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Molecular Fractionation of Organic Matter at Microscales Within Ordovician Kukersites Revealed by Nano-Infrared Spectroscopy

Abstract

Molecular fractionation of petroleum during migration through sedimentary rock matrices has been observed across length scales of meters to kilometers. These fractionation events are caused by selective adsorption of specific chemical moieties at mineral surfaces and/or by the phase behavior of petroleum during pressure changes. Molecular fractionation has been suggested as one cause of chemical compositional differences observed in petroleum from the same reservoir. Due to the recent interest in continuous shale resources, there is a current need to understand petroleum fractionation occurring during expulsion and migration at the micron scale, given the fine-grained nature of petroliferous source rocks and the observation that these processes are occurring in both petroleum source and reservoir formations.

Here we present results on recent work exploring the causes of compositional differences observed at the micron scale in organic matter (OM) within kukersite (containing acritarch Gloeocapsomorpha prisca) samples from the Ordovician Stonewall Formation. This work utilizes a combined infrared spectroscopy-atomic force microscopy (nano-IR) approach, as well as conventional Fourier transform infrared (μ-FTIR) microscopy, to evaluate the molecular fingerprint of kukersite OM across well-defined transition zones from organic-rich ‘source’ layers into adjacent carbonate ‘reservoir’ layers ~50 μm away. The nano-IR technique is a recent advance which allows for much higher spatial resolution than is capable with conventional μ-FTIR. Initial results indicate that the molecular composition of kukersite OM begins to vary immediately following expulsion from source layers, with loss of carbonyl (C=O) moieties and a concomitant increase in the CH3/CH2 ratio, indicating alkyl chain length decrease, as migration distance increases. This chemical transition correlates with a fluorescence decrease and reflectance increase in the OM. These observations are consistent with preferred retention of polar compounds during expulsion and migration due to absorption at mineral surfaces, and primary cracking and bituminization of the Gloeocapsomorpha prisca kerogen, respectively. These findings will be discussed in the context of evaluating the role that micron-scale molecular fractionation plays in petroleum composition following expulsion from shales and the results also serve to highlight the power of a nano-IR approach in evaluating heterogeneous OM within shales.