HYDROCARBON OCCURRENCE AND DIAGENETIC HISTORY WITHIN PROTEROZOIC SEDIMENTS, McARTHUR RIVER AREA, NORTHERN TERRITORY, AUSTRALIA

The stratigraphy of the Proterozoic in the McArthur River area of Northern Territory consists of the basal, non-economic Tawallah Group, overlain unconformably by dolomitic carbonates and clastics of the McArthur Group, in turn overlain disconformably by Roper Group clastics. Several shows of tarry to brittle bitumen have been reported in sandstones of the Roper Group and in dolomites of the McArthur Group. In thin sections, the bitumen commonly displays shrinkage cracks, apparently associated with the loss of volatiles. Secondary minerals are observed infilling some of the cracks, indicating those phases of diagenesis which occurred subsequent to breaching of the hydrocarbon bearing reservoir. Additionally, the contact relationships of bitumen with the secondary minerals indicate a relatively early migration of hydrocarbons into the reservoir rocks. The inferred sequence for the McArthur Group dolomites is: early dolomitization and silicification, formation of vuggy (vadose) porosity, authigenic deposition of chalcedony at shallow burial depth, cementation by quartz at deep burial depth, migration of hydrocarbons (contemporaneous with sulphide formation), breaching of the reservoir, degradation of hydrocarbons, and deposition of sparry dolomite cement. The inferred sequence of diagenesis for Roper Group clastic reservoirs in this area is: authigenic deposition of minor quartz and illite cement, migration of hydrocarbon, breaching of the reservoir, major authigenic deposition of quartz and illite, degradation of hydrocarbon, and cementation by dolomite, hematite, and kaolinite.