Geology and Hydrocarbons, Denison Trough, Australia

Abstract

The Denison trough in central Queensland has an area of about 9,000 square miles and contains up to 19,000 feet of Permian sedimentary rocks, half of them marine. The trough, which is a segment of the Bowen basin, had its most distinct expression as a depositional entity in early Permian time. Twenty-two wildcat wells were drilled in the Denison trough from 1957 to 1965. Of these, 4 produced gas from a total of six formations. Of 13 development wells at four of these locations, only 4 have been cased for production. These development drilling results show that the permeability in the reservoirs is highly variable. The Permian rocks of the Bowen basin have been grouped regionally into the Lower, Middle, and Upper Bowen Beds. The Middle Bowen rocks of the Denison trough lithologically are distinct from those in the rest of the basin. In contrast, those of the Lower and Upper Bowen are geographically more widespread, though the trough retained much of its identity during the times of Upper and Lower Bowen deposition. Middle Bowen sedimentation generally was in marine waters but five regressions occurred during which the major sandstone bodies in the sequence were deposited. The lowest four are the Riverstone and Staircase Sandstone Members of the Cattle Creek Formation and the Aldebaran and Catherine Sandstones. Each is dominantly quartzose. The sand entered the trough from the western hinge, north of Springsure, and from the southern hinge, southeast of Warrinilla. During the fifth regression, several smaller bodies of sand were deposited with coal beds in the upper half of the Peawaddy Formation. This sand appears to have entered the trough from the north. The sandstone bodies are separated, at least in areas away from the shoreline, by marine mudstones. Many of the littoral and neritic parts of these regressive sandstones were deposited in contact with possible source rocks and in a position to be sealed updip by transgressive mudstones. The structural history of the Denison trough indicates that the major folding of the Permian rocks occurred between Triassic and Jurassic times, though some evidence suggests that minor adjustments took place during Permian and Tertiary times. An intra-Permian unconformity is at the top of the Aldebaran Sandstone. A permeable sandstone at Westgrove, which was structurally high, contained only oil traces and brine. This has been interpreted to indicate that migration of oil occurred there before the major folding which formed the Westgrove structure. Moreover, where it has been possible to check, structural gas traps have not been filled completely and contain gas with lower than average percentages of higher homologues, suggesting that the gas has exsolved from formation waters as the overburden was removed. Some structural traps occur in the sandstones of coal-bearing sections, suggesting a possible alternative source for the gas from the coal. Stratigraphic traps contain wet gas at higher-than-normal pressures, so it is possible that the traps were sealed at depths greater than present. No co mercial oil has been found in the trough but gas occurs widely in its center. It is possible that oil has migrated farther than gas from the depositional center by updip spillage or that gas in structural traps has a different origin than the oil. The combination of circumstances indicates that a source for hydrocarbons existed and that, because primary migration took place before folding, only traps present during migration were in a position to accumulate oil. Future discoveries of hydrocarbons may be anticipated, in types of accumulations other than those already proved, in sandstones which always have been in favorable structural positions with respect to migrating petroleum. Such sandstones are marginal (e.g., shoreline) or enclosed (e.g., offshore bars) which may have been sealed by the compaction of the surrounding muds, after primary migration into them of any petroleum generated.