--> Near-Field Discovery and Optimisation of Complex Forelimb Structures in the Papua New Guinea Fold and Thrust Belt

International Conference & Exhibition

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Near-Field Discovery and Optimisation of Complex Forelimb Structures in the Papua New Guinea Fold and Thrust Belt

Abstract

The near-field discovery of the Agogo forelimb structure in the PNG Highlands Fold and Thrust belt was serendipitous. A suspended well was deepened below the hangingwall Neocomian Toro-Digimu reservoir sands that form the main Agogo oil and gas field, a broad gentle anticlinal structure, in search of a deeper mid-Jurassic sandstone play. Instead of penetrating these deeper sands in the hangingwall of the structure, the well drilled through a major thrust fault and into a near-vertical forelimb, containing repeated Toro-Digimu oil-filled sands. Ongoing drilling revealed an approximately 1 km high overturned forelimb, containing a ~200 m high oil column. Modelling of the Agogo hangingwall and forelimb structure, both in 3D and through constructing balanced cross-sections, has revealed a possible evolution of the structure. The Agogo field (both main and forelimb) initiated as an asymmetric detachment fold, above a mid-Jurassic decollement. Ongoing folding steepened the forelimb, before late-stage breakthrusts decapitated the main hangingwall field from the underlying near-vertical forelimb. Timing of charge of the two fields with respect to deformation is uncertain, although the hangingwall and forelimb reservoirs record different HC and water pressure gradients. Development of the forelimb has involved a further two wells with sidetracks being drilled within a ~1 km2 map area. These constrain the 3D geometry of the forelimb structure, which, on a large-scale, is relatively predictable in its along-strike projection. In detail, however, the forelimb reservoir is complexly deformed and compartmentalised by numerous smaller scale faults that accommodated ongoing folding and shearing of the forelimb. Understanding the evolution of the broader scale forelimb allows the placement of further development wells along strike from existing producers. Simultaneously, evaluating the small-scale complexity of the structure allows an assessment of possible communication and baffles between reservoir compartments; crucial to both planning well step-out and minimising structural uncertainty.