ABSTRACT

We investigated the effect of porosity, pore geometry, and diagenetic history on the elastic properties of dry, tightly cemented grainstones whose pore space consists dominantly of intragranular microporosity within micritic grains. The integration of laboratory petrophysical measurements (porosity, P- and S-wave velocity), petrographic analysis and scanning electron microscope (SEM) imaging of micropore space of 80 Lower Cretaceous microporous carbonate samples from Provence (south-east France) allows (1) the changes in porosity and pore geometry during the diagenetic history to be related to changes in elastic properties, and (2) the impact of micritic grain diagenesis on the elastic properties of microporous grainstones to be quantified by means of fitting parameters derived from equivalent elastic medium modeling. The Urgonian microporous cemented grainstones are elastically equivalent to a homogeneous calcitic host with spherical calcitic inclusions comprising spheroidal pores. The best fit is obtained when porous spheres are modelled using the differential effective medium (DEM) approach and the whole composite using the self-consistent (SC) method (DEM-SC model). At lower porosity values (<20%), when the micropore volume is controlled by intercrystalline cementation processes without compaction, the equivalent pore aspect ratio (EPAR) derived from DEM-SC modelling is nearly constant and averages 0.15. At higher porosities, changes in micropore space architecture related to leaching processes result in slightly increasing EPAR. The recognition of EPAR-preserving versus EPAR-non preserving elastic property evolution is proposed as a tool for diagenetic pattern detection in microporous carbonate reservoirs.

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