The Surat basin, in southeastern Queensland and northeastern New South Wales, is a segment of the Great Artesian basin. Deposition of fluvial quartzose sands began late in Triassic time in areas east of the Surat basin and transgressed westward to the central and northern parts of the basin. They covered folded and block-faulted Triassic and older rocks which had been nearly base-leveled. The quartzose sands were succeeded by a largely nonmarine sand and mud sequence which reached a maximum thickness of about 7,500 ft. The basin expanded south and west, but expansion was interrupted by uplift and erosion about the middle of Jurassic time. Sedimentation continued into the Cretaceous and conditions became marine. The basin contracted southward and westward in medial Cretace us time in response to the deformation which affected the area north and east of the basin.

Thirty million bbl of oil and 135 billion cu ft of gas have been proved in small Jurassic fields on both sides of the Surat basin. The source of the oil and gas was probably in nonmarine Jurassic rocks, but the marine rocks of the Permian may have contributed. In all likelihood the amount of petroleum generated was relatively small. Possible original accumulations were destroyed by middle Jurassic and post-middle Cretaceous erosion. Many structural traps, which are mostly compacted drapes on contemporaneous highs, reached their present amplification too late to receive migrating petroleum. Alternatively, tilting and flushing have emptied some traps which received petroleum. Drilling has proved that most structures (seismic closures) which do not have a stratigraphic component of closu e are barren. Some areas of primary stratigraphic trapping probably remain, undisturbed by erosion and flushing, and closed against regional, post-accumulation tilting.