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Eagle Ford Pores, Pore Throats, and Fluid Content Identified Through NMR Analysis

Abstract

Mudrock porosity is associated with both inorganic and organic matter, and hydrocarbons are found in both. The upper Eagle Ford is considered to be dominated by inorganic porosity, while the lower Eagle Ford is considered to have more organic-hosted porosity related to high organic content. The differences in inorganic versus organic pore types play a large role with regard to pore networks. This study investigates Eagle Ford mudrock pores through the use of nuclear magnetic resonance (NMR) in order to better quantify porosity values for these unconventional reservoirs. Recently, laboratory-based NMR has been used to measure fluid content and pore volume of mudrocks affordably and nondestructively. While NMR is quite versatile, important limitations exist, including the inability to directly measure pore-throat sizes or pores that are not fluid filled. However, calibration to mercury injection capillary pressure (MICP) measurements from finely grinding the same sample into powder yields interpretable results. Using a total of 28 samples from three wells in Karnes and Maverick Counties, South Texas, this study examines how pore-throat size distribution and fluid content differ vertically in the succession. The lithology and facies vary, both vertically within each well and laterally between wells. Six facies were identified among the three wells. With the aid of geochemical analysis such as XRF these lithologies can be broken down into subfacies based on calcite content and minor elements, such as molybdenum and vanadium, related to anoxia. These subfacies were used to pick the sample points. While a mixture of pore types is anticipated, samples with higher organic-hosted porosity are expected to have smaller pore throats and contain more oil. Early results indicate measurable pore throat sizes between 0.001 and 0.01 microns filled with low to medium-viscosity fluids, likely a mixture of water and oil, and intrusion corrected porosities ranging between 5.2% to 9.4% in the lower Eagle Ford. Greater heterogeneity of the pore system in the lower Eagle Ford has been confirmed through an SEM study. Using NMR to evaluate the linkage between facies changes and pore-throat-size distribution and content will aid future play-wide analysis.