Evidence for biodegradation and evaporative fractionation in West Sak, Kuparuk and Prudhoe Bay field areas, North Slope, Alaska

Oils from the West Sak, Kuparuk, and Prudhoe Bay fields on the North Slope of Alaska display geochemical evidence for a complex petroleum filling history that includes multiple sources and alteration by evaporative fractionation and biodegradation. Source-specific biomarkers in West Sak oils indicate generation from the same source rocks in approximately the same proportions as Prudhoe Bay field oil, consistent with the hypothesis that oil spilled from the deeper Prudhoe accumulation and remigrated about 10 miles laterally and 5000 feet vertically into the West Sak Field. West Sak oils are moderately biodegraded but exhibit anomalously high concentrations of gasoline-range saturate and aromatic compounds interpreted as a secondary gas/condensate charge that arrived after biodegradation of the spilled Prudhoe oil. The shallowest West Sak oils have less secondary gas/condensate charge, lower API gravities, and reduced concentrations of methylcyclopentane, benzene, cyclohexane, 3-methylhexane, heptane, methylcyclohexane, and toluene relative to the deepest West Sak zone A oils. The carbon isotopic compositions of these C6 and C7 compounds in the shallowest West Sak reservoirs are heavier than those of the deepest West Sak oil reservoirs by up to 3‰ and the isotopic compositions of saturate compounds are altered more than those of aromatic compounds. The heavier isotopic compositions of C6 and C7 compounds in the shallow West Sak reservoirs are interpreted to result from a kinetic isotope shift caused by light biodegradation of the secondary gas/condensate charge. West Sak field solution gas is also interpreted as biodegraded, based upon its dryness (>98% methane), relatively high isobutane/n-butane ratio, the presence of isotopically heavy carbon dioxide and isotopically light methane, and propane that is isotopically heavier than either ethane or butane. The isotopic compositions of methane and carbon dioxide in West Sak gas suggest that biogenic methane was generated during CO2 reduction under anaerobic conditions. The isotopic compositions of C6 and C7 compounds in the deepest, least biodegraded West Sak oils are nearly identical to those of the underlying oils from the Kuparuk Field but differ from the Prudhoe Bay Field oils. The Kuparuk Field is therefore postulated as the source of the secondary gas/condensate charge observed in oils from the West Sak Field. Ratios of toluene/heptane and heptane/methylcyclohexane in Kuparuk oils provide independent evidence for loss of gas and condensate from the Kuparuk reservoir by the process of evaporative fractionation, and the oils subjected to the most evaporative fractionation are isotopically heavier in the C6–C7 range. The isotopic compositions of West Sak zone A C6 and C7 hydrocarbons suggest that a slight ( 0.8‰) isotopic fractionation resulted from evaporative fractionation of the gas/condensate charge from the Kuparuk Field oil accumulation. North/northeast-striking faults that connect the Kuparuk and West Sak reservoirs provide a potential migration pathway for the secondary gas/condensate charge through 3000 feet of intervening shale. This model of West Sak oil biodegradation, subsequent secondary gas/condensate charge, and further biodegradation has implications for development of North Slope West Sak oil reserves, because heavily biodegraded West Sak oils that lack a secondary gas/condensate charge may be too viscous to develop by conventional waterflooding methods.