Imaging Carbonates: The Evolution of 2D Seismic Imaging Onshore in the Eastern Papuan Basin of Papua New Guinea
The Eastern Papuan Basin (EPB) is an emerging hydrocarbon province in Papua New Guinea. The EPB has a long exploration history dating back to 1913 triggered by early reports of seeps in the Vailala River area. Early efforts focused on drilling around hydrocarbon seepage. In 1937 a scientific approach arrived with the Australian Petroleum Company (APC) embarking on a regional exploration programme using photogeology, surface geological mapping and stratigraphic and biostratigraphic studies. Geophysics in the EPB arrived in the 1950s with regional gravity and seismic refraction surveys. The first well based on this work was Wana 1 (1951) targeting a gravity high, the “Wana Swell”. Although it was a dry hole, hydrocarbon shows and flow tests (18,000 bwpd) demonstrated elements of an effective petroleum system. Early success in Tertiary carbonates followed at Kuru 1 (gas, 1956), Puri 1 (oil, 1957-8) and Bwata 1 (gas 1960). Discovery of oil at Puri 1 led to the ambitious Puri Seismic Survey in 1959. Unfortunately, this seismic reflection data from along the Puri trend was unusable – an early victim of the challenges of difficult terrain and steep and complex structures. The reflection seismic technique did not return to the area until 1965 with the Wana Survey – eight surveys in three years followed, none of which were in the Fold Belt. Not until the mid-1980s did reflection seismic return to the Puri trend. These surveys were conducted by small Australian operators restricted by both technology and capital. They met with varying degrees of success. It was not until the early 2000s that the EPB’s time had come and improvements in seismic acquisition technology, advances in computing and associated processing algorithms improved seismic imaging sufficiently to see carbonate bodies in the fold belt. The initial work by InterOil in 2003 used the same seismic parameters as previous explorers. This proved fit for purpose as a reconnaissance exploration tool to define structure/trap leading to the Elk 1 (2006) gas discovery. But it was clear that data quality needed to improve and in 2007 comprehensive parameter tests led to a new suite of calibrated acquisition parameters, which undeniably improved seismic imaging. Interpretation of this improved data enabled the identification of potential shallow marine platform and reefal facies at Antelope: these were drilled by Antelope 1 (2008). This well confirmed the presence of reservoir quality carbonate in the EPB, transforming the resource potential of the Antelope field along with long-held perceptions on regions modest prospectivity. These results demonstrated for the first time in PNG, the ability of seismic in a fold belt setting to de-risk traps. Further discoveries were subsequently made at Triceratops and Raptor; however, the reservoir quality was much poorer in these fields. The Middle Miocene reservoir at Triceratops is argillaceous with stratiform intraformational marls and shales: these are interpreted to inhibit the extent and interconnectivity of the fracture system. Antelope is a cleaner, higher energy carbonate deposited remote from clastic sediment input and later dolomitised, so upon deformation has a propensity to fracture pervasively. Over the last 20 years, interpreters have demanded better quality seismic imaging as they seek to de-risk EPB opportunities through carbonate facies characterisation. Teamwork focussed on continual improvement adjustments to acquisition, processing and interpretation have enabled explorers working the EPB to focus on differentiating carbonate deposition geometries and associated facies. This realisation that not all carbonate build-ups are equal has increased the need to discriminate Antelope from Triceratops and Elk-Raptor type build-ups. Better seismic imaging provides the required exploration de-risking tool.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020