The Wheatstone gas discovery is located 110 km northnorthwest of Barrow Island in the Dampier sub-basin, northwest Australia. The field comprises two non-conformable and interconnected reservoir units consisting of shallow dipping, Triassic fluvial sandstones, and an overlying Tithonian transgressive sand. These units are partially separated by Late Triassic to Early Jurassic sediments.The Tithonian sand reservoir is instrumental in hydrocarbon accumulation in the Greater Gorgon Area due to its unconformable connectivity to underlying Triassic reservoirs, and its ability to act as a thief zone. It is therefore a significant component in hydrocarbon entrapment. This paper discusses the transgressive Tithonian sand palaeogeography, and environment of deposition as a predictive tool of reservoir risk assessment and play fairway geography.This study compares and contrasts the rock properties of the latest Triassic sequences and proposes the palaeogeographic make up within the basin. A review of the reservoir properties has identified differences between units which may impact the formation evaluation approach used. The thinly bedded character of the latest Triassic sequences necessitated the use of a non-traditional formation evaluation model that has improved the accuracy of the wireline predicted reservoir properties. Acquisition of probe permeability data through conventionally cored intervals identified the limitations associated with standard core plug sampling procedures, and ensured better net reservoir definition through the sequence.
Regional 2D seismic lines acquired in 2013 along the eastern edge of the Toko Syncline in Queensland have imaged a complexly faulted, northwest–southeast trending graben below subcropping Cambrian carbonates. At its deepest, this depocenter contains some 3,500 m of sediments that are inferred to be of Neoproterozoic age, if not older at depth. Surface outcrops of this succession are mapped as the Sylvester Sandstone. Shallow core holes confirm that the uppermost ~1,000 m of sediments are of fluvioglacial, glacio lacustrine and glacial origin, overlain by a thin carbonate of Lower Cambrian age based on the occurrence of archaeocyathids. Unconformably underlying this succession is an internally reflective sedimentary section that is up to 2,500 m which has not been drilled to date. Although the localised development of Neoproterozoic depocenters below Cambrian carbonates in the Toko Syncline has been inferred previously based on vintage seismic and gravity data, such features were not imaged as clearly as on the newly acquired seismic. Inferred sediments in this faulted graben may be correlatives of the Neoproterozoic succession in the Amadeus Basin and could be prospective for hydrocarbons as well as ore deposits.
Hydrothermal dolomitisation of carbonates can create zones of favourable porosity and permeability in otherwise tight carbonate successions. In North America, a number of fields produce oil and gas from such reservoirs that developed along and adjacent to pre-existing fault zones acting as loci for hydrothermal fluid flow. Seismic data across these North American fields are characterised by linear zones of disturbance evident along fault zones where porosity development has occurred. Similar zones of disturbance have been observed on newly acquired seismic over the Toko Syncline in Queensland. Here, these zones extend through a thick Cambro-Ordovician carbonate succession that includes platform carbonates of the Thorntonia Limestone, overlying deeper water deposits of the Arthur Creek Formation that are organic rich and hydrocarbon generative at their base, and also in younger shallow water carbonates of the Arrinthrunga, Ninmaroo, Kelly Creek and Coolibah formations. If these zones of disturbance on seismic also reflect the development of hydrothermal dolomite reservoirs, they provide a new exploration target in the southern Georgina Basin.
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