The Exotica Carbonate Debrite Deposit: A Potential Source for Thermogenic Submarine Hydrocarbon Seeps Offshore New Ireland, New Ireland Basin, PNG
The New Ireland Basin (NIB) is a 900 km x 180 km frontier offshore basin in northern Papua New Guinea (PNG). Research cruises in the 1990’s reported submarine hydrocarbon seeps of thermogenic origin emanating from scarp slopes and volcanic mounds in the offshore deepwater basin (Schmidt et al. 2002). The source of the seeps was tested in 2017 by long offset, deep tow 2D seismic during the Solomon Sea 2017 survey (Searcher Seismic, 2019). The seismic data reveal a new sub-basin (Lihir sub-basin) with 8 km of sediment overlying a basement of Eocene oceanic crust. The seafloor seeps are located above a low amplitude anticlinal structure with an apparent wavelength on the order of 6 km. Sedimentary layering in the deeper portion of the anticlinal hinge is obscured by attenuation of the seismic signal, however the limbs of the anticline show a seismic interval with discontinuous and contorted reflections consistent with a debrite deposit. Based on a limited number of lines, this seismic unit, named the Exotica Formation, can be demarcated at 45 km long, 32 km wide, 100-400 m thick and buried to a depth of 2 km. The source of the Exotica debris apron can be traced to a graben structure on New Ireland bounded by Nabuto Bay to the north, the Ramat Fault to the west and the Matakan Fault to the east. Here a 160 sq km section of Miocene Lelet Limestone (250-400 m thick) is missing from the rock strata in the evacuation area. The vertical relief from head-to-toe of slope is 2 km. Pliocene samples of bituminous carbonate breccia, collected as xenoliths ejected onto the basin seafloor by kimberlite-type eruptions, may be representative of the Exotica Formation. A model to explain the combined observational evidence is that subduction-related earthquake activity triggered a catastrophic collapse of the New Ireland coastline, transporting substantial carbonate platform sediments downslope into deepwater to form a toe-of-slope carbonate megabreccia debris flow deposit. The seep emissions and bituminous samples suggest that portions of the Exotica Formation may be charged with hydrocarbons. Potential global analogues include the Tamabra Formation in the Poza Rica field in the Gulf of Mexico (Loucks et al., 2011), the K/T carbonate breccias of the Cantarell field in the Gulf of Mexico (Grajales-Nishimura et al., 2000) and the carbonate debris reservoir of the Ruby field in the South Makassar Basin of Indonesia (Pireno et al., 2015).
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