DEVONIAN REEF PROSPECTS IN THE CANNING BASIN: IMPLICATIONS OF THE BLINA OIL DISCOVERY

Abstract

Devonian reef complexes outcrop spectacularly along the northern margin of the Canning Basin, forming what is probably the best preserved Palaeozoic reef belt in the world. The reefs extend into the subsurface, where they have been prime objectives for oil exploration since the mid 1950s. Exploration was finally successful in 1981, when Home Oil discovered the Blina oilfield in an Upper Devonian reef complex. It is the first potentially commercial field to have been found in the Canning Basin.

Blina 1 flowed oil at rates of up to 905 barrels per day from the reef complex and overlying Upper Devonian to Lower Carboniferous dolomite. Two successful step-out wells have now been drilled, and although reserves of the field are still uncertain, economic development seems likely.

The Blina reef complex is of Famennian age, belonging to the Nullara Cycle of reef development, while the overlying productive dolomite is within the Famennian-Tournaisian Fairfield Group. The Famennian reef margin is clearly displayed on seismic sections, but an earlier cycle of reefs, the Givetian-Frasnian Pillara Cycle, is more difficult to detect. These older reefs may nevertheless have better overall prospects than those of the Famennian.

Positive indications of oil associated with Devonian reefs have also been obtained in the Bonaparte Gulf Basin, where widespread showings have been reported from mineral-exploration boreholes in Famennian reefal carbonates and overlying Lower Carboniferous carbonates.

Recent seismic results have disclosed possible reef anomalies in several permit areas. The prospects for making further reef-associated discoveries seem promising along the Blina trend and elsewhere in the Canning and Bonaparte Gulf Basins. Prospective Devonian reefs are also likely to occur in the offshore area skirting the Kimberley Block and possibly extending as far south as the Carnarvon Basin.