Relationship of biomarker distribution to depositional environment: Phosphoria Formation, Montana, U.S.A.

Abstract

From biomarker distributions it is possible to distinguish extracts from four different members/facies (Meade Peak, Rex, Retort, and Tosi) of the Permian Phosphoria Formation at Little Sheep Creek, Montana. These distributions relfect changes in floral and faunal distribution in response to a changing depositional environment and suggest that the Retort and Tosi Members (the second cycle of deposition) were deposited under more anoxic conditions and at higher salinity than Meade Peak and Rex Members (the first cycle). Correlations between biomarker indicators of anoxia and salinity suggest that anoxia was in part the result of a chemocline separating normal marine waters above from more saline bottom waters. Anoxia and salinity in the bottom waters increased with time making conditions in the basin progressively more hostile to benthic organisms. Development of extreme environmental conditions in continental shelf habitats (such as those suggested by biomarker parameters at Little Sheep Creek) may have compounded spatial problems created by continental suturing suggested to be a cause of Permian extinctions. Factor analysis indicates that four factors for 75.3% of the variance in biomarker ratios. These factors are suggested to be source organisms (types of eukaryotic plankton), salinity, Eh, and mineralogy. The Grandeur Member of the Phosphoria, which is a typical reservoir rock for Phosphoria derived oils, yields an extract containing a novel C 30 nitrogen-containing steroid. This unique steroid is not abundant in any other known extracts or oils, which along with the extremely low Porphyrin Maturity Parameter of all the extracts (PMP < 0.01), confirms the absence of migrated oils in the Phosphoria at Little Sheep Creek. The biomarker ratios C 29 αββ/( gaββ + αααa steranes; Ts/(Ts + Tm); triaromatic/(triaromatic + monoaromatic) steroids; short/(long + short chain) monoaromatic steroids; and short/(long + short chain) triaromatic steroids are commonly used to determine the thermal maturity of oils and source rock extracts. However, these ratios show considerable variation in core extracts of the same thermal maturity from different facies. In contrast, the thermal maturity parameters, C 29 ααα 20 S/(20 S + 20 R) steranes and Porphyrin Maturity Parameter (PMP) show little variation, indicating that these parameters are less affected by changes in depositional environment and may be most useful for thermal maturation determinations of Phosphoria related oils and source rocks. This study supports previous work suggesting environmental effects must be considered if correct determination of thermal maturity of an oil or extract is to be made. This is especially true of oils and extracts from source rocks with high sulfur concentrations and/or those deposited at elevated salinities.