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TitleThe effect of bacterial degradation on bituminite reflectance
AuthorSynnott, D P; Sanei, H; Pedersen, P K; Dewing, K; Haeri Ardakani, O
SourceInternational Journal of Coal Geology vol. 162, 2016 p. 34-38,
Alt SeriesEarth Sciences Sector, Contribution Series 20160160
Mediapaper; on-line; digital
File formatpdf; html
AreaValley Lake
Lat/Long WENS-112.9000 -112.8833 52.6167 52.5833
Subjectsfossil fuels; geochemistry; source rocks; oil; reflectance; pyrite; core samples; maceral analyses; pyrolysis; isotopic studies; sulphur; bacterial decomposition; diagenesis; organic materials; sulphates; thermal maturation; bedrock geology; lithology; sedimentary rocks; mudstones; shales; Second White Specks Formation; Colorado Group; bituminite; bacterial sulphate reduction; Phanerozoic; Mesozoic; Cretaceous
Illustrationslocation maps; tables; histograms; photomicrographs; graphs
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
AbstractWithin bituminite particles, high reflectance oxidation rims (HROR) were observed in close proximity to framboidal pyrites. Through examining a maceral with the framboidal pyrite physically removed, a high number of reflectance measurements were made covering the entire surface of the maceral. Additional measurements were taken in the form of transects across the maceral, allowing an examination of the relationship between reflectance and distance from the bacterial sulfate reduction (BSR) zone. A baseline reflectance value of approximately 0.50% Ro was measured, with peak values of approximately 0.75% Ro measured adjacent to the pyrite. This large variability in bituminite reflectance (BRo) was not induced via thermal catagenesis; however, it could be accounted for by a diagenetic process. This process involves labile organic matter (OM) entering a BSR zone. In this zone, anaerobic bacteria reduces dissolved sulfate to oxidize OM. This process results in the formation of bacterially-derived framboidal pyrite in close proximity to HROR in bituminite macerals. Although there has been a previous report of elevated BRo associated with biogenic gas in the regional scale, there was no microscopic evidence of such occurrence reported. This study examines low reflectance bituminite particles with embedded framboidal pyrite and associated HROR.
Examination of the measured transects yielded a very rapid drop in reflectance a few micrometers away from the BSR zone. This study demonstrates a bacterial process that causes elevation of BRo on a micrometer scale and leads to a positively skewed, highly variable BRo population. We also demonstrate the magnitude of variation that can be expected when measuring BRo which could result in significant overestimation of thermal maturity.
Summary(Plain Language Summary, not published)
Bituminite is a type of organic matter commonly found in oil source rocks. This paper looks at how bituminite is altered by bacteria in a Cretaceous-aged oil source rock in Western Canada. The unit, called the "Second White Specks" is about 90 million years old. Bacteria used the bituminite as a food source shortly after the sediments were deposited.