Pore Characterization and Facies Analysis in the Geneseo Formation of New York: Can Small-Scale Mudstone Variations Relate to Pore Morphology and Distribution?
The Middle Devonian Geneseo Formation and lateral equivalents in the Northern Appalachian Basin are seeing the development of shale-gas plays with high economic potential. Mudstone properties in the Geneseo succession are highly variable. This unit displays an overall shallowing trend that reflects the westward progradation of the Catskill delta. High-resolution stratigraphy has yielded differentiation of genetically related packages, comprised of distinct lithofacies with characteristic physical, biological, and chemical attributes. In addition, nano-scale microscopy of ion-milled shale sections has led to defining three distinct pore-types.
Organic-matter enrichment appears to be focused in the lower Geneseo Formation, where pyritic and banded black shales are dominantly observed with small scale truncations, current ripples, graded silt-clay couplets, and cryptobioturbation. Upsection, dark gray silty mudstones are observed with abundant wave ripples, current ripples, graded silt-clay couplets, erosional features, bioturbation, and small brachiopods. In the middle Geneseo, a silt-rich calcareous mudstone is observed, and contains abundant wave ripples, current ripples, increased bioturbation, auloporid corals, ostracodes, and brachiopods. Above this concretionary horizon, dark gray silty mudstones with wave ripples, current ripples, graded beds, scour surfaces, and abundant bioturbation grade upsection into dark muddy siltstones with abundant wave and current ripples, deep scour surfaces, bioturbation, and auloporid corals.
Pore morphology and distribution appears to correlate with distinct mudstone lithofacies as a result of small scale compositional and textural characteristics. Phyllosilicate framework pores are small triangular openings (50-1000 nanometers wide) that are observed throughout the Geneseo succession. They are, however, better developed in the dark gray mudstones where pressure shadows of compaction resistant grains (sand, silt, pyrite, etc.) prevent compression of primary clay fabric. Organic matter porosity is also observed (10-500 nanometers wide), and dominates the lower Geneseo pyritic and banded black shale facies where organic content is highest. Carbonate dissolution pores are observed in the calcareous intervals within the Geneseo (50-500 nanometers wide), and probably reflect partial dissolution of carbonate grains (calcite or dolomite) during catagenetic formation and migration of carboxylic and phenolic acids.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California