DISCOVERY OF SALT IN THE VULCAN GRABEN: A GEOPHYSICAL AND GEOLOGICAL EVALUATION

Abstract

The Paqualin-1 well in Permit AC/P2, Timor Sea, was drilled to test a large structural closure against the flank of an interpreted piercement structure located in the Late Jurassic Paqualin Graben. Prior to drilling the well interpretation of geological, seismic, gravity and magnetic data supported both a salt diapiric and/or an igneous intrusive structural model for the origin of the piercement feature. On drilling the Paqualin-1 well in December 1988, a 627 m thick evaporitic sequence was encountered in the post-rift Tertiary sequence indicating that the well had penetrated a salt overhang close to the main diapiric stock.

The age of the evaporitic sequence is unclear but is considered coeval with Palaeozoic salt diapirs in the Bonaparte Basin to the east. Growth of the Paqualin diapir and a similar feature to the south, the Swan structure, which is also interpreted to be a salt diapir, appears to have been triggered initially by the Late Jurassic-Early Cretaceous breakup of the Australian north-west continental margin (doming and pillowing), and then again by a second major tectonic event in the Late Miocene associated with the collision between the Australian and Eurasian plates (diapir- ism and collapse structures).

A distinctive cap rock occurs at the top of the evaporitic sequence characterised by an unusual accessory suite of primary and secondary minerals including euhedral magnetite, bipyramidal quartz, biotite, chlorite, sphene, amphibole and feldspar. The high magnetite component is considered responsible for the positive magnetic anomaly observed prior to drilling. The presence of magnetite and the other minerals in the cap rock appears to be related to the presence of exotic igneous material incorporated in the salt stock and the restricted activity of sulphate reducing bacteria in the crest of the diapir.

The discovery of the Paqualin salt diapir and the interpretation of a similar structure in the adjacent Swan Graben has lead to the recognition of new play types related to the diapirs.