Macedon Detail

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MACEDON GAS FIELD

An attempt has been made to map the interpreted HRDZs over the Macedon gas field in the Carnarvon Basin, using the Macedon 3D seismic data-set (Figs 26a and b). The task is more difficult here than for the Skua-Swift oil fields because the zones of pull-up associated with the HRDZs are partly obscured by the regional depth gradient of the base Carbonate Marker horizon (Fig. 26a). It may be possible to remove this depth gradient with a suitable 2D high pass filter, thereby allowing the anomalies to be mapped more accurately.

The interpreted distribution of the HRDZs over the Macedon Gas Field is shown in Figure 26b. Their extent can be estimated from the zones of seismic pull-up seen on the base Carbonate ER-Mapper image (Fig. 26a). The anomalies extend along the faults, with the most intense development occurring at the intersection of two faults near Macedon–4 (Fig. 26b). The distribution of the seismic anomalies provides a good indication of the total area of the field (Fig. 26b), as was the case with the Skua Field in the Vulcan Sub-basin. The most intense seismic anomalies located near Macedon–3 and Macedon–4 are also clearly expressed on an ER-Mapper image of the seafloor as prominent irregularities (Figure 27).

Most of the wells drilled on the Macedon Field are located on, or near, an HRDZ (Fig. 26b). This adds to the uncertainty associated with depth conversion across the field, because the depth control points (i.e. the wells) correspond to areas of velocity pull-up on the time map. In this case, an under-estimation of the amount of pull-up will result in an under-estimation in the structural volume.* During exploration of this structure, the initial West Muiron–1A and West Muiron–2 wells were drilled off-structure, on the time high to the south-east of the field. The time high is offset from the depth structure because of a combination of the sloping sea floor and the velocity gradient in the shallow carbonates. The first two wells were drilled in 1972 and 1975, and it took another 13 years before the West Muiron–3 well was drilled in a more crestal location and discovered the gas field.

More recently, the Enfield–1 well has proved the prospectivity of the area to the west of the Macedon Field. Two HRDZs to the west of Enfield–1 described in this paper (Figs 13 and 14) suggest that there may be more discoveries to be made in this region.

Figure 26. a) ER-Mapper image of the base Carbonate horizon over the Macedon Gas Field, Carnarvon Basin (1km graticule). b) Interpreted distribution of HRDZs, Macedon Gas Field.

Figure 27. ER-Mapper image of the seafloor relief around the Macedon gas field, Carnarvon Basin.

* This sentence is changed from the original published paper which stated "In this case, an under-estimation of the amount of pull-up between areas of well control will result in an over-estimation in the structural volume.". For a given hydrocarbon column, a broad low relief structure has larger volume than a narrow high relief structure. A well drilled on seismic data affected by velocity pull-up can appear to intersect a narrower structure than actually exists in depth.