Previous studies indicate that although thermal maturation is an overwhelming control on petroleum composition within the deeper and hotter regions of the Western Canada basin, other processes significantly affect the quality and preservation of crude oil in area where the burial is less deep. The objective of our study was to investigate, with as many analytic tools as possible, the nature of those processes which alter and degrade petroleum in less mature settings.

The Mississippian oils of Saskatchewan provide an excellent opportunity for isolating and examining those processes which might result from chemical interaction between oils and formation waters. Along the entire subcrop trend, only 3 geologic parameters change significantly; formation waters become less saline from east to west, and oils become heavier and more sulfurous and eventually disappear on the west.

Methods used include chemical analyses of formation waters followed by isotopic determinations of S, C, O, and H. Crude oils were analyzed for individual gasoline-range hydrocarbons, normal paraffins to n-C³⁵, sulfur, and API gravity. In addition, C and S isotope measurements were made on saturate, aromatic, NSO, and asphaltene fractions of selected crudes. The critical environments also were checked for direct evidence of microbiologic activity.

Preliminary results suggest that the western part of the study area was invaded by fresh meteoric water which resulted in the degradation of crudes through two processes. The disappearance of light hydrocarbons toward the zone of freshwater invasion indicates removal of these more soluble compounds by water washing. Light normal paraffins disappear much faster than expected from their relative solubility, and extended range chromatograms show that heavy, insoluble,

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normal paraffins also have disappeared in the degraded zone. Biodegradation may be a plausible explanation for this additional phenomenon.

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