The Geology and Structural Style of the Juha Gas Field, Papua New Guinea

Abstract

The Juha Anticline in the jungle-covered highlands of Papua New Guinea was drilled by three crestal wells in the 1980’s and discovered gas-condensate in a Lower Cretaceous clean quartz sandstone reservoir. The Juha-4 and Juha-5 wells drilled in 2007 further delineated the structure and defined a separate North Juha compartment. The Juha structure is 25 km long and up to 8 km wide and is traversed by a number of seismic lines, some of which are of moderate to good quality allowing the structure to be interpreted. Unlike most structures in PNG the seismic lines reveal the nature of the overlying Pliocene-Pleistocene sediment which help to define the depth of burial and timing of deformation. The wells and seismic data suggest that the Lower Cretaceous sandstone reservoir was buried by 1.5 km of Cretaceous shale, the regional seal, and 1.5 km of Miocene limestone as well as more than 1.6 km of Pliocene-Pleistocene sediment prior to uplift and erosion. To constrain the timing and style of extensional and compressional deformation, 25 2D seismic lines were interpreted aided by forward modelling of the structure. The seismic interpretation revealed basement-involved structures that were predominantly influenced by two major events, rifting in the Triassic-Jurassic and compression in the late Pliocene-Pleistocene. The deep structure remains uncertain, but gravity data indicate a very deep underlying graben a concept that has recently been investigated and validated by 3D analogue modelling. A key seismic section indicates inverted basement faults beneath Juha flattening upwards into a detachment horizon creating triangle zones in the Cretaceous mudstones such that the overlying Miocene Limestone in part deforms independently. The Juha Anticline is part of the PNGLNG project operated by ExxonMobil which commenced production in 2014, 32 years after drilling of the Juha-1 discovery.