HYDROCARBON GEOLOGICAL FEATURES OF DEEPWATER BASINS ON NORTHWEST SHELF OF AUSTRALIA

The North West Shelf of Australia is a world known gas province with great potential having proved by the exploration and exploitation in recent years. It is a typical passive margin of a marginal sea consisting of four basins and an orogenic zone, namely the North Carnarvon basin,Roebuck basin,Browse basin,Bonaparte basin and the Timor-Banda orogenic zone. It has experienced three stages of evolution, i.e., the pre-rift stage from the Cambrian to Early Jurassic, the rift stage from the Early Jurassic to Early Cretaceous and the passive continental margin stage from the Early Cretaceous up to present. Correspondingly there are three suits of sedimentary sequences. Most of hydrocarbon is accumulated in the Mesozoic, with gas in the outer part and lower sequence, and oil in the inner part and upper sequence. 3 types of source rocks are recognized. They are the Triassic lacustrine shale, Jurassic paralic carbonaceous argilite, coal bearing strata and marine shale, and Cretaceous marine shale. Most of the source rocks have great capacity of gas generation. The source rocks can be summarized into four models. The main reservoir is the marine sandstone deposited in the rift stage, and the Cretaceous turbidites formed in the passive continental margin stage is a potential reservoir. The Lower Cretaceous marine shale is the main seal covering the whole region. From the view point of petroleum system, there are three reservoir-seal assemblages in the region including the marine reservoir-seal assemblage, paralic reservoir-seal assemblage and continental reservoir-seal assemblage, and the paralic reservoir-seal assemblage dominates. The major passes for hydrocarbon migration include structural ridges, faults, and unconformities. Overpressure, fault and unconformity are the major driving factors of hydrocarbon migration. In this study area major plays always occur in the lower sequence near the source with the upper parts as reservoir and seal.