Characterization and origin of permeability-porosity heterogeneity in shallow-marine carbonates: from core scale to 3D reservoir dimension (Middle Jurassic, Paris Basin, France).
Résumé
Nuclear magnetic resonance (NMR), stable isotope geochemistry of micro-sampled cores, NMR well-logs and 3D modeling are used to investigate the carbonate permeability-porosity heterogeneity along 230 m-thick limestones of the Paris Basin. Despite the global low porosity and permeability of the limestones, two aquifers units with porosity greater than 15% were identified. These two aquifers are very different in terms of pore through radii and NMR signal. The first one (A1: Aquifer 1) is a 7 m-thick mudstone unit, dominated by extended microporosity with pore throat radii of 0.25 μm to 0.3 μm. The second one (A2: Aquifer 2) is a 15 m-thick oolitic grainstone units showing macropores reaching 100 μm and pore throat radii of 32 μm. From core descriptions and wireline logs on 26 wells, a 3D static geological model is build. The fine tuning of permeability calculations from NMR logs realized along 12 of the wells, allows porosity and permeability heterogeneity to be distributed within a 3D model at the reservoir scale (area of about 2000 km2) which match the flow behavior illustrated by well tests. Associated with early meteoric calcite cements and poorly developed burial blocky calcite cements, the porous and permeable intervals may be predicted in two stratigraphic and diagenetic considerations. Firstly, the syn-sedimentary meteoric dissolution or neomorphism of the initial high magnesium calcite and aragonite particles or clasts into low magnesium calcite particles or cements prevented most mechanical and chemical compaction during the first steps of burial. Secondly, the regional stratigraphic architecture reveals the presence of local permeability barriers, which prevented Early Cretaceous lateral meteoric water circulation and the associated burial calcite cementation.