Pore network variation identified through NMR analysis : Eagle Ford Group, South Texas, USA
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Mudrock porosity is associated with both organic and inorganic matter, and hydrocarbons are found in both. The upper Eagle Ford Group is dominated by inorganic porosity, while the lower Eagle Ford Group is considered to have more organic-hosted porosity related to its high organic content. The differences in organic and inorganic pore types play a large role in the effectiveness of pore networks. This study investigates Eagle Ford Group mudrock pores through the use of nuclear magnetic resonance (NMR) in order to more accurately describe porosity. Laboratory-based NMR has been used to measure the fluid content and pore volume of mudrocks affordably and nondestructively. Although NMR is versatile, it has limitations, in that pore-throats and pores must be fluid filled in order to be detected. However, calibration to Mercury Injection Capillary Pressure (MICP) measurements yields interpretable and valuable results. Using a total of 28 core samples from three wells in Karnes and Maverick Counties, South Texas, this study examines how pore networks differ vertically within the succession. The lithology and facies vary vertically within each well and laterally between wells. The facies groups defined with visual core examination were then modeled with wireline logs. While the facies groups show significant overlap in petrophysical character measured by NMR, scanning electron microscopy (SEM), total organic carbon (TOC), x-ray fluorescence (XRF), and x-ray diffraction (XRD), the samples taken from the same facies group nonetheless have similar MICP intrusion corrected porosities. When samples are compared without regard to facies groups, both MICP and TOC strongly correlate with NMR. While NMR detects differences in the pore network including the relative amount of porosity associated with clay sized particles between the upper and lower Eagle Ford; and SEM shows that much of the upper Eagle Ford organic porosity appears to be associated with migrated organic matter. Vertically segregating the Eagle Ford by facies groups that can be modeled by wireline log, while difficult, with further sampling, may prove useful to play-wide mapping.