Abstract

Middle Devonian Marcellus Shale and late Devonian New Albany Shale samples were argon-ion milled and studied by scanning electron microscope for petrographic features and pore development. Petrographic observations revealed that a finite number of pore types exist in spite of considerable variability in composition, depositional setting, and compaction history. The four major pore types are framework pores, framework shelter pores, dissolution pores within inorganic grains, and organic matter pores. Framework pores occur in open spaces of the grain fabric. The most common framework pores are defined by phyllosilicate (clays, micas) and carbonate grains (biogenic, diagenetic) and also occur in areas with abundant diagenetic silica. Framework pores associated with carbonate grains can be well developed where abundant skeletal debris provides shelter porosity. Phyllosilicate framework pores increase in abundance with increasing clay content, but more critically, their abundance in compacted shales hinges on the presence of pressure shadows generated adjacent to mechanically competent grains (quartz, feldspar, dolomite, calcite, pyrite) that resist compaction. Dissolution pores in inorganic grains were predominantly encountered in association with calcite and dolomite grains. In places, other minerals, such as pyrite, also can show dissolution effects, but do not contribute significantly to overall porosity. Dissolution pores probably reflect decreased pH associated with the formation of carboxylic and phenolic acids at elevated temperatures (about 80 to 120° C). At low carbonate contents (a few percent), dissolution pores constitute only isolated porosity, but in shale intervals that contain abundant carbonate, or where carbonate grains were concentrated into laminae, this pore type may be an important facilitator of gas storage and transmission. Pores within organic matter are maturity dependent and restricted to amorphous organic matter (bituminite/amorphinite) in thermally mature samples (>0.6%R_o). Grain and fabric shrinkage during core storage and sample processing produced pore artifacts as well (false pores), and their correct identification is critical for accurate petrographic pore assessment.