AUTHORS: Di Cuia R., Gout C., Balzagette L., Masse P., Vieban F.
YEAR: 2005
International Petroleum Technology Conference held in Doha, Qatar, 21–23 November 2005. IPTC Paper Number IPTC-10817-PP
Understanding reservoir connectivity in a complex, low-porosity, fractured carbonate reservoirs is fundamental for the correct assessment of the potential of a reservoir. In this kind of reservoirs fracturing is essential. It is very difficult, using only subsurface data, to correctly define the 3D distribution of the fracture network because the criteria to distribute fracture density and type are often doubtful.
The Upper Cretaceous Rudist Carbonates of the Apulian Inner Platform represent a superb reservoir for some of the most prolific southern Italy HC fields. This reservoir is deeply buried underneath a thick pile of allochothonous thrust sheets but the outcrop equivalent in the Maiella Mountain allow to study its 3D architecture and characteristics.
Fracturing is not randomly distributed but the pervasive fracture distribution at bed scale (background fractures) is closely related to the distribution of different sedimentary facies.
This relationship is a key starting point to correctly model the reservoir behaviour in terms of three-dimensional connectivity (fracture connectivity network) and of identification of potential fracture-related good hydrocarbon storage intervals.
The multi-disciplinary outcrop acquisition method and integration used for this study are innovative: they are designed to get the most interactive contribution between sedimentary, stratigraphic, mechanical and structural analysis.
Generally, fine-grained sedimentary facies (the more abundant and continuous in the inner platform) are characterised by higher fracture densities (for all fracture orders). In such sedimentary facies, typical fracture sets are often sub-parallel and bed-confined. These facies, with higher fracture porosity could represent important HC storage intervals within the reservoir.
Laterally discontinuous coarse rudist associations and coarse-grained sedimentary facies are characterised by scarce fracturing but also by relatively good matrix porosity. These facies can also be considered as good HC storage intervals which could be isolated from the rest of the reservoir.
All observed sedimentary facies are connected by fractures which are not confined to one single bed but can cross several beds within different sedimentary facies. In this way, a dense network of fractures and matrix porosity can be connected and can increase reservoir characteristics.
In this framework, large-scale faults and fracture corridors also play an important role. While small-scale bed-confined fracture could represent bed-parallel conduits, and medium-scale fractures connect several intervals, fracture corridors ensure the vertical connection over several tens of meters. Only with this kind of hierarchical organisation the reservoir is entirely connected.