Publication Name: PESA Journal No. 26
Authors: Simon C. George, Peter J. Eadington, Mark Lisk and Robinson A. Quezada
Publication Volume: 26
Date Published: December 1998
Number of Pages: 19
Reference Type: Journal Article
Abstract:
The oil trapped in fluid inclusions in the main reservoir ofthe Blackback oilfield, the top of the Latrobe Group, has
been compared with currently reservoired oil from the same
horizon using detailed molecular geochemistry, and also
with other Gippsland Basin oils. Blackback 2 inclusion oil
has some geochemical similarities with the MDT oil
analysed, including moderately waxy n-alkane profiles,
bicyclic sesquiterpanes and small amounts of bicadinanes
and oleanane/?lupane, biomarkers typical of terrestrial
organic matter input. However, the inclusion oil has a lower
Pr/Ph ratio, lower C2/C27 sterane and diasterane ratios, lower
diahopane/hopane ratios and greater amounts of 28,30-
bisnorhopane, extended homohopanes and methylhopanes
than the MDT oil. Collectively, these and other biomarker
parameters suggest that the inclusion oil was derived from a
marine source rock, perhaps with a calcareous influence,
deposited in a more reducing depositional environment than
the source rock for the MDT oil. The majority of maturity
indicators suggests the inclusion oil is considerably less
mature than the MDT oil. The presence of 25-norhopanes in
the inclusion oil indicates that some oil was biodegraded
prior to trapping, although the co-occurrence of n-alkanes
and other more readily biodegraded hydrocarbons in the
inclusion oil suggests trapping of pristine, non-biodegraded
oil also.
A marine source rock probably started generating oil about
10 Ma and resulted in early charge of the Blackback
structure with low maturity oil. Part of this initial charge
was subjected to heavy biodegradation, which may have
occurred progressively during inclusion formation, thus
enabling both fresh and biodegraded oil to be trapped.
Alternatively, fresh oil may have mixed with an earlier
biodegraded residue, prior to trapping in oil inclusions. The
biodegradation episode in the reservoir may have been
terminated by the higher temperatures due to continued
burial of Latrobe Group sediments, or by the on-going
transgression, which would have reduced fresh water
recharge to the reservoir. Large volumes of oil were generated from Latrobe Group terrestrial source rocks once
the upper oil window was reached. This oil migrated to the
Blackback structure and was volumetrically more abundant
than the early biodegraded charge, so that little or no trace
of it can be discerned in the currently reservoired oil due to
dilution. Some of the main oil charge may have been
trapped as oil inclusions, thus explaining the waxy n-alkane
signature and the presence of some terrestrial biomarkers in
the inclusion oil; however, oil inclusion formation was
relatively inhibited by this stage.