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 (similar to 5.5. MaBP), the co llision of the Australian and Eurasian plates resulted in proto-foreland de velopment and significant structural reactivation in the Timer Sea, north-w estern Australia. Flexural extension, resulting from the down-warping of th e Australian plate into the developing Timer Trough, resulted in the dilata tion of the major Jurassic and older extensional faults and the formation o f shallow Mio-Pliocene fault arrays. An integrated, multidisciplinary study of hydrocarbon traps from this region using 2-D and 3-D seismic data, stab le isotope geochemistry, fluid inclusion measurements and apatite fission t rack data has revealed that this fault reactivation produced three categori es of traps: high (HIT), moderate (MIT) and low (LIT) integrity traps. Thes e have characteristic hydrocarbon fill-spill, fluid flow and thermal histor ies. In MITs and LITs, the dilatation was moderate to intense respectively, and allowed hot (90-130 degrees C), highly saline (200,000(+) ppm salinity ) brines from deep Palaeozoic evaporites to migrate up the reactivated faul ts and chemically and thermally affect the reservoir and shallower interval s. Apatite fission track data suggest that fluid migration lasted for betwe en 100,000 and one million years in the case of the MITs, but for only 10,0 00-100,000 years in the LITs. This major fluid flow event resulted in the d evelopment of a prominent, localised Late Tertiary heating 'spike' in the M ITs, which can significantly affect the accuracy of modelled thermal histor ies. In the LITs, the thermal effect is less marked, due to the more transi ent nature of the fluid flow event. HITs were largely unreactivated and hen ce conduits for brine migration from depth were absent. Consequently, these traps are the most representative of the thermal histories of the source r ock depocentres. Where MITS or LITs were charged, the associated loss of fa ult seal integrity facilitated hydrocarbon loss from the Mesozoic reservoir s, which co-migrated with the brines up through the Mio-Pliocene fault netw ork. Upon entering a shallow, elastic aquifer system (the Eocene Grebe Form ation), bacterial oxidation of the hydrocarbons liberated CO2 which, in tur n, resulted in significant and very isotopically light carbonate cementatio n. This cementation produces sufficient acoustic impedance with the surroun ding uncemented sands that it allows these hydrocarbon-related diagenetic z ones (HRDZs) to be mapped seismically. Since both the size and acoustic res ponse of the HRDZs are directly proportional to the amount of hydrocarbons that have leaked from the traps, their presence or absence provides a power ful indicator, predrill, of both trap integrity and the likely thermal regi me that that traps have experienced. An important observation is that the l eaky fault segments over partially breached traps typically only extended f or 200-1000 m, whereas over the breached traps, leaky segments extended for 3000-5000 m. Consequently, exploration programs acquiring remote sensing g eochemical data (such as geochemical sniffer and airborne laser fluorosenso r (ALF) techniques), should have closely spaced line spacings if leaky, pot entially commercial fields are to be detected reliably. Potential analogues exist between the processes documented during HRDZ formation, namely the m ixing at shallow depths of basinal brines, hydrocarbons and connate waters, and processes occurring during the formation of Pb-Zn and other, low tempe rature ore deposits. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.