Timing of multiple fracture reactivations using micro geophysics, geology, and isotope geochemistry

Fault and fractures properties are responsible of a large part of the fluid transfer properties and associated seismicity, at all scales. Multiple reactivations can deeply modify the initial fracture properties all along the rocks diagenetical history, leading to complex alternate periods of fractures sealing and seismic instability. For instance, each reactivation step may be associated with cementation/dissolution processes that can be traced through combined geophysical and geochemical analyses.

For that purpose, we studied a parallelepiped (2.5x1.5x1.1m) carbonate quarry block with a detailed structural and diagenetical characterization (fractures, karsts and stylolites digitalization; thin section and plug porosity). The block is affected by two en-échelon fracture clusters, the first one being simply formed in mode 1 and cemented, the second one being polyphased (multiple reactivations, cementation and karstification phases). We performed 1298 acoustic P-wave velocity measurements on a vertical cross section along with geochemical analyses of carbon and oxygen isotopes ratios on fracture fillings/cements.

Preliminary key results show that the fracture diagenetical evolution induces an anisotropic Vp variation regarding the dip angle of the raypaths:

- Fracture initial cementation, likely related to meteoric diagenesis during eogenetic event, leads to the obliteration of facies initial acoustic heterogeneity.

- Multiple reactivations phases relate both to burial mesogenetic geochemical signature leading to angular anisotropy, Vp crossing the reactivated fractures being in average 500m/s lower compared to non-reactivated fractures.

- Fracture meteoric karstification leads to a dramatic decrease of Vp, but slightly increases the overall Vp anisotropy.

Thus, ultrasonic anisotropy evolution may help detecting the degree of fracture sealing which is a crucial point in better understanding fluid natural and induced movements in the sub-surface fracture networks.