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Geochemical (Rock Eval, δ13C and Pyrolysis GC) and Textural Characterization of Amorphous Organic Matter (AOM) in Carboniferous Mudstones From the Pennine Basin (UK)

Abstract

Amorphous organic matter (AOM), sedimentary organic matter that appears structureless at the scale of light microscopy, is considered as an important hydrocarbon precursor although very little is known about its exact origin, composition or texture. This is surprising given that AOM dominates the kerogen composition of the majority of source rock sediments. For example, palynofacies analyses from the Barnett Shale (Fort Worth Basin, Texas, USA), the second largest shale gas resource in the U.S., revealed the presence of 95–100% of AOM in the kerogen fraction. Similar abundances of AOM were reported in Namurian (latest Mississippian, Carboniferous) mudstones from the Bowland Shale and the Morridge formations in the United Kingdom. Even though these formations are of comparable age and composition to the Barnett Shale they were deposited in a more proximal mosaic of interlinked syn-rift basins, collectively termed the Pennine Basin. We present an AOM-focused palynofacies scheme to characterize the kerogen content of Namurian (Mississippian, Carboniferous) mudstones using transmitted light microscopy. Using this scheme in a series of randomized palynological counts, a covariance of terrestrially sourced material and homogeneous AOM was revealed. Scanning transmission electron microscopy (STEM) showed that the texture of the amorphous particles consists mostly of mineral imprints suggesting a diagenetic overprint on the original texture. This observation has potentially far-reaching consequences as the reactionary surface of these organic particles is much larger than originally expected. To geochemically characterize the AOM we developed a method to enrich AOM in the kerogen fraction by sequentially sonicating and sieving the kerogen. These AOM concentrated kerogens were then used for Rock Eval pyroloysis and pyrolysis gas Chromatography and compared to whole rock analyses. In this way we show that AOM has variable geochemical signatures throughout the Namurian and that assessments in a heterolithic system, like the Bowland Shale mudstones from the Pennine Basin should be conducted on a very high resolution to identify potentially prospective intervals.