DIAGENESIS AND RESERVOIR QUALITY OF THE ALDEBARAN SANDSTONE, DENISON TROUGH, EAST-CENTRAL QUEENSLAND, AUSTRALIA

The Lower Permain Aldebaran Sandstone is the principal hydrocarbon reservoir in the Denison Trough (Bowen Basin), east-central Queensland, Australia. It accumulated in a wide range of fluvio-deltaic and nearshore marine environments. Detailed petrological study of the unit by thin section, X-ray diffraction, scanning electron microscopy, electron microprobe and isotopic analysis reveals a complex diagenetic history which can be directly related to depositional environment, initial composition and burial-temperature history. Early diagenetic effects included the precipitation of pyrite, siderite and illite-smectite rims (delta-O-18 (SMOW) = + 8.9 to + 11.3 parts per thousand. Deep burial effects included physico-chemical compaction and the formation of quartz overgrowths, siderite (delta-C-13 (PDB) = -34.0 to + 11.5 parts per thousand, delta-O-18 = -0.7 + 22.7 parts per thousand), illite/illite-smectite and ankerite (delta-C-13 = -9.3 to -4.9 parts per thousand, delta-O-18 = + 7.6 to + 14.4 parts per thousand). Involved fluids were in part 'connate meteoric' water derived from compaction of the underlying freshwater Reids Dome beds. Important post-maximum burial effects, controlled by deep meteoric influx from the surface, were ankerite and labile grain dissolution and formation of kaolinite (delta-O-18 = + 7.8 to + 8.9 parts per thousand, delta-D = - 115 to - 99 parts per thousand), calcite (delta-C-13 = -9.5 to 0.9 parts per thousand, delta-O-18 = + 9.0 to + 20.0 parts per thousand) and dawsonite (delta-C-13 = - 4.0 to + 2.3 parts per thousand, delta-O-18 = + 9.8 to + 19.8 parts per thousand), the formation of dawsonite reflecting eventual stagnation of the aquifer. Entrapment of contained hydrocarbons was a relatively recent event which may be continuing today. Reservoir quality varies from marginal to good in the west to poor in the east, with predictable trends being directly linked to depositional environment and diagenesis.