AUTHORS: DI CUIA R., SCIFONI A. (G.E.Plan Consulting, Italy)
GIC Annual Conference (Milan, IT)
Carbonate reservoirs are liable to display large variability in their characteristics which can affect their performance and economic viability. Primary facies distribution and properties, sequence stratigraphic framework, diagenesis and fracturing are amongst the main factors that exert important controls on reservoir properties. Different subsurface disciplines must contribute to the characterization of carbonate reservoirs with the aim of understanding and predicting reservoir dynamic performance to help defining further exploration plans and reservoir development and depletion strategies.
Complex carbonate reservoirs are widely exploited around the world (Russia, Middle East, Europe, Americas). In these reservoirs the interaction between matrix characteristics (facies, layering, poro-permeability, wettability) and fractures characteristics (timing, style, sizes, distribution, orientation) invariably control fluid flow by enhancing or imparting primary reservoir properties. As a result the understanding of fracture distribution and in particular the relationship between fractures and sedimentary facies is fundamental for an adequate description of fractured carbonate reservoirs. Understanding these relationships is a key to correctly model matrix storativity and fracture connectivity, and to identify potentially highly productive intervals.
To achieve such comprehensive reservoir descriptions the geoscientist must make use of outcrop analogues, the evidence from which has to be integrated with subsurface datasets from the specific reservoir under study. For the models to have predictive capability the geological evolution must also be understood which is greatly helped by disposing of field observations in well constrained tectonic settings analogue to the setting of the reservoir under study.
With this type of complex reservoirs exploration, appraisal and development strategies have to decided based on:
• The spatial distribution of HC storage intervals;
• The spatial distribution of preferential fluid pathways;
• The dynamic behaviour of the reservoir;
• The company business plan.
Only the correct understanding of these elements and a clear geological model will end up in the correct chose of the appraisal and development wells locations and the better performances of the reservoir production.
In this framework, outcrop analogues often supply a complete spatial distribution of the main key elements that also interact in subsurface where data resolution could be limited to well locations and to seismic resolution.
The Cretaceous Apulia Platform
The Cretaceous Apulia Platform carbonates outcropping in the foreland of the Southern Apennines Thrust Belt (southern Italy), are characterized by superb large scale outcrop where this type of study can be carried out and represents also an outcrop analogue of some of the largest European onshore HC fields. On the eastern side of southern Italy the Cretaceous Apulia carbonates massively outcrop in several large quarries and along the cost.
The outcrop used as an example for this study is a deep quarry (>130m) located in the Murge Plateau area. In this quarry the section belongs to the Altamura Formation (Upper Cretaceous). From the bottom the sequence is characterized by:
• grey argillaceous chalk and interbedded white mud-supported lithofacies, with cryptalgal laminitic horizons, forming characteristic peritidal cyclothems;
• massive, fine-grained peloidal packstone-wackestone, with scattered bioclasts and bioclastic packstone, with benthic foraminifers, bivalve (rudist) debris and ostracods.
• featureless, locally cross-bedded medium-grained skeletal calcarenite. A few biogenic beds with sparse rudists,
• very fine-grained pelletal and skeletal packstone, both massive and thin bedded. A characteristic feature of this interval is the occurrence of embrionic tabular colony, composed of juvenile forms of radiolitids in life position
• a repetition of biogenic units (radiolitid colonies) interbedded with poorly cemented, fine-grained pelletal, skeletal and foraminiferal sediment (packstone)
Only the skeletal calcarenites and the biogenic unit show interesting petrophysical properties and can be interpreted as storage intervals in the sequence.
The quarry is characterized by various faults with variable displacement (from few meters to some tend of meters). In general, fracturing is less abundant than in other quarries of the area and more localized in correspondence of faults. Fault zones are relatively narrow (few meters) and characterized by intense fracturing in some cases developed only on one side of the fault plane. Coarse grained intervals (biogenic rudist bodies and calcarenitic beds) tend to show a lower fracture density than fine grained intervals, while faulting seems not to be influenced by lithological texture of the rock. Fractures and faults act as preferential fluid pathways.
In this kind of geological framework the trajectory of a deviated well will have a completely different response in the dynamic/production behavior that the trajectory of a vertical well because of the petrophysical characteristics of the units encountered. Moreover the location of the well trajectory (intersecting a major fault or not) will also have a different impact on the well performances.