NI- AND CU-BEARING FRAMBOIDS IN THE WOODFORD SHALE, ARBUCKLE MOUNTAINS OK—HINTS AT EARLY PORE WATER COMPOSITION

Petrologic investigations of cryptobioturbated chert and siliceous mudstones of the Devonian-Mississippian Woodford Shale (world-class unconventional petroleum system) exposed in outcrops in the Arbuckle Mountains, Oklahoma, reveal multiple, texturally distinct generations of pyrite. Framboidal pyrite occurs in both cherts and mudstones with individual framboids ranging from 4-10 µm in diameter, based on scanning electron microscopic (SEM) investigations. Other pyrite textures include aggregates of framboids into small concretionary masses (<500 µm across) in mudstones, euhedral crystals (50 µm across) scattered throughout the matrix in either lithology, euhedral crystals that partially fill the interiors of radiolarian and acritarchs(?) in cherts, and masses of anhedral pyrite in small (up to 1 cm across) concretions in mudstones. All generations of pyrite appear to be authigenic, and most are likely to have formed during early diagenesis, based on their relationship to other authigenic minerals, compaction history, or other petrologic features.

Total organic carbon (TOC) contents are relatively low in cherts (< 6 wt %) as compared to mudstones (< 22 wt %), so some variability in pyrite texture may be a function of original TOC content. Nevertheless, individual matrix framboids are commonly nickeliferous or cupriferous, based on SEM analyses. Only matrix framboids are Ni- or Cu-bearing, which makes them compositionally unique among all pyrite generations in the Woodford Shale.

The presence of Cu and/or Ni in the framboids indicates that these metals were present in interstitial pore fluids and available for substitution for iron during framboid formation. However, once these early diagenetic framboids formed, the availability of Ni and Cu was apparently exhausted, as these metals are not observed in subsequent pyrite generations. Thus, Ni and Cu enrichment of pore fluids appears to have been transient, possibly only during a short period of time when the sediment was directly in contact with the overlying water column. The Cu and Ni were seemingly not continuously supplied through diffusion or other mechanisms from seawater during subsequent diagenesis following matrix framboid formation. Any Ni or Cu in oils generated from the Woodford must have been derived from sources other than the framboids given that these metals remain fixed in this early-diagenetic pyrite.