Mineral/organic Matter Associations and Pore Microtextures in the Marcellus Formation, West Virginia

Abstract

Understanding detailed reservoir-rock mineralogy and pore structure is important for elucidating water-rock interactions that occur during hydraulic fracturing of Marcellus shale. Core samples from a depth of 7451.5 feet from an active production well (MIP 3H) in Morgantown, West Virginia have been interrogated by light and electron microscopy. We examined fresh cleavage fragments as well as rock surfaces that were gallium ion beam polished. Fragment orientations, based on rock cleavage planes, are observed both parallel and perpendicular to bedding. The goal is to assess the natural pathways for fluid migration and to link the resulting produced water chemistry to mineral and organic materials that contribute to that chemistry.

At least three texturally distinct types of organic matter (OM) are identified in Marcellus shale samples. These include relatively large pods visible with the digital light microscope, and two finely-disseminated occurrences associated with illitic clay or clustered (some framboidal) pyrite that are only resolved with scanning electron microscopy (SEM). Backscattered electron (BSE) images and energy dispersive X-ray spectroscopy (EDXS) reveal dendritic and needle-like Fe-Mg rich clay (consistent with chlorite) intimately intermingled with the pods, which are roughly circular and approximately 200 microns in diameter, as measured parallel to bedding. The matrix clay surrounding the pods is mostly illitic in composition. In addition, the pods also contain euhedral quartz and calcite crystals, which are interpreted to be diagenetic and possibly represent biologically-mitigated crystallization.

Dual-beam focused ion beam (FIB)/SEM slices acquired with secondary electron detectors reveal different pore textures within these organics. The first, large, pod type OM does not show obvious pores. The second and third types (illitic clay- and pyrite-associated OM) have obvious pores and occur in much smaller clusters. The porous OM types range from approximately 100s of nm to a few microns in size, and contain pores ranging in size from single to tens of nm. Nitrogen gas sorption measurements are planned to help assess the pore size distribution of the connected pore network.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016