Pennsylvanian coal and associated bitumen at Johnson Mills, Shepody Bay, New Brunswick, Canada

Abstract

Pennsylvanian fluviatile sandstones at Johnson Mills, New Brunswick, host fossil logs preserved as a result of the paragenetic succession: pyritization, carbonatization, carbonization and overlapping bitumenization. Carbonatization is the dominant process. The coalified material is all overmature as reflected by temperature of maximum pyrolysis ( T max) in a range of 460–464 °C. Veinlets of once liquid, now solid bitumen, extend outward from the logs into the surrounding sandstones for several meters. Present in the bitumen are relict coal macerals that contribute to a microscopic distinctly structured coaly appearance of the material. Reflectance measurements of coal range from Ro max=0.78% to Ro min=0.61%. Two distinct populations of bitumens are recognized (Ro max=0.78% to Ro min=0.65% for the main population; Ro max=0.60% to Ro min=0.48% for the second population. Transmitted and reflected light microscopy, long-count energy-dispersive spectroscopy (EDS) and Rock Eval analysis indicate that bitumen was derived from the logs as a result of late stage diagenesis. Exsudatinite-like material is the probable precursor of the bitumen. Thermal maturity of the bitumen increases directly with inferred migration distance from the logs. Across the spectrum, solid bitumen closest to the logs and within the oil window is least mature ( T max=449 °C), and bitumen furthest from the logs is most mature (with T max=493 °C). This phenomenon results from differences in the chemistry of the derivative bitumen, which is relatively more aliphatic and NSO-enriched close to the log source, and more aromatic distally. These differences in bitumen composition may have arisen as a consequence, respectively, of pre-gas and post-gas generating conditions.