Families of Oils and Their Sources in Southeastern Saskatchewan

L. R. Snowdon, K. G. Osadetz, P. W. Brooks

Four compositionally distinct families of oils are characterized by gross geochemical and C₁₅₊ saturated hydrocarbon fraction compositions obtained by gas chromatography and mass spectrometry-gas chromatography. Family A oils (two pools) are characterized by low pristane/nC₁₇ and phytane/nC₁₈ ratios, high diasterane/regular sterane ratios, and low C₂₃/C₃₀ terpane ratios. This family of oils is restricted to Ordovician pools and is identical to the oils from Ordovician reservoirs in the United States portion of the Williston basin. Family R oils (two pools) are characterized by low pristane/phytane and high diasterane/regular sterane ratios. This family of oils is restricted to Winnipegosis Formation pinnacle-reef rese voirs. Family B oils (nine pools) are characterized by crossplots of the ratio of diasteranes/regular steranes against either the ratio of saturate/aromatic hydrocarbons or the ratio of pristane/phytane. Within family B, a group of oils (four pools) with low C₂₃/C₃₀ terpane ratios may represent a mixture of oils from families B and A. Family B oils are restricted to pools in the Birdbear Formation, the Bakken Formation, and the Madison Group. The most abundant family of oils, family C (22 pools), is identified by the same criteria used to characterize family B oils, although they have low diasterane/regular sterane ratios and low pristane/phytane ratios in comparison to family B. Family C occurs in the Madison Group and younger reservoirs. A distinctive subgroup of ils in family C has high saturate/aromatic hydrocarbons ratios. One oil with an intermediate diasterane/regular sterane ratio may represent a mix of oils from families B and C. Oils occur in reservoirs lying stratigraphically close to their sources. Family A is sourced from Ordovician kukersites in the Big Horn Group and family R is derived from organic-rich laminites of the Elk Point Group. Oils in families B and C have been previously included in a single family of oils attributed to a Bakken source rock. However, the stratigraphic distribution and compositional characteristics of family C oils preclude such a derivation. The Madison Group is a more likely source for family C. The Bakken Formation remains the source for family B.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988