Sedimentary facies control on mechanical and fracture stratigraphy in turbidites

Abstract

Natural fracture networks exert a first-order control on the exploitation of resources such as aquifers, hydrocarbons, and geothermal reservoirs, and on environmental issues like underground gas storage and waste disposal. Fractures and the mechanical stratigraphy of layered sequences have been intensively studied to unravel the relationships between bed thickness and fracture spacing, but less attention has been paid to intrabed fracturing patterns due to the intrinsic local variability of sedimentary processes and products. Among sedimentary rocks, turbidites show great lateral and vertical variability of textural characteristics and depositional facies, which are expected to strongly influence the location and density of fractures. To better understand the contribution of stratigraphic, sedimentologic, and petrophysical properties on fracture patterns, we performed a high-resolution study on a selected stratigraphic interval of jointed foredeep turbidites in the Miocene Marnoso-Arenacea Formation (Northern Apennines, Italy). Cumulative statistical relations of field and laboratory structural, sedimentologic, and petrophysical data significantly improved when analyzed at the sedimentary facies scale. In particular, for facies recording different cross-flow (i.e., longitudinal to the paleocurrents) depositional conditions within the parent turbidity currents, we observed three-dimensional anisotropies of rock hardness (i.e., uniaxial compression) that were positively correlated with normalized fracture intensities, indicating a primary sedimentary control on fracture distribution. This type of intrabed joint distribution has crucial practical implications for the lateral prediction and evaluation of mesoscale fracture patterns in turbidite sequences.