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The Significance of Framboid-Hosted Porosity in the Marcellus Shale of the Appalachian Basin, USA


The occurrence of hydrocarbon associated with organic-rich mudstones has long been recognized. Indeed, the Marcellus Shale demonstrates a clear relationship between total organic carbon and porosity. The link between TOC and natural gas reservoirs was not definitively established until the work of Loucks et. al., (2009) wherein they demonstrated the presence of organic matter-hosted pores. Moreover, they noted that pyrite framboid intercrystalline pores although less common than OM-hosted pores, were the second most common pore type. Here, we quantify framboid-hosted porosity of the Devonian Marcellus Shale. We surveyed 12 SEM images of framboids and quantified pore space to test using Cubic Close Packing to model framboidal porosity. SEM images were converted to binary images of white crystallites and black pores where the quantity of black pixels relative to the total pixel count represents porosity. The results suggest that the CCP model accurately represents porosity within framboids. A positive correlation between framboid diameter and microcrystallite size also exists. As framboid and microcrystallite size increase so too does the size of the pores, ranging in size from >200 nm to >1200 nm. To quantify framboidal porosity, all forms of pyrite were noted in SEM images and classified as porous framboidal, welded framboidal, or non-framboidal pyrite, and their occurrence as a percentage of the image was calculated. This method yields the fraction of total pyrite that is pore-bearing framboidal pyrite, which is compared to XRD-derived pyrite volume in order to determine total framboidal porosity. In one investigation, framboidal porosity ranges from ̴0-1% with an average of 0.5% or 1.5-15.5% of the total pore system. Lastly, while the occurrence of framboidal pore space is not nearly as abundant as OM-hosted porosity, it is important to note that pyrite largely co-varies with TOC, meaning that the greatest occurrence of these two pore types often occurs in the same stratigraphic interval. Moreover, often times framboidal porosity is already filled with pore-bearing bitumen. While these framboidal pores are not contributing to the measured porosity, they are evidence of a local pore system into which early mobile bitumen migrated.