The equivalent pore aspect ratio as a tool for pore type prediction in carbonate reservoirs

Abstract

The equivalent pore aspect ratio provides a tool to detect pore types by combining compressional and shear wave velocities, porosity, bulk density, and mineralogical composition of carbonate rocks. The integration of laboratory measurements, well logs, and petrographic analysis of carbonate rocks from various depositional and diagenetic settings (Lower Cretaceous presalt nonmarine carbonates from offshore Brazil; Lower Cretaceous shallow-water platform carbonates from southeast France; and Upper Cretaceous deep-water carbonate from the Gargano Peninsula, Italy) allowed the acoustic signature of a wide range of pore types to be quantified. The equivalent pore aspect ratios are shown to be independent of pore volume and mineralogy. They can, therefore, be regarded as relevant parameters for discriminating selected pore network architectures in carbonate reservoirs. Interparticle and intercrystalline microporosity and macroporosity may include a variety of distinct pore network structures and a wide range of acoustic signatures depending on particle shape, nature of contacts, packing, and cementation (intergranular macroporosity in a densely packed granular sediment, secondary pseudofenestral, etc.). The acoustic response of moldic pores appears to be mainly controlled by the shape of the dissolved allochems. The present work reveals also that correct pore type detection from acoustic and porosity measurements is highly dependent on the pore type classification used. A selection of petrographic and diagenetic features to be considered for the construction of a suitable pore type classification is proposed to improve the detection of the pore type effect on the acoustic properties of carbonate reservoirs at plug and well log scale.