Gareth T. Cooper

The Porgera Transfer Zone (PTZ) is a major crustal and probably lithospheric structure across Papua New Guinea recording >50 km offset of ophiolites and very different patterns of geology and topography on either side. In the Late Jurassic, the PTZ probably separated oceanic crust and thick Jurassic Om shales to the west from a continental promontory to the east. During the Late Miocene to Recent orogenesis, the differential compression of these features is interpreted to have created a dextral strike slip fault across the fold belt with pull-apart basins at sites of fault relays. This facilitated the ascent of intrusions and mineralization at Porgera. The acquisition of high-resolution LIDAR data semi-regionally around the Porgera Gold Mine greatly improved interpretation of the regional geology and particularly the recognition of normal faults. By correlating with sparse dip data and paly-dated samples, it was possible to create stratigraphic sections and interpret structural cross-sections using the LIDAR data. As the area involved strike–slip offsets, it was important to construct sections in multiple orientations in order to interpret the 3D geology. Both dips and fault orientation could be directly inferred from the LIDAR data such that sections could be constructed orthogonally to them. A balanced, restored and forward-modelled cross-section illustrates the interaction between thrust faults and normal faults during compression and that it was synchronous with the development of a pull-apart basin. A semi-regional 3D geological model, which was developed mainly from the LIDAR data, supports the hypothesis of inversion of the thick Om beds to the west before or during compression of the continental promontory to the east resulting in dextral strike–slip offsets across the PTZ. A jog or relay in the faults occurred and caused a pull-apart collapse basin to develop in the area of the Porgera mine. Similar pull-apart graben, or negative flower structures, were detected nearby and may be areas for future exploration.

The Taroom Trough in Queensland is the main synclinal depression and southern extent of the Bowen Basin. Both the Bowen Basin and the overlying Surat Basin have been the focus of Australia’s coal seam gas (CSG) liquefied natural gas (LNG) export industry since 2014, but with CSG production now in decline and with a greater focus on domestic gas, it is timely to review the potential of new plays in this region. Although exploration and development in the Taroom Trough over the past decade have focussed on shallow Permian and Jurassic CSG resources, the early hydrocarbon potential of the trough was realised through conventional discoveries. Regardless of the reservoir, all conventional hydrocarbons in the trough have been typed to the same prolific Permian coals. These source rocks produce enormous volumes of oil and gas, yet only a small percentage of those volumes have been trapped in known conventional structures. This implies that a significant volume of oil and gas could remain trapped in tight reservoirs, including Fractured Thermally Mature Coals. Elixir Energy Limited plans to commence a drilling program in 2023 in ATP2044 (Grandis Gas Project) to unlock the gas potential of the Taroom Trough. The project will investigate the potential of Permian coals and sandstones at >3700 m depth using new understandings developed from recent studies into the critical role of elastic rock properties and stress anisotropy on coal fracture growth. This paper describes the opportunity that Elixir has identified and details the legacy technical work in the area and new research and ideas which have since evolved.