Nano-Scale Pore Structure Evolution of Middle Devonian Organic-Rich Black Shale Through Thermal Maturation

Abstract

The pore structures and the evolution of porosity were analyzed in samples from three wells that penetrated the Marcellus Shale in Appalachian basin. The thermal maturity ranges from Ro (vitrinite reflectance) 1.36% to Ro 2.89%. Total organic carbon (TOC) of Mahantango Formation to Marcellus Shale samples from West Virginia and Pennsylvania used in this study varies from 0.25 to 9.12 wt.%. Subcritical N2 adsorption and Scanning Electron Microscope (SEM) techniques were utilized to test nano-scale pore structures (pore sizes, pore volumes, and pore-size distributions) qualitatively and quantitatively. Also, X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis were conducted to study the heterogeneity of mineral composition and its influence on the pore structures. Before running N2 adsorption, shale samples were crushed, then degassed under high vacuum with heating. In order to find an appropriate procedure for sample preparation, one set of rock sample was crushed and hand-grounded, then degassed under five different temperatures (25°C, 80°C, 120°C, 200°C, 300°C). Based on the results, size and more critically temperature for degassing are important factors. A size of <250 microns (60 mesh) degassed under 120°C for 24 hours is the recommended procedure. After the measurements, BET, t-Plot, H-K, and BJH models were used to interpret the results. The results, combined with SEM image analysis indicate that, as thermal maturity increases, specific surface areas (SSA), pore volumes, and pore-size distributions varies significantly. Among all the factors we tested, thermal maturity appears to be the major control on evolution of pore structures of organic-rich mudrocks.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016