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SKUA-SWIFT OIL FIELDS

The Skua 3D data-set covers the Skua and Swift oil fields, as well as the as-yet undrilled Spruce Prospect, which is located east of the Skua Field and north-east of the Swift trap (O’Brien et al, 1998a). The top Paleocene horizon is a very strong reflector in the area and is located just beneath the focus of HRDZ formation (which is principally within the Eocene Grebe Formation sands; O’Brien and Woods, 1995). As this horizon is relatively flat, the amount and distribution of time pull-up present on the 3D seismic data accurately reflects the size and shape of the velocity anomalies. This effect is evident from the ER-Mapper image of the top Paleocene Horizon, shown in Figure 24a.

The distribution of HRDZs in the Skua 3D area is controlled closely (O’Brien and Woods, 1995) by the faulting (Fig. 25). Three main fault trends affect this region, all of which, to a greater or lesser extent, affect the location of the HRDZs:
1. An old NNW–SSE trending, probably basement-involved (?Proterozoic; O’Brien et al, 1993) system which is seen only as subtle linear features on enhanced images of the deeper top Lower Cretaceous horizon (Fig. 24b). This structural trend is imaged more clearly further to the northeast, on the Buffalo and Bayu-Undan 3D seismic surveys, where it is expressed as discrete faults within the Tertiary section.
2. A later Tithonian-aged system trending northeast to southwest and forming the tilted fault blocks which define the Skua and Swift structures. The Spruce fault is part of this system, but is oriented more in an ESE–WNW direction.
3. A Late Miocene to Pliocene-aged, ENE-trending fault system which is centred mostly over the Skua Tithonian fault. These faults have a well-developed en-echelon appearance (Fig. 24a).

The three principal HRDZs located over the Skua Field actually provide an accurate indication of the length of the field. These HRDZs also appear to form preferentially at the intersection of different fault systems (Figs 24a and 25). The anomalies appear to be offset by some 500 to 1,000 m to the southeast of the field, probably because leaking hydrocarbons migrated up the Miocene fault arrays, which dip to the NNW.

The HRDZ southwest of Skua–2 is aligned along the oldest, NNW-trending structural grain (Fig. 24a). No discrete fault cut can be mapped on this trend in the shallow section using seismic data; all faults within the Late Tertiary section are clearly ENE-trending Mio-Pliocene faults. Nevertheless, it appears that in this case, the NNW-trending structural grain has in some way controlled the hydrocarbon leakage. Perhaps leakage was enhanced at the intersection of the NNW trend with the younger fault trends. The anomaly south of Skua–7A is aligned along the Mio-Pliocene fault trend, and again leakage may be related to the intersection of this trend with the Tithonian fault (Fig. 24a). There is also some alignment of the anomaly with the older NNW–SSE fault trend.

The HRDZs associated with the undrilled Spruce structure lie along the Tithonian-aged Spruce fault, which bounds one side of the trap (Figs 24a and 25). The anomalies extend along the fault to the level of the spilling contour, suggesting a high degree of trap fill. Using the criteria proposed by O’Brien and Woods (1995), the size of the anomalies is not large enough for the underlying accumulation to be breached, These observations significantly upgrade the prospectivity of the Spruce structure, which is a stratigraphic trap with a potential oil-in-place volume of up to 70 MMBBL.

The prominent HRDZ associated with the Swift structure is the most intense and largest in area, and runs along a significant length (>4,000 m) of the Tithonian-aged bounding fault. The size and intensity of this HRDZ indicates a high rate of seepage over a large area (O’Brien and Woods, 1995), a proposal supported by the fact that Swift–1 encountered only a small oil leg, and a considerable residual column (Lisk et al, 1998).

Figure 24. a) ER-Mapper image of the top Paleocene horizon mapped on the Skua 3D seismic survey (1 km graticule). b) ER-Mapper image of the top Lower Cretaceous Horizon mapped using the Skua 3D seismic survey (1 km graticule).

Figure 25. Interpreted distribution of HRDZs near the Skua Field.