New insight into the microtexture of chalks from NMR analysis

Ophélie Faÿ-Gomord, Jeroen Soete, Konstantina Katika, Serge Galaup, Bruno Caline, Fanny Descamps, Eric Lasseur, Ida Lykke Fabricius, Jessica Saïag, Rudy Swennen, Sara Vandycke

Research output: Contribution to journalJournal articleResearchpeer-review

215 Downloads (Pure)

Abstract

An integrated petrographical and petrophysical study was carried out on a set of 35 outcrop chalk samples, covering a wide range of lithologies and textures. In this study various chalk rock-types have been characterized, in terms of microtextures and porous network, by integrating both geological, sediment-petrological and petrophysical data, including porosity, permeability, low-field NMR (Nuclear Magnetic Resonance), MICP and specific surface area (BET) measurements. The data allow an in depth understanding of the NMR signal of chalks, with a focus on tight chalks, including all low reservoir quality chalks independently of their sedimentological and/or diagenetic history. The study aims to develop an NMR-based approach to characterize a broad range of chalk samples. The provided laboratory low-field NMR chalk classification can be used as a guide to interpret NMR logging data.Based on the petrographical and petrophysical analysis, 6 groups of samples were identified, each of them characterized by a unique NMR signature: (1) micritic chalks, (2) grainy chalks, (3) cemented chalks, (4) marl-seam chalks, (5) argillaceous chalks and (6) silicified chalk. NMR T2 distributions were linked to pore body size and T2 logarithmic (T2lm) was calculated. It is apparent that tight chalks, whether their characteristics are sedimentological or diagenetic, yield smaller pore body sizes (T2lm <20 ms), as well as narrower pore throats (average radius <150 nm) and lower permeability values (typically below 0.2 mD). Grainy chalks possess T2 distributions reflecting larger pore sizes (T2lm > 60 ms) and pore throats (average radius > 290 nm) and higher permeabilities (up to 13 mD). The marl-seam chalk samples yield bimodal T2 distributions, with a first peak related to the micritic matrix pores and a second peak related to intraparticle pores within fossils. For all samples, permeability was inferred from NMR spectra using SDR (Schlumberger Doll Research) model.
Original languageEnglish
JournalMarine and Petroleum Geology
Volume75
Pages (from-to)252-271
Number of pages20
ISSN0264-8172
DOIs
Publication statusPublished - 2016

Keywords

  • NMR
  • Chalk
  • Petrography
  • Petrophysics
  • Microporosity
  • Porecasts
  • Marl-seams
  • MICP

Cite this

Faÿ-Gomord, O., Soete, J., Katika, K., Galaup, S., Caline, B., Descamps, F., Lasseur, E., Fabricius, I. L., Saïag, J., Swennen, R., & Vandycke, S. (2016). New insight into the microtexture of chalks from NMR analysis. Marine and Petroleum Geology, 75, 252-271. https://doi.org/10.1016/j.marpetgeo.2016.04.019