INFLUENCE OF HYDROCARBON MIGRATION AND SEEPAGE ON BENTHIC COMMUNITIES IN THE TIMOR SEA, AUSTRALIA

Abstract

The Ashmore Platform–Timor Sea region of Australia’s North West Shelf is an area of significant petroleum exploration potential, with several large commercial oil fields present. Moreover, exploration activity seems likely to continue at current levels for the foreseeable future, and may also extend into deeper water, given high oil prices and improved drilling technologies. The area is also one of high conservation value, with both the Cartier Marine reserve and Ashmore Reef (a Category 1 marine park), as well as numerous other genetically-rich carbonate seed bank systems, closely associated spatially with exploration activities. Balancing the conservation and resource values in this area will present a key challenge into the future.

The magnitude of this challenge has been highlighted by recent work undertaken by AGSO, which involved the acquisition and interpretation of assorted remote sensing data, such as high-resolution bathymetry (including sidescan sonar), satellite synthetic aperture radar (SAR), Landsat, water column geochemical sniffer, airborne laser fluorosensor, seismic data and seafloor sediment sampling. These studies have shown that, at both a regional and local scale, the development of these important carbonate systems appears to directly relate to the geological development of the area.

At a regional scale, the collision between the Australian and Eurasian crustal plates in the Pliocene (<5 MaBP) induced the formation of the Timor Trough. The rapid subsidence associated with the formation of the trough provided a range of ideal habitats within which rapidly growing carbonate communities could effectively outcompete other types of sediment deposition/accumulation processes on the continental shelf. Moreover, this trough focussed the location of (what would become) the Indonesian Through Flow (ITF). The conjunction of the ITF with the Indian Ocean ultimately provided a range of diverse genetic opportunities, a situation which reinforced the favourable growth conditions created by the rapid subsidence. As a result of these processes, reefal growth in the Timor Sea took place almost exclusively over the last five million years, with most occurring over the last three million years or so.

At a local scale, new data also strongly suggest that the locations of the majority of reefs and carbonate banks and build-ups in the area are associated with active and palaeo-hydrocarbon seeps. These seeps are localised over either fault systems which tap the reservoir, along migration fairways, or at the pinch-out of the regional Cretaceous top seal. Our interpretations suggest that the reefs and the build-ups formed by a sequential process. Firstly, hydrocarbon seepage (induced by collisionrelated faulting) localised small seafloor (chemolithotrophic) biological carbonate communities, which ultimately formed topographically positive features. These higher relief features were subsequently preferentially colonised by an assortment of reef-building biota, whose rapid growth progressively kept up with rising sea-level (which was driven principally by collisionrelated subsidence). The most favourable conditions for initial reef colonisation probably occurred during periods of relatively low sea-level, when the areas around the reefs were located at much shallower water depths (<40m) than today, and ample sunlight reached the seafloor.

Clearly, the fact that the genetically rich carbonate communities in this area are probably causally related to natural hydrocarbon seepage (and the attendant processes which drove that seepage) will present a series of almost unique exploration, development (especially engineering) and conservation challenges.