Unravelling PNG’s Frontier Basins: New Structural-stratigraphic Insights in Analogous Subduction Margin Settings

Abstract

With over 250 wells drilled, >5600 MMBOE gas and >1200 MMBOE liquids produced to date, the Papuan fold/thrust belt has proven to be a ‘world class’ hydrocarbon province. However, despite an almost 100-year exploration history PNG remains as a frontier hydrocarbon destination. The onshore Papuan basin is currently the only producing region, and a third of PNG’s gas reserves are within the one onshore Hides field. Further, only sparse legacy 2D and 3D seismic data exists both onshore and offshore, whilst only 4 exploration wells have been drilled offshore in the last 20 years. With numerous obstacles in place inhibiting conventional exploration methods, the requirement to integrate multiple exploration techniques and technologies is critical to identify more subtle closures within proven intervals onshore and de-risk new plays across PNG’s basins. One of the challenges in unlocking the hydrocarbon potential across PNG will be to unravel the complex interaction between structural elements and sediment distribution. Given the style and distribution of sediments in active subduction settings being fundamentally influenced by the underlying structural style, understanding and integrating multiple data types to constrain the structural setting is critical. Onshore to the south-west of the producing Papuan fold/thrust belt exists the Fly Platform, composed of Cretaceous to recent sediments overlying rotated fault blocks composed of crystalline basement. Continuing a southeastward transect offshore, along strike to the Central Highlands and Owen-Stanley Range, from the Gulf of Papua through to the Coral Sea, are a series of ridges and troughs. These basins exhibit both extensional and compressional fault systems with dominantly Mesozoic and Tertiary fill (subsequently intruded by a series of igneous complexes) onlapping basement highs. With limited penetrations through this basin fill, little is known about the spatial and vertical variation in sediment type, or timing and source of sediment supply during different tectonic phases. Further, given the limitations of legacy seismic imaging, the depth and fill of sub-basins along this transect are poorly defined, and as such studies investigating petroleum systems have been limited, given heat flow assumptions etc. Similar complexities in the interaction between structural elements and sediment distribution are being investigated across the Hikurangi Subduction Margin and Taranaki Basin, New Zealand. This project integrates onshore fieldwork, offshore seismic data and processed gravity and magnetic data to understand the structural evolution across the region and the impact upon deposition of pre-, syn- and post-kinematic sedimentation. As with onshore PNG, multiple onshore oil and gas seeps occur in the subduction wedge of eastern North Island, NZ, indicating at least one active petroleum system. Only two wells have been drilled offshore, making this complex tectonic region also vastly underexplored. As those wells were drilled on structural highs in the near-shore region, little can be said regarding the type of sediments and their architecture in the prospective offshore sub-basins. As a result, characterization of the outcropping syn-subduction strata was conducted to document lithofacies and map seismic-scale architectural elements. This, integrated with >15,500 km² of newly acquired 3D and >5000 km of 2D seismic data offshore, is providing a proxy calibration for sediments and stratigraphic architecture and, as a result, generating new insights into the highly prospective offshore basins. This sheds light on seismic-scale architectural element distribution within the trench-slope sub-basins, their reservoir properties, and trap geometries. Additionally, this study investigated the timings and volumes of bypass phases in the outcropping sub-basins, which may lie on the same fairway as prospective offshore sub-basins. Further, comparing the Pliocene to recent Gulf of Papua and Taranaki Basin sedimentation rates (with >3km of siliciclastics deposited within the last 5 Myr. in both depocentres) provides analogous insights into the interaction of sediment deposition and tectonic weathered hinterland and implications for regional scale basin sedimentation and starvation patterns- with obvious implications for late maturation of source rocks, remigration of existing accumulations etc. Herein we present and illustrate how integrating multiple seismic and non-seismic exploration methods, proved critical in de-risking exploration concerns in analogous settings, which can be used to help guide tailored exploration campaigns going forward in PNG.

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