Biomarkers And Diamondoids Improve
Understanding of Oil Cracking, Mixing, And Geochemical Correlation on The
Wang, Ye1, J. Michael Moldowan1,
Kenneth J. Bird2, Leslie B. Magoon2, Kenneth E. Peters2
(1) Stanford University, Stanford, CA (2) U.S. Geological Survey, Menlo Park,
CA
Due to high thermal stability, diamondoids resist oil cracking and become more
concentrated, while biomarkers crack and decrease in concentration. Therefore,
the extent of oil cracking can be investigated by plotting both diamondoid and biomarker concentrations. Furthermore,
mixtures of high- and low-maturity oil can be identified when concentrations of
both biomarkers and diamondoids are high.
Forty-one crude oil samples from the
North Slope of Alaska have variable diamondoid and
biomarker concentrations, which indicates different extents of oil cracking.
Some of the samples are mixtures of high- and low-maturity components because
they contain high concentrations of both diamondoids
and biomarkers. These mixtures were de-convoluted using compound-specific
isotope analysis (CSIA) and age-related biomarker analysis. CSIA of individual diamondoids and light hydrocarbons was used to correlate
the high-maturity components, while biomarkers, especially those providing
information on the age of the source rock, were used to correlate the
lower-maturity components.
Five source-rock intervals for the oil
samples can be identified by their biomarker characteristics:
Carboniferous-Permian Lisburne Formation, Triassic Shublik Formation, Jurassic Kingak
Shale, Lower Cretaceous Hue/GRZ Shale, and Tertiary Canning Shale. For example,
the NDR ratio [24-nordiacholestane/(24- and
27-nordiacholestane)] distinguishes oil samples that originated from Hue/GRZ
Shale from those that originated from older source rocks, such as the Shublik and Kingak source rocks.
The TA-DMD ratio (triaromatic 23,
24-dimethylcholesteroids/triaromatic stigmasteroids)
distinguishes oil samples from the Lisburne, Shublik, and other units. The possible contribution of Lisburne gas and condensates to mixed-oil accumulations
will be investigated using the CSIA methods.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California