Oils from the Mississippian Mission Canyon 5 (MC5) carbonates of Saskatchewan, on the northern flank of the Williston basin, have been changed from sweet, medium- to high-gravity crudes, to sour nonproducible tars where the MC5 subcrop trend is intersected by a zone of fresh meteoric formation water. These waters are recognized by lower salinities and oxygen isotopic compositions which resemble water from present-day precipitation.

The alteration of petroleum by fresh water may involve three processes: inorganic oxidation, water washing, and biodegradation. Inorganic oxidation (by molecular oxygen) is restricted largely to near-surface reservoirs and is not documented by the present study of crudes from depths of 3,500-6,000 ft. Water washing has occurred where formation waters undersaturated with hydrocarbons have dissolved the more soluble lighter hydrocarbons in the reservoired oils. This has left more sulfurous, heavier crudes with lower solution gas-oil ratios.

Bacterial degradation has occurred within the area of freshwater invasion, presumably through the introduction of microorganisms, molecular oxygen, and nutrients. Normal paraffins and smaller naphthenes have been preferentially degraded and nonhydrocarbon NSO's and asphaltenes produced with resulting low-wax, heavier, more sulfurous crudes. Isotope data provide additional evidence of bacterial activity in the subsurface.

In the study area, water washing and biodegradation have reduced crude oil gravities by more than 20 degrees API and increased the sulfur content from less than 1 percent to more than 3 percent.