Geology
Typical geological cross section – Gunnedah Basin, Bohena Trough Area
Click here to enlarge map
Source: ESG
The CSG resources being explored and developed within PEL 238 lie in the Permo-Triassic aged Gunnedah Basin, which connects the Sydney Basin to the south and the Surat/Bowen Basin to the north.
Structurally, the Gunnedah Basin can be divided by the north-north-west trending Boggabri Ridge into the Mullaley Sub-basin to the west and the Maules Creek Sub-basin to the east. The most important structural element within PEL 238 is the Bohena Trough, covering an area of approximately 3,500 km2. It is bounded by the Rocky Glen Ridge to the west and the Boggabri Ridge to the east, the Narrabri High to the north and the Walla Walla Ridge to the south. This trough contains an active petroleum system, with conventional gas discovered at Bohena Number One, Wilga Park Number One and in the Coonarah gas field, from which ESG has been feeding gas to the Wilga Park Power Station since 2004.
The Bohena Trough contains two well developed coal measure sequences that are the main focus of ESG’s CSG exploration and development programme. The Late Permian aged Black Jack coal measure contains several minor coal seams in its upper sequence, and one approximately 6-10 metres thick, laterally extensive coal known as the Hoskissons Seam that lies at a depth of less than 700 metres. The Early Permian aged Maules Creek coal measure contains a 2-5 metre thick upper coal seam and the primary CSG target in the lower part of the sequence known as the Bohena Seam that is laterally extensive, up to 22 metres thick, and lies at depths of 600-1,200 metres.
The Bohena Seam comprises mostly dull coal that has unusually high permeability for its depth and coal type. This is due to the coal’s hardness and strength and the prevalence of open sub-vertical fractures that are generally confined to coal bands. The seam is commonly over-saturated with gas that comprises a high percentage of methane. There are, however, localised areas where high levels of carbon dioxide have been encountered.
In some parts of the Bohena Trough, Tertiary aged igneous intrusions have moved up through the Permo-Triassic sediments as dykes and predominantly formed as sills near the base of the Triassic sequence. Although difficult to identify in seismic, the igneous dykes are believed to be associated with faults that connect economic basement with the overlying sedimentary sequence, and are suspected to be one cause of localised carbon dioxide accumulations in the coals.
Follow the links to view our Gas Reserves and Targets page.
