Source Rock Mapping using Fluid Inclusion Geochemistry in the Offshore Perth Basin

Volk, Herbert¹; Kempton, Richard²; Gong, Se¹; Ahmed, Manzur¹; George, Simon C.³; Boreham, Chris⁴; Grosjean, Emmanuelle⁴
¹Earth Science and Resource Engineering, CSIRO, North Ryde, NSW, Australia.
²Earth Science and Resource Engineering, CSIRO, Kensington, WA, Australia.
³Department of Earth and Planetary Sciences, Macquarie University, North Ryde, NSW, Australia.
⁴Energy Division, Geoscience Australia, Canberra, ACT, Australia.

The fill history of petroleum reservoirs is often a prolonged process where oils represent multiple oil charge events. Oils trapped in fluid inclusions (FI) are contained in sealed time capsules and can be used to track petroleum systems through geological time. This case study from the offshore Perth Basin presents geochemical data from seven FI and crude oils, together with new data on source rocks and oil stain which were used for the mapping of effective source rocks in this region.

In the Abrolhos Sub-basin, Cliff Head FI and reservoir oil from Permian strata sealed by Kockatea Shale (KS) are very similar and sourced by the Hovea Member of the basal KS. Evidence for a KS source includes a high wax content, a low Pr/Ph ratio, high abundance of extended tricyclic terpanes, and the highly diagnostic C33 n-alkylcyclohexane (ACH 33) anomaly.

FI oils from Permian strata of three wells to the west and northwest of the Cliff Head oil field come from a region where the KS was previously thought to be an ineffective source rock. Hadda-1 FI oil is extremely waxy, shows the ACH 33 anomaly and other features consistent with a major KS input. FI oils from Flying Foam-1 and Leander Reef-1 are less waxy but also show typical KS markers. However, FI oils from Hadda-1 and Flying Foam-1 also differ from the Cliff Head oils, e.g. have higher abundances of C19 tricyclic and C24 tetracyclic terpanes. This may reflect facies variations of the KS or charge from an additional source unit. Direct evidence for the latter comes from Leander Reef-1 FI oil that has a heavy n-alkane 13C isotope signature more consistent with a deeper terrestrial source.

In post-Triassic successions, FI oils from Houtman-1 in the Houtman Sub-basin and from Gage Roads-2 in the Vlaming Sub-basin have different source characteristics. Houtman-1 FI oil from the Middle Jurassic Cadda Formation may be derived from the Early Jurassic Cattamarra Coal Measures, with a possible contribution from an unknown lacustrine source. Gage Roads-2 FI oil from the Early Cretaceous Carnac Formation is derived from source rocks of relatively low maturity containing a high proportion of Araucariacean-type land-plant organic matter typical for the Early Cretaceous Parmelia Group and Late Jurassic Yarragadee Formation.

Collectively the geochemical evidence indicates that the KS is an effective source rock in the Abrolhos Sub-basin, greatly extending the proven distribution of this oil-prone source rock.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012