Geochemical comparison of oil trapped in fluid inclusions and reservoired oil in Blackback oilfield, Gippsland Basin, Australia

The oil trapped in fluid inclusions in the main reservoir of the 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 C₂₉/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 CSIRO Division of Petroleum Resources, PO Box 136, N. Ryde, NSW 1670, Australia 64 PESA journal No. 26 1998 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.