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AAPG Bulletin

Abstract

DOI:10.1306/07231212048

Organic matter–hosted pore system, Marcellus Formation (Devonian), Pennsylvania

Kitty L. Milliken,1 Mark Rudnicki,2 David N. Awwiller,3 Tongwei Zhang4

1Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas; kittym@mail.utexas.edu
2ExxonMobil Upstream Research Company, Houston, Texas; mark.d.rudnicki@exxonmobil.com
3ExxonMobil Upstream Research Company, Houston, Texas; david.n.awwiller@exxonmobil.com
4Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas; tongwei.zhang@beg.utexas.edu

ABSTRACT

The Marcellus Formation of Pennsylvania represents an outstanding example of an organic matter (OM)–hosted pore system; most pores detectable by field-emission scanning electron microscopy (FE-SEM) are associated with OM instead of mineral matrix. In the two wells studied here, total organic carbon (TOC) content is a stronger control on OM-hosted porosity than is thermal maturity. The two study wells span a maturity from late wet gas (vitrinite reflectance [Ro], sim1.0%) to dry gas (Ro, sim2.1%). Samples with a TOC less than 5.5 wt. % display a positive correlation between TOC and porosity, but samples with a TOC greater than 5.5 wt. % display little or no increase in porosity with a further increasing TOC. In a subset of samples (14) across a range of TOC (2.3–13.6 wt. %), the pore volume detectable by FE-SEM is a small fraction of total porosity, ranging from 2 to 32% of the helium porosity. Importantly, the FE-SEM–visible porosity in OM decreases significantly with increasing TOC, diminishing from 30% of OM volume to less than 1% of OM volume across the range of TOC. The morphology and size of OM-hosted pores also vary systematically with TOC.

The interpretation of this anticorrelation between OM content and SEM-visible pores remains uncertain. Samples with the lowest OM porosity (higher TOC) may represent gas expulsion (pore collapse) that was more complete as a consequence of greater OM connectivity and framework compaction, whereas samples with higher OM porosity (lower TOC) correspond to rigid mineral frameworks that inhibited compactional expulsion of methane-filled bubbles. Alternatively, higher TOC samples may contain OM (low initial hydrogen index, relatively unreactive) that is less prone to development of FE-SEM–detectable pores. In this interpretation, OM type, controlled by sequence-stratigraphic position, is a factor in determining pore-size distribution.

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