The First Oil Seep Discovery in South East Png and its Implications to Torres Basin Petroleum System Analysis.
The Torres Basin is located within the northern Papuan Plateau which constitutes the south east peninsula of Papua New Guinea. It is a frontier basin, in respect that there are no deep exploration wells in the region. In fact there has not been any record of any petroleum or stratigraphic drilling, shallow or deep, for hydrocarbon, in sedimentary rocks of the region. The deepest available cuttings are from road works or village wells. The first confirmed oil seep discovery was in 2017, and the implications to the Torres Basin petroleum system are discussed here. This seep was in the foot hills of the Aure Thrust Belt, specifically in the very front of the Bogoro Shear Zone where the major over-thrust faults come to the surface. The results of GCMSMS-QQQ analysis confirm the recovered sample to be very light raw crude Paleogene oil of conventional petrogenic origin. The geochemistry of the oil does not place it among any previously identified fluid family and it is not from either (a) an upper Jurassic source rock as for many fold belt oils and condensates (b) a Neogene marine source rock as for some Gulf of Papua condensates. Assuming average heat flows and geothermal gradients the oil is from a depth of about 4 kilometres as the oil has experienced no significant liquids to gas cracking. The characteristics of the geology of the seep location as well as the oil characteristics modify the understanding of the petroleum systems in the wider region. The oil is from a clastic (clay rich) containing a mixture of marine algal and land-plant organic matter. The likely depositional environment is a paralic-marine deltaic setting with transported land-plant organic matter deposited under open-water (sub-oxic to oxic) marine conditions. These environmental factors point to the upper geological section of northern margin of the Papuan Plateau as still being the north facing post rift section of the Tethyan rift margin, with the preserved and undisturbed syn-rift section below. Note the seep location is over 200 km north of the Coral Sea which opened around a pole of rotation in the Late Cretaceous-Palaeocene, with a consequent lack of thermal uplift at the margins (much like the Woodlark Basin currently). As a consequent the Paleogene rocks are not a part of the Coral Sea Basin post rift sequence. The Paleogene sequence has eroded in part by the Oligocene Unconformity, as seen from seismic data south of the area. The remaining section has been gradually buried since the late Oligocene when the East Papuan Composite Terrane (EPCT) collided with the Papua Plateau. There has been significant mountain building and crustal thickening due to the over thrust of oceanic crust and ophiolite. The southern Papuan Plateau is now under deep water in a foreland basin setting. In summary the oil age as pre-EPCT collision age and post Tethyan Rift indicates the entire Jurassic Rift and Cretaceous post rift section is probably preserved and now covered by younger rocks and rocks of the Owen Stanley Obduction Complex. The oil seep is encouraging for a Paleogene oil petroleum systems and basin modelling indicates there would have been a recent and current early phase oil generation from the Tethyan rift system and a late gas generation with accumulating depth of burial.
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