Geoffrey O'Brien

Plate convergence, foreland development and fault reactivation: primary controls on brine migration, thermal histories and trap breach in the Timor Sea, Australia

During the latest Miocene and Early Pliocene (∼5.5. MaBP), the collision of the Australian and Eurasian plates resulted in proto-foreland development and significant structural reactivation in the Timor Sea, north-western Australia. Flexural extension, resulting from the down-warping of the Australian plate into the developing Timor Trough, resulted in the dilatation of the major Jurassic and older extensional faults and the formation of shallow Mio-Pliocene fault arrays. An integrated, multidisciplinary study of hydrocarbon traps from this region using 2-D and 3-D seismic data, stable isotope geochemistry, fluid inclusion measurements and apatite fission track data has revealed that this fault reactivation produced three categories of traps: high (HIT), moderate (MIT) and low (LIT) integrity traps. These have characteristic hydrocarbon fill–spill, fluid flow and thermal histories. In MITs and LITs, the dilatation was moderate to intense respectively, and allowed hot (90–130°C), highly saline (200,000+ ppm salinity) brines from deep Palaeozoic evaporites to migrate up the reactivated faults and chemically and thermally affect the reservoir and shallower intervals. Apatite fission track data suggest that fluid migration lasted for between 100,000 and one million years in the case of the MITs, but for only 10,000–100,000 years in the LITs. This major fluid flow event resulted in the development of a prominent, localised Late Tertiary heating spike in the MITs, which can significantly affect the accuracy of modelled thermal histories. In the LITs, the thermal effect is less marked, due to the more transient nature of the fluid flow event. HITs were largely unreactivated and hence conduits for brine migration from depth were absent. Consequently, these traps are the most representative of the thermal histories of the source rock depocentres. Where MITS or LITs were charged, the associated loss of fault seal integrity facilitated hydrocarbon loss from the Mesozoic reservoirs, which co-migrated with the brines up through the Mio-Pliocene fault network. Upon entering a shallow, clastic aquifer system (the Eocene Grebe Formation), bacterial oxidation of the hydrocarbons liberated CO2 which, in turn, resulted in significant and very isotopically light carbonate cementation. This cementation produces sufficient acoustic impedance with the surrounding uncemented sands that it allows these hydrocarbon-related diagenetic zones (HRDZs) to be mapped seismically. Since both the size and acoustic response of the HRDZs are directly proportional to the amount of hydrocarbons that have leaked from the traps, their presence or absence provides a powerful indicator, predrill, of both trap integrity and the likely thermal regime that that traps have experienced. An important observation is that the leaky fault segments over partially breached traps typically only extended for 200–1000 m, whereas over the breached traps, leaky segments extended for 3000–5000 m. Consequently, exploration programs acquiring remote sensing geochemical data (such as geochemical sniffer and airborne laser fluorosensor (ALF) techniques), should have closely spaced line spacings if leaky, potentially commercial fields are to be detected reliably. Potential analogues exist between the processes documented during HRDZ formation, namely the mixing at shallow depths of basinal brines, hydrocarbons and connate waters, and processes occurring during the formation of Pb–Zn and other, low temperature ore deposits.

Hydrocarbon charge history of the northern Londonderry High: Implications for trap integrity and future prospectivity

Re-appraisal of the oil charge history of the northern Londonderry High has identified numerous palaeo-oil columns of up to 80 m in height. An integration of the oil charge history, stress field analysis and contemporary seepage data allows a subdivision of the well results into three distinct provinces. These each have distinct charge histories that reflect differences in potential source kitchens and all have been adversely affected by the Neogene collision of the Australian and Southeast Asian plates. Traps located on the northern and northeastern Londonderry High have experienced high oil charge rates at the Mesozoic level, with nearly all valid traps showing evidence of prior oil accumulation. Breaching of these oil columns in the Neogene appears to be related to the orientation of the contemporary stress field, which promotes shear failure on the faults reliant for seal. Present day hydrocarbon migration indicators, such as Synthetic Aperture Radar (SAR) data show differences in seepage response between the northern and northeastern Londonderry High, with prolific current day seepage restricted to the northern province. Rapid subsidence associated with plate collision has accelerated maturation in the northern province to create these strong seepage anomalies over this region. The absence of seepage over the breached oil columns of the northeastern province indicates that either, oil charge has ceased to this area or that hydrocarbon leakage is episodic in nature. In contrast, results from the northwestern province show no evidence of prior oil accumulation, despite many wells having tested valid traps. These data point to either a lack of connected oil migration pathways or an impoverished source kitchen for liquid hydrocarbons. Low levels of seepage in the northwestern Londonderry High detected by the SAR data are minor compared with other parts of the Timor Sea and consistent with migration continuing at the current day. The overall prospectivity for fault bound traps in the study area appears to be low, due to extensive fault reactivation producing low fault seal integrity. Stratigraphic plays that do not rely on faults for seal, particularly in the northern and northeastern provinces, represent an alternative play concept at the Jurassic level. At shallower levels in the Cretaceous, subtle four-way dip closed structures are often enhanced by the reactivation process and could be ideally positioned to receive remigrated oil from breached Jurassic oil accumulations.

Basin-scale fluid flow in the Gippsland Basin: implications for geological carbon storage

Petroleum systems analysis has been carried out to better understand the geological CO2 storage potential of the Gippsland Basin. From a regional perspective, the hydrocarbon migration architecture of the basin is interpreted to be dominated by two highly connected, filled-to-spill, hydrocarbon fairways; the northern (gas-dominated) and southern (oil-dominated) fill-spill chains, forming a convergent system that extends onshore along the Golden Beach Fill-Spill Chain (GBFSC). A separate oil-dominated Fill-Spill Chain, the Dolphin-Perch Fill-Spill Chain (DPFSC), is identified offshore to the southwest. Two broad flanking provinces, the Northerly Migration Province (NMP) and Southerly Migration Province (SMP), are also identified. Both provinces have broadly ramp-like geometries and relatively low dips. Migration across these provinces is not focused, and hence multiple pathways are present across a wide area. An understanding of the hydrocarbon systems in the basin can be used for characterising the potential for CO2 storage. Previous studies have shown that the top seal potential of the offshore Gippsland Basin is suited to geological carbon storage and that large areas are prospective as storage regions. However, the linked nature of the fluid flow systems and the focused fluid flow fairways between areas of high storage potential and leaky systems onshore will require both a good regional geological understanding and informed resource management.