WAXINGS AND WANINGS IN STRATIGRAPHY, PLAY CONCEPTS AND PROSPECTIVITY IN THE CANNING BASIN

Abstract

The prospectivity of the Canning Basin is by no means exhausted. Furthermore, low product prices will offer those with the will and the wherewithal some relatively low cost opportunities to drill seismically well- defined, selected plays in selected acreage. There may never be a better time to invest in the Canning Basin.

The Canning Basin rock record includes at least 16 distinct regional episodes of onlapping, quiet- water conditions that transgressed higher energy reservoir- type facies. These vertical successions often constitute correctable seismic sequences and represent apt horizons at which to map prospective, shale- capped trap configurations. All of these 16+ top- sealed reservoir levels are associated with oil and/or gas shows in some part of the Canning Basin. Indeed, the majority of Canning Basin wildcats are associated with reports of petroleum shows.

There are seven separate petroleum discoveries (four developed) in the Canning Basin. These span seven different formations and three distinctive trap- types: draped bioherms, anticlinal culminations and tilted horsts. While the overall historical ratio of discoveries to wildcats is low (~1:19), the most successful joint venture in the Canning Basin can claim a 1 in 5.3 rate of discovery leading to development since its first wildcat (Blina 1) in 1980.

The most effective oil source rocks in the Canning Basin are thought to be Arenigian to Llanvirnian (Ordovician) marine shales, the Givetian to Frasnian (Devonian) Gogo Formation and the Late Devonian to Early Carboniferous Fairfield Group, in particular the Tournaisian Laurel Formation.

The most consistently permeable reservoirs that are most frequently in favourable juxtaposition to source rocks and seals are the Permo- Carboniferous glacigenic quartzose sandstones of the Grant Group. Most other Palaeozoic reservoirs that are judged to have adequate top seals are less regularly porous. All significant porosity in carbonates in the Canning Basin is apparently diagenetic and irregularly distributed. Those carbonates most likely to be permeable are leached and/or dolomitised and/or fractured. Regressive carbonates, carbonates interfingering with permeable siliciclastics, carbonates adjacent to major faults, and carbonates that either lie above or are cut by unconformities are those apparently most frequently dolomitised. Fenestrate (especially algal) and oolitic fabrics provide excellent habitats for high levels of secondary dolomite and subsequently leached porosity. The Nita, Mellinjerie (lowermost Pillara), uppermost Nullara and Yellow Drum Formations are those units that most frequently exhibit these characteristics in the Canning Basin.

Reefs, salt domes and anticlines have enticed, and will probably continue to attract, explorers to the Canning Basin. Traps including (1) intra- Grant Group palaeo- monadnocks, (2) Carribuddy salt pillows and salt evacuation- related turtle- backs, (3) low- stand submarine fan sandstone complexes in the Frasnian Gogo Formation and (4) tilted horsts at Ordovician levels are additional recognised play types that warrant continued interest and will probably be further explored, if product prices permit.