Abstract: Some world-class gas fields have been discovered in the Middle to Upper Triassic, which is complete and widely distributed, on the Northwest Shelf of Australia. The typical sedimentary sequence is characterized by:(1) The shallow braided stream delta is the commonest depositional system which consists of a broad deltaic plain and narrow deltaic front; (2) The sedimentary sequence of the shallow braided stream delta shows a "sandwich" structure by the alternation of the distributary sandstone and the dark interdistributary bay mudstone. The distributary sandstone which contains large amount of carbonaceous laminations, is medium-grained up to coarse-grained and could be a kind of reservoir with high quality, and the interdistributary mudstone rich in terrigenous organic matter is a kind of good gas source rock; (3) The seed ferns and fern spores from the nearshore lowland is always mixed with Alpine pine pollen from the mountain. The typical sedimentary assemblage mentioned above is the response to the specific palaeoclimate and paleogeography. The Triassic occurs upon a shallow marine background on the northern margin of the stable Australian-Antarctic craton. Because of the shielding of the outside islands, current process is the main depositional process of the Triassic delta. The Australia-Antarctic ancient land mainly composed of Proterozoic-Archean granites provided a large amount of coarse clastics for the Middle to Upper Triassic delta. In the Middle-Late Triassic, Australian North West Shelf lain in the tropical -subtropical climate zone and was influenced by the Central Tethys Ocean monsoon. The climate was warm and humid, the vegetation was flourishing, and flood was frequent. The coarse sandstones formed during the flood period were interbeded with dark mudstone formed during the flood interval. Offshore lowland spore-pollen are always mixed with Alpine spore-pollen.