Halibut Field, Southeastern Australia

Abstract

Australia's first commercial offshore field, located in the Gippsland basin of southeastern Australia, was discovered in 1965. Further exploratory drilling in the area led to the discovery of additional gas and oil fields in subsequent years. Among these, the Halibut oil field was discovered in August 1967 and drilled for development in 1969-1970. This field is 40 mi offshore in 238 ft of water and encompasses an area of 10.4 sq mi. The structure, first delineated seismically in 1962, was mapped further seismically during 1966, and then confirmed for development on the strength of only the discovery well. Confidence in the seismic interpretation was sufficient to initiate construction of a 24-conductor drilling platform without the support of delineation drilling. Oil in the Halibut field is present in rocks of the fluvial-deltaic Latrobe Group (Eocene-Cretaceous) and the field reservoir has been subdivided into 10 mappable units. The southwest-dipping Latrobe beds are truncated by a late Eocene erosion surface and sealed by the overlying Oligocene mudstones of the Lakes Entrance Formation. The field has a common oil-water contact even though permeability barriers may restrict the vertical movement of fluids between individual sandstone units. Closure in excess of 500 ft at the unconformable Latrobe surface provides a trap for the oil between depths of 7,350 and 7,856 ft subsea. The field was developed by directional drilling techniques, with some wells angled more than 45° from the vertical and reaching up to 6,100 ft laterally from the platform. With only 24 conductors available, optimum drainage points were selected with care. The highest, nontruncated, structural position of individual sandstone units was determined from the structure and isopach maps to find the optimum drainage position for each well. After the original seismic anomaly was recognized and further seismic surveys made, a successful wildcat confirmation test was drilled. Following construction of a drilling platform, the development of the field was routine until the Halibut A-9 well was drilled. This well, drilled on the apex of the structure, revealed a late Eocene erosion channel cutting deep into the intra-Latrobe reservoir beds. Additional seismic control and the reinterpretation of existing data successfully delineated the channels, and the field was developed to completion without further significant changes in interpretation of the reservoir configuration. The field was put on stream in March 1970. Cumulative production to October 1, 1970, from the 19 producing wells, was over 26 million bbl of stock tank oil. The Halibut field is contributing significantly in Australia's effort to become a self-sufficient oil-producing country.