The Vocontian Aptian and Albian Syndepositional Clastic Sills and Dikes: A Field-based Mechanical Approach to Predict and Model the Early Fracturing of Marly-limy Sediments

Shaly formations are the focus of many research programs and consortia sponsored by petroleum companies and/or waste management organizations; given that they act as seals for oil- or gas-bearing reservoirs or as host rock for underground waste disposals, their integrity (e.g., the possible presence of water-bearing fractures) is a critical factor in risk assessment. To model their rheological properties through time, the observed clastic injectites are used as markers of their mechanical evolution. Aptian–Albian marly formations of the Vocontian Basin (southeast France) are the basis of this study; massive turbidite systems associated with large-scale clastic injectite networks have been described in exceptional outcrops. Field data have permitted the identification of early fracturing in the host formation; the injection of sand is an early event, contemporaneous with the deposition of massive sand bodies. The paleocompaction curve has been calculated, and the porosity evolution of the sediments has been restored from sea floor to about 500 m (1640 ft) burial. Then, the original configuration of dikes can be reconstituted. Boundary conditions of various numerical modeling have been derived from this extensive reliable data set. Numerical static simulations of the behavior of marly formations are presented, testing the possible function of heterogeneous lithology, bedrock geometry, or loading by sudden massive sand deposition; they indicate that early fracturing is physically possible in the presented scenarios. The next step will be to simulate in dynamic conditions the opening and filling of some of these cracks by hydrofracturing. We dedicate this chapter to the memory of our colleague and friend, Stephen T. Horseman.