Palaeo-stresses inferred from macro- and microfractures in the Balazuc-1 borehole (GPF programme). Contribution to the tectonic evolution of the Cévennes border of the SE Basin of France

Abstract

The Scientific borehole Balazuc-1 was drilled on the Ardèche margin of the SE Basin of France for the purpose of studying fluid-rock interactions in stratiform reservoirs in a tectonically controlled area. A structural study was made of oriented cores from the drill hole between the depths of 1218 and 1729.5 m; the various types of fracture that could have been used during fluid transfers have been described and analysed. Four major faults and 335 minor faults and tension gashes mineralized in carbonates, anhydrite and sulphides were intersected. At the bottom of the hole, one of the major faults brings into contact rocks of Triassic age and a formation attributed to the Upper Palaeozoic; the other faults have caused thinning of certain units of the Lower-Middle Triassic. Micro-structural study of the fault planes has enabled characterization of the tectonic episodes by which the Triassic and Lower Liassic sedimentary rocks were affected. These episodes, discussed in the context of the Ligurian Tethys, were: (1) E-W synsedimentary extension during the Early-Middle Triassic that caused tilting to the southeast of the Upper Palaeozoic and Lower-Middle Triassic successions; this continued into the Late Triassic. (2) An episode of N-S extension during the early Lias, characterized by hydroplastic-type faults, probably the result of a local variation of the stress-field in a trans-tensional regime. (3) The major period of activity on the Uzer fault, with downthrow of more than 1000 m, that continued from the Lotharingian (Late Sinemurian) to the Early Bathonian. It was not possible to calculate the stress tensor for this period, because no secondary fault system developed in the lower block traversed by the borehole. Brittle deformation at this time seems to have been concentrated on the major structures., (4) Approximately N-S extension, possibly end-Jurassic to Earliest Cretaceous, during which the tension gashes, mineralized in carbonates, and pyrite were formed in the Rhaetian and Hettangian deposits. (5) Three compressive and/or strike-slip episodes attributed to the Cenozoic Pyrenean and Alpine phases of deformation. The Oligocene extensional phase was not recognized in the borehole.