NMR Relaxation Measurements in Carbonates: Impact of Early Diagenesis from Pore-Space and Rock-Typing Perspective

Benoit Vincent¹, Marc Fleury², Yannick Santerre¹, and Mei Han^2
1Sedimentology-Stratigraphy, IFP, Rueil-Malmaison, France
²Reservoir Engineering, IFP, Rueil-Malmaison, France

The use of rock-types in petroleum exploration greatly simplifies reservoir descriptions, and improves the prediction of petrophysical properties distribution in static geological models. Measurement of the pore throat size with MICP (Mercury Injection Capillary Pressure) is most often used, as the necessary complement to thin sections and permeability data, to group the samples. In carbonates, a measure characterizing porous network is mandatory since true porosity-permeability relations are rare. The NMR (Nuclear Magnetic Resonance) technique also brings information about the pore space, but can moreover be realized during wireline logs acquisition. Such a technique is exceptionally rich, but still not used enough because the NMR information differs from the MICP one, and difficulties remain in the NMR signal interpretation in complex media.

The scope of this work is to elaborate a guide for interpreting NMR signal of carbonates in wireline logs, and we present here the first set of results. In this study, we performed an integrated petrographic (microscope, SEM, scanner) - petrophysic (NMR, MICP, BET) analysis on 16 samples of carbonate from outcrops. These samples illustrates the variability of sedimentary facies (textures and depositional environments), and can be considered as petrophysical analogues of reservoirs (good Φ/K).

We first observe an almost systematic decoupling of sedimentary facies and petrophysical properties. Different facies may display identical NMR/MICP signals and reversely. This study also illustrates the main impact of eogenesis (early diagenesis), i.e. micritization and early marine cementation, on the reservoir properties and their physical expression. It also appears that NMR and MICP information are mostly in accordance, except when a pore coupling phenomenon occurs leading to a misinterpretation of the pore network from NMR.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas