Dynamic Redox Conditions in the Marcellus Shale as Recorded by Pyrite Framboids-Implications for Reservoir Quality
Understanding the effects of preservation and dilution on organic matter concentration in organic-rich mudtsones is crucial to maximizing hydrocarbon production in those plays where the organic material is the main site of porosity development. To assess redox conditions, a primary control on organic matter preservation, pyrite framboid diameters were examined in 31 samples taken from two Marcellus Shale cores recovered from Greene County, Pennsylvania, and Upshur County, West Virginia. Analysis of framboids diameters in those samples from the more proximally located Upshur County core suggests that anoxic to anoxic-euxinic conditions persisted during accumulation of the transgressive-regressive cycle (MSS1) that comprises the Union Springs Member of the Marcellus Shale, which was interrupted by intermittent episodes of dysoxia. An increased abundance of large framboids documented from the overlying MSS2 transgressive-regressive cycle, which comprises the bulk of the Oatka Creek Member, tells of improved bottom water conditions. Redox conditions recorded by framboid diameters of the MSS1 cycle of the Greene County core are generally similar to those of Upshur County; however, conditions in that region of the basin from which the Greene County core was recovered appear to have remained anoxic to anoxic-euxinic. Framboid diameter trends established for both cores enhances our understanding of just how much redox conditions varied both spatially and stratigraphically during accumulation of the Marcellus Shale. Further, the presence of small syngenetic framboids and large diagenetic framboids in the same thin section samples suggests that redox conditions fluctuated at a scale beyond the resolution of what can be observed at the thin section scale, where one thin section may represent more than 1000 years.
AAPG Datapages/Search and Discovery Article #90195 © 2014 Eastern Section Meeting, London, Ontario, Canada, September 27-30, 2014