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The Southwest Ampa field; geological and structural setting

The Southwest Ampa field is situated at the southern end of the large southwest - northeast trending Ampa anticline, bounded by the Inner Shelf syncline to the southeast and the Middle Shelf syncline to the northwest. The anticline overlies a deep-seated counter-regional fault system, accompanied by a clay ridge in its footwall. Delta progradation into this syn-depositional counter-regional fault system resulted in a strongly diverging depositional sequence, with aggradational, foresetted shelf edges in the downthrown block.

Increasing sediment supply through development of the Baram delta system, and/or cessation of counter-regional fault activity, resulted in subsequent advance of the delta over the Southwest Ampa area, during the Late Miocene. Delta progradation initiated a series of growth faults, progressively younger from south to north. These growth faults, and their accompanying, subsidiary fault systems, are the main compartmentalising fault systems across the hydrocarbon-bearing areas of the field. Most of the faults are down-to-basin, like the Betty Baram fault. The main exception is the 21-Area fault, which is a major counter-regional fault (Figure 5.37). Shallow marine and coastal (topset) sediments were progressively followed by lower coastal plain series, as the Baram delta advanced further to the northwest.

Pliocene transpressional overprint caused the formation of the anticline, initially syn-depositional with considerably thinning towards the crestal part of the anticline, later increasingly post-depositional, with erosional truncation at the depositional surface over the area of uplift. The largest erosional truncation occurred to the east of the Southwest Ampa field, in the central part of the large-scale anticline. This is the area where Ampa-1, the first offshore exploration well in Brunei waters was drilled (Figure 5.36). The well was unsuccessful because it was located in the area where most of the reservoir-bearing overburden has been deeply truncated.

The earlier growth faults have been deformed and partly re-activated by the Pliocene folding, as the axis of the large-scale fold is oblique to their predominant easterly trend. Moreover, the early faults were overprinted by a hinge collapse graben over the outer arc of the anticline. Interference between the earlier growth faults and the mainly northerly trending hinge collapse faults caused the very high degree of fault compartmentalisation at intermediate to shallow parts of the structure. The strike of the hinge collapse faults at an angle to the axis of the large-scale anticline reflects involvement of a (small) wrench component.

Very dense fault interference in the western part of the 21-Area resulted in the evolution of a stock-like clay diapir, by buoyancy of highly undercompacted clays, through the overlying topset sediments (Figure 3.13). Accompanying fracture networks and cataclastic shear bands cause considerable deterioration of reservoir conditions in its immediate surroundings.

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