David N. Awwiller

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

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], 1.0%) to dry gas (Ro, 2.1%). 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.

Diagenetic Resetting of Sm-Nd Isotope Systematics in Wilcox Group Sandstones and Shales, San Marcos Arch, South-Central Texas

ABSTRACT Sm-Nd analyses of sandstones and shales from the Wilcox Group (Upper Paleocene-Lower Eocene) suggest that the whole rock Sm-Nd signature is changed by burial diagenesis. The samples are from the San Marcos arch, south-central Texas, at depths ranging from outcrop to 15,000 feet. Sandstones from above the depth of intense feldspar reactions (ca. 10,000 feet) have whole rock Nd model ages of 1400 - 1500 Ma, whereas sandstones buried deeper than ca. 10,000 feet have Nd model ages of 1500 - 2000 Ma. Subsurface shale samples have whole rock model ages that increase from 1300 Ma at 5,000 feet of burial to 1500 Ma at 15,000 feet. Model ages increase most between 5,000 and 10,000 feet, within the zone of intense smectite illitization. The similar depositional age of all samples makes change in provenance an unlikely cause of the observed Nd model age variation. Sm and Nd reservoirs within the shale change considerably with progressive diagenesis. Outcrop samples contain subequal concentrations of Sm and Nd in the acid-soluble and silicate fractions. The acid-soluble reservoir in deeper samples contains progressively less Sm and Nd, and higher Sm/Nd. In all samples, the acid-soluble Nd is more radiogenic than the Nd in the corresponding silicate fraction, and the difference between the two reservoirs increases with depth. Diagenetic enrichment of Sm relative to Nd in the authigenic phases is the most likely explanation for the observed increase in Nd model ages in the sandstones. Decrease in Nd causes the increase in model age in the shales.