History of Antelope Discovery

Abstract

The Antelope discovery in the Eastern Papuan Basin (EPB) changed the understanding of the region that dated back to the pioneering exploration work by the Australasian Petroleum Company (APC) in the 1950’s and 1960’s. The discovery was less than 20km east of Puri 1, drilled by APC in 1959, the first oil discovered in PNG. In hindsight, the nature of limestones penetrated in Puri 1 have had a disproportionate influence on exploration of the region. The penetration of reefal limestones and dolomites by Antelope 1, so close to this original well, not only transformed the resource potential of the Antelope gas field, it challenged prevailing wisdom. Changing concepts of the palaeogeography, depositional environments and reservoir potential of carbonates in the EPB transforming its prospectivity. The discovery at Antelope was the culmination of, and testament to, decades of exploration by small petroleum companies who toiled, and persevered, in an unfashionable frontier basin essentially written off by majors. InterOil, who discovered Antelope had a business model framed on monetisation of discoveries by lowering the commercial threshold by utilising trucking and barging for a domestic market at the refinery built in Port Moresby. With a modest capitalisation and large acreage position exploration costs needed to be kept low - utilising a Western Papuan Fold Belt (WFB) “template” – based on surface geological mapping. The results of the early work were promising. Geochemical analysis of bitumen residues in core of late Cretaceous quartz sandstones with the Subu stratigraphic wells and residual live oil from the Moose wells defining a potentially prolific Late Cretaceous to Paleogene petroleum system distinct from the WFB Jurassic Family A system. However, results at Sterling Mustang 1, the next target defined by surface geology, were more complex. Highlighting rapid lateral thickness changes in the Mid. to Late Miocene Aure and Orubadi clastics. This observation echoed pioneering work by Stanley (1941), who after a decade of mapping in the EPB elegantly captured the future frustrations of many workers… “field geologists”….”are wandering amidst a veritable forest of columnar sections and each new survey only results in the forest becoming thicker.”….”The temptation to make paleogeographic studies using scant (and often misinterpreted) facts at present available, should be resisted”….”until the time arrives when the reasoning can step by step be based on firmly established observations, and stout bridges have been built across the yawning chasms, which at present daunt all..” By comparison with work by APC, little new detailed field mapping has been carried out. However modern biostratigraphy on key wells and outcrops has refined and improved the accuracy and confidence of correlations. More work remains. Seismic imaging has improved since the first reflection seismic survey by APC over the Puri Anticline trend in 1959. In 2004 Puri-Eri-Elk Seismic line (PEES) linking the Puri trend with the Moose wells identified the “Elk” thrust sheet isolated from the Puri Anticline. The line also revealed intense and ductile deformation mapped at surface was detached from the reservoir level - the WFB “template” was not applicable and surface geology alone could not identify traps. An exploration methodology tailored to the EPB needed to be defined - seismic would need to play a key role. However, the caveat being seismic in the jungles of PNG is expensive and had to be diligently applied. A low-cost reconnaissance and regional scale exploration tool needed to be found that could define the basin framework and architecture to lay the foundations for the “stout bridges”, yearned for by Stanley, and identify specific targets for prospect scale investigation. Potential field data, airborne gravity specifically, proved to be this tool and the 2005 Sander Geophysics Airborne Survey over the “Puri” area became the “Rosetta Stone” which unlocked the EPB. After the Elk discovery, it allowed the design of optimised seismic programs, in the initial case targeting the much larger Antelope gravity anomaly. The focussed program kept costs within InterOil’s budget, and the resulting 2007 Elk Trend Seismic Survey identified a potential reefal objective within the Antelope closure and an attic to the initial discovery at the Elk 4 well. The Antelope 1 well targeted and subsequently penetrated and confirmed reefal reservoir.

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