Injection triggering mechanisms inferred from AMS and microtectonic Analysis. Example of Injectites in the Blue Marls formation, SE basin of France
Résumé
Understanding the way fracture networks evolve in space and time in low-permeability rocks is required in several applied problems. The turbiditic system of the SE Basin of France is associated with injectites that constitute markers of early deformation in the Blue Marls formation. It thus offers a unique opportunity to characterize the onset dynamics and evolution through time of such a 3D fracture network into low permeability rocks. This study explores the relative timing between (i) Blue Marls deposition, (ii) hostrock deformation, (iii) sand injection into fractures and (iv) regional changes in paleostresses, thanks to both microtectonics and magnetic fabric (Anisotropy of Magnetic Susceptibility) analyses in both the hostrock and injectites.
These two coupled studies put forward that the hostrock was submitted to a N150-N160 layer parallel shortening which was not recorded or preserved by injectites. Injection, probably triggered in several pulses, was synchronous with a N/S compression responsible for strike-slip faulting in the study area. This compression persisted after cementation of injections triggered by the first pulse. The AMS signal in injections is more characteristic of hydrodynamic processes at the time injection took place than compaction. Fracturing in injections is decoupled from that recorded in hostrock.