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.