Characterisation of the Structural Control on Fluid Flow Paths in Fractured Granites

Abstract

The characterisation of fluid flow in fractured media is complex due to the fact that the access to the reservoirs is restricted to the boreholes, and the resolution of geophysical methods decreases with depth. In addition, the structures in a sedimentary cover of a granite mass targeted for geothermal exploitation reflect only a partial amount of the structures affecting the hot basement. In this respect, two fractured variscan granites have been selected to investigate the fluid flow paths; both of them are located within the European Cenozoic Rift System (ECRIS).
The first case study is the experimental geothermal site at Soultz-sous-Forêts (Rhine Graben). Borehole seismic data (VSP) have been used to map some generally permeable structures developed at the hectometre scale in the vicinity of the wells. The pattern of structures revealed by this way suggests a clear expression of the variscan inheritance rather than a faulting linked to the opening of the graben. The activation of some of these structures can be due to the current stress field controlled by the Alpine push, and explains the major hydraulic connections between the wells.
The second case study is a batholith outcropping in the Catalan Coastal Ranges, allowing a continuous structural analysis from centimetre to kilometre scales. A network of carbonate veins has been considered as a witness of past circulations. Both the extent and the conditions of precipitations of these carbonates allow to consider the outcrop as a fossil geothermal reservoir. The veins studied in this analogue outcrop and at Soultz-sous-Forêts allowed to identify different drainage patterns developed in a fractured granite. At the kilometre scale, the drainage may either be localised in the major structures, or be more homogeneously or randomly distributed, implying an important contribution of the protolithe.