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Quantification of pore structure and its effect on sonic velocity and permeability in carbonates

¹ University of Miami, Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, 4600 Rickenbacker Causeway, Miami, Florida 33129; rweger{at}rsmas.miami.edu
² University of Miami, Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, 4600 Rickenbacker Causeway, Miami, Florida 33129; geberli{at}rsmas.miami.edu
³ University of Miami, Rosenstiel School of Marine and Atmospheric Science, Division of Marine Geology and Geophysics, 4600 Rickenbacker Causeway, Miami, Florida 33129
⁴ Gerencia Geología y Estudios Integrados, Dirección Exploración y Desarrollo de Negocio, Macacha Güemes 515, (C1106BKK), Puerto Madero, Buenos Aires, Argentina
⁵ Department of Geology and Geophysics, Texas A&M University, College Station, Texas 77843

Ralf J. Weger was a postdoctoral researcher with the Comparative Sedimentology Laboratory at the University of Miami when the article was written. He received his B.S. degree in systems analysis (2000) and his Ph.D. in marine geology and geophysics (2006) from the University of Miami. His dissertation focuses on quantitative pore- and rock-type parameters in carbonates and their relationship to velocity deviations. His main interests range from processing and visualization of geophysical data to petrophysical characterization of carbonate rocks.

Gregor P. Eberli is a professor in the Division of Marine Geology and Geophysics at the University of Miami and the Director of the Comparative Sedimentology Laboratory. He received his Ph.D. from the Swiss Institute of Technology (ETH) in Zürich, Switzerland. His research integrates the sedimentology, stratigraphy, and petrophysics of carbonates. With laboratory experiments and seismic modeling, his group tries to understand the physical expression of carbonates on log and in seismic data. He was a distinguished lecturer for AAPG (1996/97), Joint Oceanographic Institutions (1997/1998), and the European Association of Geoscientists and Engineers (2005/2006).

Gregor T. Bächle graduated from the University of Tübingen in 1999 with a Diploma (equivalent to M.Sc. degree) in geology. In 2001, he joined the Comparative Sedimentology Laboratory (CSL) with a Scholarship of the German Academic Exchange Service to obtain a Ph.D. from the University of Tübingen. From 2004 to 2008, he was a research associate in the CSL, managing the rock physics laboratory. He is currently working for ExxonMobil Upstream Research Company, Quantitative Interpretation, Houston, Texas.

Jose Luis Massaferro is a geology manager in Repsol YPF's exploration office in Argentina. He received his Ph.D. from the University of Miami in 1997. He was a Fulbright Fellow while pursuing his studies in Miami. Prior to his Ph.D. studies, he worked for Texaco as a geologist. In 1998, he joined Shell E&P and was involved in different projects, including 3-D seismic volume interpretation, high-resolution sequence stratigraphy, and kinematic modeling of compressional structures. In 2005, he joined Repsol in Madrid.

Yue-Feng Sun is an associate professor at Texas A&M University. He received his Ph.D. (1994) from Columbia University. He has 25 years of experience as a geoscientist in the industry and academia. His professional interests include carbonate rock physics, poroelasticity, poroelectrodynamics, reservoir geophysics, and petroleum geology. He is a member of AAPG, the American Geophysical Union, American Physical Society, and the Society of Exploration Geophysicists.

ABSTRACT

Carbonate rocks commonly contain a variety of pore types that can vary in size over several orders of magnitude. Traditional pore-type classifications describe these pore structures but are inadequate for correlations to the rock's physical properties. We introduce a digital image analysis (DIA) method that produces quantitative pore-space parameters, which can be linked to physical properties in carbonates, in particular sonic velocity and permeability.

The DIA parameters, derived from thin sections, capture two-dimensional pore size (DomSize), roundness (), aspect ratio (AR), and pore network complexity (PoA). Comparing these DIA parameters to porosity, permeability, and P-wave velocity shows that, in addition to porosity, the combined effect of microporosity, the pore network complexity, and pore size of the macropores is most influential for the acoustic behavior. Combining these parameters with porosity improves the coefficient of determination (R²) velocity estimates from 0.542 to 0.840. The analysis shows that samples with large simple pores and a small amount of microporosity display higher acoustic velocity at a given porosity than samples with small, complicated pores. Estimates of permeability from porosity alone are very ineffective (R² = 0.143) but can be improved when pore geometry information PoA (R² = 0.415) and DomSize (R² = 0.383) are incorporated.

Furthermore, results from the correlation of DIA parameters to acoustic data reveal that (1) intergrain and/or intercrystalline and separate-vug porosity cannot always be separated using sonic logs, (2) P-wave velocity is not solely controlled by the percentage of spherical porosity, and (3) quantitative pore geometry characteristics can be estimated from acoustic data and used to improve permeability estimates.