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Near-Field, Sub-Thrust and Deep Reservoir Tests of the Kutubu Oil and Gas Fields, Papua New Guinea

Hill, Kevin C.; Iwanec, Jeremy B.; Lund, David
Exploration, Papuan Oil Search Ltd, Sydney, NSW, Australia.

The Kutubu oilfield is a double-humped, thrust-faulted anticline in the Papuan Fold-Belt and is en echelon to the smaller Agogo oilfield. The structures were first drilled in the mid-80's and have since produced over 300mmBBL of oil from the lowermost Cretaceous Toro and Digimu sandstone reservoirs. It is estimated that 50mmBBL of oil and >1 TCF of gas remain to be produced. The reservoirs are overlain by ~1 km of Cretaceous shale and ~1 km of Miocene limestone which crops out in the mountains at surface. There, the limestone is highly karstified and covered in equatorial jungle making acquisition of surface dips and seismic data difficult. In 2009, an Agogo well was sidetracked and deepened in order to test the Oxfordian Koi-Iange sandstone within the hangingwall. Instead, the well drilled through the main thrust and penetrated a 1 km sub-vertical limb of Toro and Digimu reservoirs that proved to be oil-bearing. The geological structure was re-evaluated and found to be a large asymmetric fold that was cut by a late nearly-planar break-thrust that decapitated the crest. The break-thrust has ~2km of offset and therefore separated and sealed the hangingwall and footwall reservoirs. The geometry seen at Agogo was applied to the Kutubu field using thrust outcrop data and a handful of subsurface fault penetrations to define the geometry of the main thrust within a 3D structural model. The subthrust structure was drilled in 2011 with the fault being encountered close to prognosis with ~2 km of offset. The Toro-Digimu reservoirs were overturned in the footwall and cut by small faults. The Toro was found to be oil-bearing while the Digimu appears water wet. Both the Agogo and Kutubu steep footwall limbs are on production with further appraisal wells planned. Meanwhile, another Koi-Iange test was attempted further back on the Kutubu field. After drilling through a backthrust with ~300m of offset and a fault with ~100m of normal offset, the interbedded sands and shales of the Koi-Iange Formation were encountered, extending previous interpretations of the sand fairway. These sands will be tested in 2012. Following an update of the 3D structural model, it is hoped that further near-field hydrocarbon discoveries will be made to augment production from these fields.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012