LATE TERTIARY FLUID MIGRATION IN THE TIMOR SEA: A KEY CONTROL ON THERMAL AND DIAGENETIC HISTORIES?

Abstract

An integrated study of the fluid flow, hydrocarbon charge and thermal histories of a suite of traps from the Timor Sea has revealed the presence of a key fluid flow event in the latest Miocene/Early Pliocene. This event, which coincided with, and was driven by, the extensive tectonism and fault reactivation associated with the collision of the Australian and Eurasian plates, involved the flowage of hot (90-120°C), saline (>200,000 ppm) brines (probably from deeply buried Palaeozoic evaporite sequences) up major faults and through the Mesozoic and Tertiary sequences. The passage of these hot brines produced a Late Tertiary (<5 Ma), transient heating event evident in fission track and fluid inclusion data, which is best seen in moderately reactivated traps, such as the Challis and Jabiru oil fields. These apparently leaked for a relatively extended period (>100,000 to 1 million years). In contrast, strongly reactivated, low integrity traps such as East Swan appear to have been breached too quickly (<100,000-10,000 years) to have equilibrated thermally with the transient heat pulse. High integrity traps, such as Oliver and Montara, did not experience late stage brine migration (because of a lack of fault reactivation and attendant fluid flow pathways) and consequently are now at their maximum temperatures. These traps probably most accurately reflect the maturation history of the source rock kitchens in the Timor Sea. As such, failure to recognise the transient and localised (fault-related) nature of this fluid flow/thermal event could lead to significant 'over-modelling' of maturation histories in the Timor Sea. Similar late stage, local fluid flow events may be an important component of the much discussed Late Tertiary heating event along the entire WA margin.