Definition and Development of the Mackerel Field, Gippsland Basin

Abstract

The Esso-Hematite Mackerel oilfield is located approximately 70 kilometres offshore in the Gippsland Basin, south-eastern Australia. It is one of a number of fields in this basin in which the reservoir occurs beneath an unconformity at the top of the Latrobe Group which is of Paleocene-Eocene age. The trap has been formed within a topographic erosional feature, sealed by calcareous shales and mudstones of the Oligocene Lakes Entrance Formation. This paper covers two topics. The first is the history of, and problems related to, the definition of the irregularly eroded Latrobe surface. In particular, the use of migrated seismic data to particular, the use of migrated seismic data to define the Top of Latrobe is discussed. The second topic of the paper is the use of seismic stratigraphy to help plan the development of the field. Progradational seismic character in the Latrobe reservoir section is observed on the seismic data at Mackerel. Seismic stratigraphic work was integrated with the results from the four exploration wells to build a model of the reservoir section prior to field development. This model was then used to help plan the development well program. Finally, the results of development drilling to date are discussed and an evaluation made of the predevelopment geophysical and geological interpretation. predevelopment geophysical and geological interpretation Introduction The Esso-Hematite Mackerel oilfield is located approximately 70 kilometres offshore in the Gippsland Basin, south-eastern Australia (Figure 1). It is one of a number of fields in this basin in which the reservoir occurs at an unconformity at the top of the Latrobe Group which is of Paleocene-Eocene age. The Mackerel "structure" is not a structural closure in the strict sense but rather a combination of a number of elements. Dip to the south east is the result of the Latrobe paleoslope with its strike running along the south-eastern extremities of the Mackerel and Kingfish fields. The Latrobe section progrades from the north-west to the southeast. Dip progrades from the north-west to the southeast. Dip to the north-east and north-west is provided by erosion of the Marlin channel and by a feeder channel to the Marlin channel respectively. Finally closure to the south-west is the result of structural dip. The trap is sealed by calcareous shales and mudstones of the Oligocene Lakes Entrance Formation. THE DEFINITION The Mackerel feature was first mapped from seismic data in 1966 as a nose extending to the north-east from the Kingfish structure. It was first mapped as a closed feature after the 1967 seismic survey. The Mackerel-1 location was chosen to be on or near the crest of a large single northeast to south-west trending anticlinal closure (figure 2). Possible reserves were estimated to be up to 700 MMB. Possible reserves were estimated to be up to 700 MMB. In 1969, Mackerel-1 was drilled, intersecting the top of Latrobe within 0.5m of prediction. However, the results of Mackerel-1 were very disappointing, as the well intersected a gross oil column of only 18.5 metres reducing the estimated reserves to 30 MMB. In the following years, interpretation of new seismic data indicated that the Mackerel-1 discovery well had missed the crest of the structure and had drilled into a local erosional channel on what was now interpreted to be an irregular topographic surface, and reserve estimates for the field were upgraded to 140 MMB. In 1971, further seismic was shot over Mackerel. The structure (figure 3) was interpreted to consist of two crestal areas, the larger just to the south-west of Mackerel-1 and the smaller to the north, on the downside of a northwest-southeast trending fault.