AUTHORS: R. Di Cuia* & A. Riva (G.E.Plan Consulting, Italy), B.Caline (Total, France), C.Pabian-Goyhenche (Total, France)
Geo2010 conference in Manama (Bahrain) in March 2010
Diagenesis plays an important role over most of the calcareous reservoirs in the best HC productive basins of the world. Dolomite sequences and intervals often show the best reservoir potentials and are considered as key productive zones. It is difficult to completely unravel the diagenetic evolution of a carbonate sequence because of the complexity and variety of the processes that affect the rocks through their evolution. This is mainly due to the interactions between different processes and, in subsurface, because of the lack of complete datasets or the limited spatial representativity of well data.
Structurally-controlled, partially dolomitised reservoirs are proven hydrocarbon reservoirs in the subsurface; yet their origins and spatial variability in reservoir quality are poorly understood because of their complexity. The use of outcrop analogues for better understanding subsurface reservoirs is essential to reduce some of the main reservoir uncertainties.
The geometry, internal heterogeneity and petrophysical properties of dolomite bodies were studied in a Jurassic partially dolomitised outcrop analogue in the Southern Alps using an integrated, multidisciplinary approach. Dolomitisation of the lower part of the studied section 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 determined a highly variable porosity network controlled by the original facies, the degree of dolomitisation and the structural framework.
Near open fracture swarms or faults, the dolomitisation front tends to uprise, sometimes generating vertical chimneys that can cross the overlying sedimentary succession. In these zones the dolomite is massive, with a complete reworking of the original limestones, sometimes with strong evidence of hydrofracturing related to overpressured fluids.
From these vertical dolomite bodies, high porosity and permeability bedding-parallel dolomitic bodies develop with lenticular or planar shape. These bodies can be 10’s of meters in length and 1-3 meters in thickness and are often stacked one on top of the other along major fault zones
Based on core samples the porosity associated to these dolomitic bodies can be up to 25-30% with an extremely good connectivity. Matrix porosity and permeability, directly measured on plug analysis, vary respectively between 0.5-25% and 0.05-40 mDarcy. These petrophysical data appear strongly related to the diagenetic facies associations.