HYDROCARBON-RELATED DIAGENETIC ZONES (HRDZs) IN THE VULCAN SUB-BASIN, TIMOR SEA: RECOGNITION AND EXPLORATION IMPLICATIONS

Abstract

Within very localised areas of the Vulcan Subbasin, the Eocene Grebe Formation sandstones are strongly cemented with carbonate. These cemented sands are recognisable on seismic data as zones of anomalously high velocity, and result in both time 'pull-up' and deterioration of the stack response in the underlying section.

To determine the nature and origin of these cemented zones, their isotopic, mineralogical and petrologic compositions have been characterised, their seismic response and areal distribution established, and these observations integrated with ~2,730 km of AGSO water column geochemical ('sniffer-type') data.

The carbon isotopic compositions of the carbonate within the cemented Grebe sands are diagnostic of carbonates formed principally via the oxidation of migrating, thermogenic hydrocarbons. Oxidation of the hydrocarbons took place in two stages: an earlier phase led to calcite precipitation, whereas a later phase produced (generally subsidiary) ferroan dolomite/ankerite cementation.

Areas of known, present-day hydrocarbon seepage from the seafloor, such as over major faults on the Skua Horst and along the Vulcan Sub-basin/ Londonderry High boundary zone, are invariably associated with zones of highly cemented Eocene sands. Similarly, areas of known Tertiary hydrocarbon seepage, such as those associated with the residual oil columns on the Eider Horst, also contain strongly cemented Eocene sandstones.

These observations have established a causal relationship between the presence of these Hydrocarbon-Related Diagenetic Zones (or HRDZs) in the Eocene sandstones and Tertiary-Quaternary hydrocarbon seepage. It is likely that most of the cementation occurred during the Late Miocene/Early Pliocene, when the Grebe Formation sands were at a shallow depth of burial(<300-500 m), and were acting as an aquifer. The collision of the Australian and Eurasian Plates in the Late Miocene to Early Pliocene resulted in the partial to complete breaching of many charged traps and facilitated the vertical migration of hydrocarbons through the Tertiary sequences. Bacterial oxidation of the migrating hydrocarbons within the Grebe sands, probably via sulphate reduction, liberated C02 for subsequent carbonate precipitation.

Recognition of this causal association has allowed several insights to be gained into the exploration potential and reactivation history of structures within the Vulcan Sub-basin. Mapping of the areal distribution of the cemented zones can effectively define hydrocarbon migration pathways. More importantly, however, predictable relationships exist between the seismic expression of the HRDZs, the total amount of hydrocarbons that have leaked from the traps, and the obliquity between the Jurassic and Late Miocene fault trends over the respective structures. A continuum exists between highintegrity accumulations, in which the fault trends are parallel and the HRDZs are small or absent, and breached accumulations, in which a significant obliquity exists between the respective fault trends and the HRDZs are large and seismically-intense.

These observations provide a potential predictive tool for evaluating undrilled structures. It may be possible to determine, from the integration of seismic structural mapping and the characterisation of the seismic expression of the HRDZs, not only whether an individual structure is ever likely to have had a hydrocarbon column, but whether that column is likely to be preserved.