Abstract

A new gas hydrate province has been identified in the north Makassar Straits between the islands of Borneo and Sulawesi. The database for the interpretation of the bottom simulating reflector (BSR) comprised 21,000 kilometres of newly-acquired and reprocessed multi-client marine 2D seismic data encompassing an area of approximately 100,000 km². The majority of BSRs have been identified within an 8000 km² area in the deep-water West Sulawesi Fold Belt (WSFB; Fraser et al., 2003).

Sediments in the WSFB were sourced from the Mahakam Delta until the late Pliocene, when a tectonic event in Sulawesi reversed the direction of sediment transport from eastward to westward. The same tectonic event was also responsible for the initiation of the west-verging fault propagation folds which created the fold belt. The fold-belt is comprised of numerous thrust sheets creating long anticlinal structures and intervening mini-basins in which numerous high amplitude reflection packages are observed, indicating the presence of coarse-clastic turbidite facies. The deposition of the fill-and-spill turbidite successions had two significant effects for gas hydrate accumulation in the WSFB. First, coarse-grained turbidites provided an effective reservoir for hydrates in addition to a transport medium for migrating gas and a steady supply of water. Secondly, the turbidites were likely to transport terrigenous plant matter into the deep-water where biogenic processes produced the methane required for gas hydrate formation.

Most of the BSR anomalies are concentrated on the east side of the study area in the vicinity of the WSFB approximately 300 milliseconds below the seafloor. Fault propagation folds concentrate free gas below the hydrates, resulting in a dramatic BSR which is almost continuously present within the fold-belt. Although the BSR fades in the intervening synclines, the hydrates are still likely to be present.

On the eastern side of the fold-belt, the BSR can not be identified with certainty since there are many high amplitude turbidite slope sands and unconformities parallel to the seafloor that truncate reflectors in a manner similar to a BSR.

On the west side of the study area, in the vicinity of the toe-thrusts associated with gravity sliding in the Mahakam Delta, there appear to be fewer BSRs, partially due to recent sedimentary processes that make identification difficult. The problem of BSR identification may be similar to that found in the Gulf of Mexico where, rather than the conventional through going reflector, the BSR is represented by a lineation of steeply-dipping, high-amplitude nomalies separated by a significant thickness of non-anomalous sediment.

BSRs are difficult to identify on the abyssal plain between the WSFB and the toe-thrusts of the Mahakam Delta, due to the flat-lying sediments near the seafloor. However, several BSRs can be identified where a through-going reflector is observed in an area of pervasive block faulting near the seafloor. This observation provides evidence that there may be a more widespread distribution of gas hydrate in the abyssal plain than indicated by BSRs.