the Rheingraben. Moreover, it is demonstrated that the thermal anomalies along the western rim of the graben (such as Pechelbronn, France or Landau, Germany) can be convincingly explained by a basin-wide, deep rooted E–W groundwater circulation that locally enhances a background basal heat flow density of about 80 mW m-1 on average by 50 per cent and at individual sites by as much as 120 per cent.