Characterizing the pore structure of a shale-gas reservoir is significant for calculating the original gas in place and fluid-flow characteristics. To better understand the impact of organic matter accumulation, redox condition, and depositional environment on pore structure and storage capacity, integrated geological and petrophysical characterization of the Devonian organic-rich shale was conducted. Core samples from a newly drilled science well from the Marcellus Shale Energy and Environment Laboratory (MSEEL) project and other wells in the Appalachian basin were selected to undertake this research. X-ray fluorescence (XRF), X-ray diffraction (XRD), and pyrolysis were performed to understand variations in composition, mineralogy and total organic carbon (TOC). Samples were examined from an interval including the overlying Tully Limestone, organic-lean Mahantango Shale, organic-rich Marcellus Shale and top of the underlying Onondaga Limestone.

We introduce the application of subcritical N2 adsorption to measure pore volume, pore-size distribution, and pore-surface area, which are critical properties in characterization of the nano-scale pore regime of mudstone reservoirs. Results of the test are used to build models of the mudstone pore systems. With variations of TOC and mineralogy, changes in the characteristics of pore structure are observed. Middle Devonian shales have complex, heterogeneous pore size distributions as identified by subcritical N2 adsorption. XRD results suggest a high content of clay minerals (mainly illite) through both Mahantango and Marcellus shales. Hysteresis of N2 adsorption isotherm indicates slit-shape pores between 2nm and 50nm, possibly formed by clay particles. Organic matter shows strong influence on pore volume and pore surface area (BET specific surface area), which strongly influences storage mechanisms of shale-gas reservoirs. Carbonate-rich intervals show very low pore volume especially micropore (pore width smaller than 2nm) volume, and surface area. The results of N2 adsorption are compared with NMR log to upgrade the evaluation.

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