Joint Distribution in a Fractured Carbonate Reservoir in the Umbria-Marche Anticlines (Central Italy)

Abstract

A detailed structural analysis of the discontinuities of the Calcare Massiccio Fm. (Early Liassic) of the Umbria-Marche fold and thrust belt (central Italy) was carried out in order to define the fracture pattern typical of a massive limestones from a carbonate shelf at the core of an anticline structure, This research was made with the aim to apply the results to the modelling of the Cavone oil field fractured reservoir (Po Valley subsurface, northern Italy). The attitudes of rock discontinuities, including bedding, faults and joints were surveyed and their temporal and spatial relationships recorded.

Joints are the most common structures and in average represent about 85% of the mapped discontinuities. The joint pattern is independent of the position within each anticline. Cross-joints, sub-perpendicular to the axial trend of the host anticline, are the most common type of joints. Longitudinal joints, sub parallel to the axial trend, and diagonal joints, oblique to the axial trend, are also present although less abundant. The latter are usually associated with zones of strike-slip deformation. Bedding joints are very rare.

The overall characteristics of the analyzed jointing are regionally consistent and fit well into the two-stage history of deformation of the Umbria-Marche fold and thrust belt. Thus, the cross and longitudinal joints can be interpreted as extension joints developed at the end of the folding stage (Late Miocene), whereas the diagonal joints can be interpreted as shear joints formed mainly during the thrusting and shearing stage (Early Pliocene).