Pore Characteristics in Refractory Kerogen vs. Solid Bitumen and Pore Systems in the Dry-Gas Window Marcellus Formation, Appalachian Basin, Northeastern Pennsylvania
The Marcellus Shale in the Appalachian Basin is the most expansive shale gas play in the U.S. Its pore systems contribute to gas storage and affect estimated sorbed and free gas capacities. In this work, we systematically investigated pore systems and their relationships to organic matter (OM) and mineral matrix in the dry-gas window from core samples at near 3.0% Ro maturity using a combination of HIM (helium-ion microscopy), FE-SEM (field-emission scanning electron microscopy) imaging, petrography, XRD (X-ray diffraction), LECO carbon analyzer, helium porosimetry, and GRI pressure-decay method for permeability. At the dry-gas window, OM is typically found as refractory kerogen and solid bitumen (pyrobitumen). We hypothesize that most pores would be OM spongy pores hosted by solid bitumen and form the predominant pore network. Both SEM and HIM imaging reveal that OM spongy pores whose sizes are mostly < 200 nm are predominant. Mineral pores are present, many associated with carbonate dissolution. The majority of OM is solid bitumen which hosts spongy pores. Correlative relationships between total organic carbon (TOC) and porosity (R2=0.5) and TOC and permeability (R2=0.76) suggest that both solid bitumen and OM spongy pores form a connected network. The basal Marcellus Shale member (BMS) is more siliceous (<40 wt.% total clay minerals) and has higher average TOC than the lower and upper shale members (LMS and UMS). Observations from thin-sections confirm that the BMS has abundant siliceous algal cysts, calcified radiolarians, authigenic quartz, and contains less detrital components than the LMS/UMS. The total porosity does not vary much throughout the Marcellus; however, the measured matrix permeability of the BMS is two orders of magnitude higher than that of the LMS/UMS. The significant increase in permeability might be related to higher TOC, more Type II kerogen, and a less-compacted siliceous framework that helps preserve interparticle pores in the BMS than the argillaceous framework in the LMS/UMS. A connected pore network provides
an effective flow pathway for bitumen/oil to migrate, resulting in formation of a more effective OM pore network. Using SEM petrography, we observed qualitatively that OM nanopores in solid bitumen are more equant (spongy shape) and larger than those in refractory kerogen. Continuing works include using micro-FTIR (Fourier Transform Infrared Spectroscopy) and correlative light and electron microscopy to further identify and differentiate solid bitumen and kerogen.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019