Development and compartmentalization of chalky carbonate reservoirs: The Urgonian Jura-Bas Dauphiné platform model (Génissiat, southeastern France)

The chalky units are presently compartmentalized by horizontal layers of compact limestones characterised by a total porosity between 2.9 and 4.5%, a threshold radius between 0.04 and 0.09 μm, a trapped porosity of 92.4-92.8% and a gas permeability of 2 mD. Like the chalky limestones, the present-day reservoir qualities and transport properties of compact limestones are controlled by the intercrystalline microporosity of microcrystalline areas. The petrophysical differences between the chalky and compact limestones result from differences in the pore network of microcrystalline areas due to an increase in the average size of particles and a modification of crystal-crystal contacts. These petrographical differences reflect an evolution of micritic areas of original sediments during submarine lithifications (precipitation of microcrystalline cements within the pore space and/or an increase in solid-solid contacts by crystal growth) which preceded the aggrading neomorphism induced by the meteoric porewater input.