oa Origin, Distribution and Petrophysical Properties of High Porosity/Permeability Sub-Horizontal Drains within a Dolomitised Sequence: Lessons Learned from Outcrop Analogue

Dolomite sequences and intervals often show the best reservoir potentials and are considered as key<br>productive zones. It is difficult to completely unravel the diagenetic evolution of a carbonate sequence<br>because of the complexity and variety of the processes that affect the rocks through their evolution.<br>This is mainly due to the interactions between different processes and, in subsurface, because of the<br>lack of complete datasets or the limited spatial representativity of well data. The origins and spatial<br>variability of reservoir properties in structurally-controlled, partially dolomitised reservoirs are poorly<br>understood because of their complexity. The use of outcrop analogues for better understanding<br>subsurface reservoirs is essential to reduce some of the main reservoir uncertainties. The geometry,<br>internal heterogeneity and petrophysical properties of dolomite bodies were studied in a Jurassic<br>partially dolomitised outcrop analogue in the Southern Alps using an integrated, multidisciplinary<br>approach. Dolomitisation of the lower part of the studied section led to the development of good<br>petrophysical properties for a potential hydrocarbon reservoir, in particular by the formation of porosity<br>systems interconnected with fracture and fault networks, hence assuring a consistent permeability<br>through the entire sequence. The dolomitisation process determined a highly variable porosity network<br>controlled by the original facies, the degree of dolomitisation and the structural framework. Near open<br>fracture swarms or faults, the dolomitisation front tends to uprise, sometimes generating vertical<br>chimneys that can cross the overlying sedimentary succession. In these zones the dolomite is massive,<br>with a complete reworking of the original limestone, sometimes with strong evidence of hydro<br>fracturing related to overpressured fluids.From these vertical dolomite bodies, high porosity and<br>permeability bedding-parallel dolomitic bodies develop with lenticular or planar shape. These bodies<br>can be 10’s of meters in length and 1-3 meters in thickness and are often stacked one on top of the<br>other along major fault zones.Based on core samples the porosity associated to these dolomitic bodies<br>can be up to 25-30% with an extremely good connectivity. Matrix porosity and permeability, directly<br>measured on plug analysis, vary respectively between 0.5-25% and 0.05-40 mDarcy. These<br>petrophysical data appear strongly related to the diagenetic facies associations.