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Pore Size and Textural Analysis of Carbonates from Nuclear Magnetic Resonance Logging : an Arbuckle Case Study

John Doveton

The T2 relaxation times recorded by nuclear magnetic resonance logging (variously acronymed as NMR, MRI, and CMR) are measures of the ratio of the internal surface area to volume of the formation pore system. While standard porosity logs are restricted to estimating the volume, the NMR log partitions the pore space as a spectrum of pore sizes. These logs have great potential in carbonate sequences which can have single, double, or triple porosity systems and whose pores have a wide variety of sizes and shapes. Continuous coring and NMR logging of the Arbuckle in KGS #1-32 Wellington gave a great opportunity to compare core textural descriptions to NMR T2 relaxation time signatures. Grainstones, packstones, and mudstones showed a distinctive bimodal pore-size distribution with a dominant large pore-size in grainstones transitioning towards dominant small pore-size in mudstones. When the bin counts of the T2 distribution log are plotted as percentages, grain-supported facies are easily distinguished from mud-supported facies. This distinction is also strongly reflected by the computed gamma-ray (CGR) log which records contributions from thorium and potassium and therefore assesses clay content. T2 relaxation times of greater than one second are considered to pick up visible vuggy porosity and this property was evaluated by comparing core observations of vugs, ranging from pin-point to fist-size with pore volumes recorded by slower times. There is a good overall match and, not surprisingly, intervals of missing core generally coincided with long T2 relaxation times. 

AAPG Search and Discovery Article #90176©AAPG Mid-Continent Meeting, Wichita, Kansas, October 12-15, 2013