Fracture stratigraphy and DFN modelling of tight carbonates, the case study of the Lower Cretaceous carbonates exposed at the Monte Alpi (Basilicata, Italy)

Abstract

Abstract This work focuses on the role played by depositional/diagenetic heterogeneities on the vertical growth of sub-seismic, high-angle structural elements such as joints, sheared joints, small faults, and well-developed faults in Lower Cretaceous tight carbonates exposed along the 100's m-high, km-long, western cliff of the Monte Alpi massif. The study carbonates pertain to the Inner Apulian Platform, and form a surface structural analogue of the fractured carbonate reservoirs currently exploited for hydrocarbon production in nearby areas of southern Italy. By combining traditional field stratigraphic and structural methods with photogrammetric analysis of a large portion of the aforementioned cliff, we assess the abutting and crosscutting relationships among the sub-seismic structural elements and different types of flooding surfaces. At smaller scale, bed interfaces extend laterally for a few tens-of-metres, bound tens-of-centimetres-thick beds characterized by amalgamation and bifurcation geometries, and form sequence boundaries of the 4th-5th order. At larger scale, transgressive surfaces extend laterally for hundreds-of-metres, include mm-thick terrigenous material, bound multiple metre-thick bed packages made up of multiple beds, and form sequence boundaries of the 3rd order. Field evidences are consistent with joints and sheared joints often forming stratabound and non-stratabound elements, respectively. At a reservoir scale, prominent transgressive surfaces are laterally continuous for kilometres, include up to a few centimetres-thick, clay-rich levels and collapsed breccia, bound tens-of-metres-thick bed package associations, and form sequence boundaries of the 2nd order. There, small faults with up to 10's of cm-offset mainly localize within individual bed package associations, whereas well-developed faults with up to a few m-offset crosscut multiple prominent transgressive surfaces. Results after DFN modelling of representative geo-cellular models highlight the control exerted by the study structural elements on the fluid accumulation and flow properties of the fractured carbonates. These data are useful to fill the gap between core and seismic data, as they provide information on the sub-seismic domain (i.e. below seismic resolution).