The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry:
Maturing Through Exploration and Production

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New Insights into the PNG Margin from the Roho Non-Exclusive Airborne Gravity Gradiometer and Magnetic Survey

Abstract

The region of Papua New Guinea has experienced a complex tectonic history and encompasses the full spectrum of tectonic process, including continental rifting, terrane accretion, ophiolite obduction, ultrahigh-pressure rock exhumation and plate collision. In particular, the impact of the collision of the northward moving Australian craton with island arc terranes in the Miocene has developed a complex fold and thrust belt with both deep and thin-skinned thrusts that are responsible for the present-day PNG terrain. Active exploration in the region has been encouraged by significant oil and gas reserves identified along this fascinating margin as well as a number of mineral discoveries. Fold and thrust belts often provide excellent outcrop data (e.g. European Alps) to map and image structures, however the dense vegetation of the PNG region hampers effective field mapping. In addition, onshore structural mapping and ‘shallow’ drilling typically yields geological insights into the relatively young fold and thrust belt, but little information into the underlying pre-compressional tectonic architecture. Structural inheritance has long been recognized as an important influence in how margins develop, yet the understanding of the Mesozoic rift structures underlying the fold and thrust belt within PNG remains in its infancy. A number of studies have recognized significant structural variation along the margin of PNG, however studies are typically based on widely spaced 2D and sparse 3D seismic coverage, poor quality onshore seismic data and a selection of wells that extend to Mesozoic depths. The Roho Non-Exclusive Airborne Gravity Gradiometry and Magnetic Survey was acquired by Searcher Seismic to provide additional insight into the structural evolution of the margin of the Eastern PNG Gulf, particularly in nearshore regions where seismic surveys are not possible. The survey was designed to enable imaging of shallow features, as well as deep basement structures typically not imaged on seismic data sets. The Roho Airborne Survey collected magnetic, gravity (gD) and gravity gradiometry (GDD) data from the FALCON® AGG system with data from the sGrav (strap-down gravimeter) also utilized for improving the long-wavelength gD data. In total 52,684 line kms of data were acquired with flight line spacings of 1km (northern portion) and 2km (southern portion) – covering a total of 60,600 km2 of onshore and offshore PNG. A structural interpretation of the data was undertaken, encompassing a number of existing 2D seismic lines, well data, onshore geological mapping and existing structural models. Selected 2.5D lines were modelled to test depth responses to the airborne geophysical data and to suggest shallow and/or Mesozoic/basement architecture to account for the data anomalies. In addition to providing insights into deep structures, the interpretation identified volcanics, carbonate complexes, and/or structural highs of higher density material. Delineation of Late Miocene to Pleistocene foreland basin sequences and Pliocene graben and pull apart basins in the east of the survey area was also possible. In shallower areas, such as on the Fly Platform, high frequency, near surface magnetic responses are suggestive of volcanogenic sediments shedding from the Recent Pleistocene-Pliocene volcanics. Broader, higher intensity magnetic responses are interpreted to reflect lithology changes in the basement. Together with developing a structural interpretation along the entire PNG margin, the data were also used to estimate late Cenozoic sedimentary thickening, identification of Mesozoic sub-basins and mapping of possible carbonate reef buildups. With the broad variation in tectonic domains recognized along the PNG margin, including extensional and compressional domains, a number of opportunities for working petroleum systems are present which highlights the possibility of additional discoveries within this under- explored region.