New Geochemical Technologies to Reduce
The Risk in Gas Exploration
Moldowan, J. Michael1, Zhibin
Wei1, Jeremy Dahl1 (1)
Several new technologies have been
developed that can be used to improve gas prospecting, as follows:
(1) Quantitative diamondoid analysis of
oils, sediments and piston cores, for recognizing oil to gas cracking,
evaporative fractionation and reservoir seal leakage. Petroleum that carries
signatures of extensive cracking (abundant diamondoids) together with
significant stigmastane concentrations is recognized using quantitative
diamondoid-biomarker analysis as mixed from post-mature and normally-matured
sources. Liquids showing high cracking profiles provide evidence of gas
migration.
(2) Compound specific isotope analysis of
diamondoids (CSIAD) provides a fingerprint that may neither be altered by
extreme catagenetic conditions in the source rock nor by alterations in the
reservoir, such as those that occur during thermochemical sulfate reduction
(TSR), thermal cracking and biodegradation. Post-mature, TSR-altered and
biodegraded liquids may be correlated with non-altered oils of normal maturity.
CSIAD is the most specific way to correlate oil with highly mature condensate,
as a proxy for associated gas, and thus provide information about gas
provenance in a basin. Mixtures of black oil and cracked condensate often
occur, but are seldom recognized (except using diamondoids). The cracked
component in such mixtures can be correlated by using CSIAD.
(3) Thiadiamondoids can be used for
determining and quantifying the extent of TSR, which can play an important role
in oil destruction. Clues to the occurrence of TSR are available from a variety
of methods, but thiadiamondoid analysis offers a definitive approach, since the
formation of thiadiamondoids requires TSR conditions.
Applications from several regions will be
used to illustrate.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California