FLUID EVOLUTION INTERPRETED FROM DIAGENETIC ASSEMBLAGES AND SALINITY DATA IN PERMO-TRIASSIC SANDSTONE, NORTHERN PERTH BASIN, WESTERN-AUSTRALIA

Shallow-marine Permo-Triassic sandstone of the northern Perth Basin, W estern Australia, shows a progressive range down-dip in pore-water sal inities, and in the mineralogy of diagenetic assemblages. These featur es have been interpreted as resulting from a history involving three h ydrocarbon injections, and the introduction of mete oric waters. The s andstone units, some containing oil and gas, dip and thicken southward toward the basin depocenter. Salinity in both irreducible water of th e hydrocarbon column and pore water of the water leg increases southwa rd. Isohalines in the hydrocarbon column are broadly parallel to those in the water leg, but water salinity in the oil column is higher than in the water leg. In the north, typical diagenetic mineral assemblage s include kaolinite, siderite, pyrite, Ti-oxides, and quartz, but fart her south they in elude chlorite, pyrite, albite, Ti-oxides, and quart z. Detrital feldspar and mica in the north are partly dissolved and al tered to kaolinite, whereas to the south, they show chlorite alteratio n. The authigenic phosphate xenotime increases from north to south. Th e contrast in diagenetic assemblages from north to south, together wit h the increase southward in water salinity, point to southward how of introduced water, and siderite zonation indicates that the introduced water ranged from meteoric to saline. The water probably originated on basement near the northern margin of the basin, which, although now e xposed, may have been repeatedly submerged and exposed since the Permo Triassic. It Bowed down dip, generally diluting saline con nate water s of the marine sequence as it penetrated the basin. Flow began during early diagenesis. There were at least two episodes of oil emplacement . The presence of the first oil halted most mineral diagenesis in the reservoir, and solid bitumen envelopes developed around radioactive mo nazite grains. As the oil leaked out, mineral diagenesis recommenced, and traces of residual oil were trapped in the growing quartz. Emplace ment of the second (present) oil again halted mineral diagenesis, and a little of the new oil adhered to envelope surfaces or penetrated env elope cracks. A late gas inflow displaced the oil-water contacts. The fresh-water influx continued below the hydrocarbon column, lowering sa linity in the water leg and promoting quartz cementation. The influx m ay still be going on.