Triassic Source Facies in High Paleo-Latitude Petroleum Systems

By

A. Holba (Phillips Petroleum Co.), W.D. Masterson (Phillips Alaska, Inc.), L. Ellis (Terra Nova Technologies), and L. Dzou (BP Upstream Technology Group)

Oils derived from the Triassic Shublik Formation of the North Slope of Alaska are prolific and well studied. However, the Shublik sourced system is not unique; other Triassic petroleum systems exist in high northern latitudes. Triassic oils are found from the Chukchi Sea, across Alaska through Canada and as far east as the Barents Sea of Norway. Age and environmental diagnostic geochemical indicators were used to describe and differentiate oils of the Triassic petroleum system from oils from other possible sources, particularly Jurassic derived oils worldwide. Triassic oils have some distinct characteristics when compared to Cretaceous, Jurassic, and Upper Paleozoic derived oils. Marine Triassic-derived oils are isotopically negative (<-29), have high extended tricyclic terpanes, and generally possess dinosteranes and 4-methylsteranes. They contain little or no 24- norcholestanes (C26 steranes) or oleanane.

We report a new ratio to be a reliable source age parameter when evaluating oils and rock extracts: Extended Tricyclic Terpane Ratio (ETR) [sum of C28 and C29 tricyclic terpanes /18-a(H)- trisnorhopane (Ts)]. Both the numerator and denominator are thermal maturation and biodegradation resistant, thus minimizing these effects. This is a more robust source related age and correlation parameter, useful for a wide range of applications. Note that Triassic oils tend to have a high ETR (>2.0) and that Jurassic oils are not observed to have a high ratio.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.