THE GEOLOGY OF THE HARRIET OILFIELD, OFFSHORE WESTERN AUSTRALIA

Abstract

The Harriet Oilfield, discovered in November 1988, is situated within offshore permit WA-192-P in the Barrow Sub-basin. Following the Harriet 1 discovery well, detailed seismic surveys were recorded and a further ten wells were drilled on the structure between 1988 and 1985. Nine of the wells were completed as producers and one was plugged and abandoned as a dry hole.

The oil accumulation occurs in a low relief, fault-dependent closure on the upthrown side of the Lowendal Fault. The trap is mainly structurally controlled but stratigraphic barriers are believed to be locally present, based on differing oil-water contacts in Harriet B-3 and Harriet A-5. These indicate the presence of three hydrocarbon pools separated by permeability barriers.

The massive Flag Sandstone reservoir of Lower Cretaceous (Neocomian) age was deposited in a submarine fan environment, northward of the advancing Barrow Group delta. Reservoir quality is very good, with average core porosity of 22 per cent and permeabilities mainly in the range 800-2 000 md. However, a broad oil-water transition zone is developed above the oil-water contact. A residual oil zone is present below the oil-water contact in the northeastern area of the field, suggesting tilting of the structure after initial accumulation of the oil. The gross oil column in the main, Central Pool is 19-21 m with a gas cap up to 10 m thick. The 37° API crude is a relatively unaltered, high quality, paraffinic oil probably sourced from the Jurassic Dingo Claystone.

The Harriet Field is the first commercial development of a Barrow Group hydrocarbon accumulation. Recoverable oil reserves are currently estimated at 21 million barrels. The field came on stream in January 1986 and by October 1986 oil production was averaging 10 000 barrels/day.