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AUTHORS: Di Cuia R., Scifoni A., Riva A., Moretti A., Caline B.

YEAR: 2006

Bathurst Conference Norwick (UK)

Geometry and internal heterogeneity of subsurface dolomite bodies remain difficult to predict and model. Therefore, a study of outcrop analogue has been carried out in the Southern Alps in order to understand the key parameters (depositional, diagenetic and structural) that constrain both the external envelop and the facies organization of fault-related dolomite bodies.
A dedicated field work has been carried out in the Asiago Plateau (Southern Alps), concerning the dolomitisation of a variable basal portion of the Calcari Grigi Group, deposited in an early Jurassic carbonate platform s.l.. Dolomitisation affects the three formations of the Calcari Grigi Group in different ways, because of the presence of contrasted carbonate lithologies, fracture networks and the stratigraphic position of each Formation (i.e., Mt Zugna, Loppio and Rotzo Fm). Dolomitisation is interpreted as a medium burial dolomitisation, with fluids ascending through the main fault and fracture systems.
The available data highlight a great lithological variability, ranging from pure limestones, slightly dolomitised limestones and a wide range of dolostones. Accordingly, textures span from fine-grained unimodal to coarse grained strongly polymodal dolomites that represent the effect of a “multi-phase” process. The texture variations, concerning both dolomite to dolomite and dolomite to limestone changes, are abrupt and transitional with vertical and horizontal directions, and take place both at the reservoir/formation and at the core/outcrop scales. Some of these variations are controlled by bedding (which drives the dolomitising fluids) and by the pre-existing facies distribution.
The geometry of dolomite bodies can be characterized at different scales: they range from spectacular dolomitic pinnacles (tens to hundred meters), clearly related to the presence of large scale fault systems, to bed-parallel bodies linked to more local conditions, such as facies variations, bedding and fracturing
Dolomitisation of the lower part of the Calcari Grigi Group led to the development of good petrophysical properties for a potential hydrocarbon reservoir, in particular by the formation of porosity systems interconnected with fracture and fault networks, hence assuring a consistent permeability through the entire sequence. The dolomitisation process is thought to determine a highly variable porosity network mainly depending from the degree of dolomitisation. In some places, the pores are also characterized by the genesis of late saddle dolomite and calcite cements which partially fill the vugs. Notwithstanding, in some cases, dolomitisation led to the development of good porosity, up to 15-20%, usually confined to lenticular bodies.