Sedimentary facies control on fracture and mechanical stratigraphy in siliciclastics: Marnoso-arenacea formation, Northern Apennines, Italy

Jointing in bedded siliciclastic sedimentary successions has been mainly analyzed in terms of fractured layers thickness-normalized joint spacing and related probability distributions, commonly neglecting the influence of primary structures on the petrophysical properties, joint height patterns, spatial arrangements and best-fitted spacing distributions. Taking into account the depositional controls on mechanical and fracture stratigraphy is crucial for improving the predictive potential of geological modelling in exploration and exploitation of underground resources, gas injection and storage. Here, we present a high-resolution, structural-stratigraphic study carried out on a 72 m-thick succession of the Marnoso-arenacea foredeep turbidites, exposed in the Northern Apennines of Italy. The systematic integration of the structural and sedimentological characterization of this basin-plain turbiditic succession, coupled with petrophysical and geomechanical measurements, represents the fracture and mechanical stratigraphy database onto which we tested the standardly used fracture density parameters and the best-fitted probability distributions, i.e. normal, log-normal, gamma and exponential, applied to fracture spacing analysis. Our results document that cumulative analysis does not identify the variability of joint height patterns and spacing distributions in siliciclastic turbidites, which instead requires considering single sedimentary facies types. Specifically, sandstones are affected by facies bounded joints and gamma spacing distributions while clay-rich and poorly sorted facies show a higher abundance of top bounded and throughgoing joints, and are best fitted by log-normal spacing distributions. We suggest that the fracture and mechanical stratigraphy should be studied at the facies scale resolution to unravel the strong control of the sedimentary facies for improving the characterization of siliciclastic reservoirs.