AUTHORS: Di Cuia R., Gout C., Sarti M., .Balzagette L.
AAPG International Conference & Exhibition – Paris, 2005
The Upper Cretaceous Apulian Carbonate of the Murge foreland and of the Maiella Mountain represent very good reservoir analogues for some of the Italian most prolific oil fields that lie at great depth underneath the Southern Apennines Thrust Belt. In some of these fields, the amount of available subsurface geological data, to understand the lateral and vertical distribution of facies and fracturing is scarce and architecture and reservoir characteristics of the reservoir must be assessed from outcrop analogues.
The two selected outcrops analogues present same age interval, similar depositional system, and were deposited at the same paleo-latitude or in the same paleo-climatic conditions than the subsurface reservoirs. The architecture of the platform is provided by the stacking of multiple-order stratigraphic cycles of tidal origin. Peritidal sub-environment and associated rudist build-ups established over the depositional site and alternated through time in cyclic fashion, obeying to short-term relative variations of the sea level. Rudist build-ups established and flourished at times of maximum flooding, being replaced by extensive mud flats during subsequent platform fill-up. At the lithozone scale, the variability in thickness and facies is in the range of tens of kilometers: the dense buildup system enclosed in peritidal strata show persisting characteristics of size, geometry, frequency and density of elementary units; the calcarenitic and biogenic lithozones characteristics vary smoothly turning into sensibly finer and pelletal some 20-25 kilometers away. At the facies scale, variability is much more pronounced, recognizing sensible variations within the range of distances of an individual site (100-300 meters). Individual depositional elements may significantly change their geometric and facial characters in 10 (mounds and buildups)-100 (biostromes) meters at most. Even cryptalgal laminitic layers display modest lateral persistence, characteristic of patchy mudflats.
Fracturing is less pervasive in the foreland Murge outcrops than in the Maiella mountain sections. Generally, fine-grained sedimentary facies are characterised by higher fracture densities. In such sedimentary facies, typical fracture sets are often sub-parallel and bed-confined and could represent bed-parallel conduits. Laterally discontinuous coarse rudist associations and coarse-grained sedimentary facies are characterised by scarce fracturing. 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 and can connect several intervals. In this way, a dense network of fractures and matrix porosity can be connected and can increase reservoir characteristics.
The relationships between facies and fracturing show similar characteristics in the two selected outcrop analogues even if they lie in two completely different tectonic settings (foreland and thrust belt). This observation is fundamental for subsurface reservoirs because implies that fracture distribution is relatively independent by the tectonic regime but is strongly influenced by facies.