Testing the Tertiary Basin Floor Fan Play in the Gulf of Papua, PNG

Abstract

The Gulf of Papua (GoP) is situated offshore on the southern coast of Papua New Guinea. Present-day morphology of the continental shelf consists of a broad platform that extends 180 km outboard off the coast in shallow water of approximately 100m. The dominant litho-stratigraphy is the prograding Plio-Pleistocene delta front deposits accommodated in the actively growing Moresby Trough foreland basin and early Tertiary rift basins. Over the past four decades oil and gas exploration has focused on Mesozoic clastic and Miocene carbonate in the GoP. Whilst the extension of the prolific onshore Mesozoic reservoirs of Toro and Digimu fields are limited as it progressively sub-cropped and deeply eroded due to the uplift and rifting at northern end of the Coral Sea, the Mesozoic play is proven unprospective. The Miocene carbonate play was successfully tested by a number of small gas and gas/condensate discoveries made in isolated reefal build-ups. Exploration for the Plio-Pleistocene basin relied on availability of good quality 3D seismic to understand turbidite sand fairway and delineate channels, feeders and fan sheets geometry to de-risk reservoir presence. It was only made possible by the recent acquisition and processing of 6,000 km² of modern 3D seismic data in 2010. Advanced seismic interpretation techniques were applied including attribute analysis, AVO analysis and seismic AVO inversion using true-amplitude processing. The interpretation supported the model of deep water turbidite deposition in the GoP with likely presence of porous sandstone reservoirs fan sheets geobodies encased in mudstone and claystone. The fans typically exhibited Class III AVO signatures, consistent with gas-bearing sandstone reservoirs determined by rock physics forward models. A three well drilling campaign was carried out in 2013 to test this new play, targeting the seismic amplitude anomalies which ranged in depth from 2300m to 3200m. In each of the wells, quartz-rich high quality reservoir sandstone turbidites were intersected. Although similar in seismic character and AVO response, some of the reservoirs contained low saturation ‘fizz gas’ whilst others accommodated higher saturation free gas or a combination of the two. However, the discovery of gas in each of the reservoirs has proved that an active and effective petroleum system exists within the Plio-Pleistocene clastic intervals of the Gulf of Papua and has opened up a new play fairway.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015