Origin of Anomalous Structures and their implications for Hydrocarbon Accumulations on the Exmouth Plateau, Western Australia
Michael E. Gray - 2012
the Honours Degree of Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
The WA-434-P tenement, located on the outer Exmouth Plateau on the North West Shelf of Western Australia, is currently being explored by Woodside Energy Pty Ltd. Woodside, to date, has drilled four wells in the tenement and completed the Claudius 3D seismic survey over the shallowest region of the permit.
While analysing the Claudius survey numerous circular depression features underlain by zones of chaotic reflections were noted. The tops of these anomalous structures occur in Jurassic carbonate sediments just above the Late Triassic Mungaroo Formation. The primary concern of Woodside is that these anomalous structures may indicate paleo gas migration, with implications for the integrity of traps in the Mungaroo Formation and possible migration of gas out of the system. This project characterised these features, with the aim of determining, how they formed and the possible impact on prospectivity in the area.
The anomalous structures at their top appear as circular depressions on the Late Jurassic J50.0 horizon, with an average diameter of 266 metres and an average area of 89, 217 m3. The average depth extent of the anomalous structures, including the chaotic reflector zone is 378 metres. Some of the anomalous structures appear to coalesce, and many form distinct linear patterns possibly associated with minor north northwest/south southeast faulting.
Analyses of anomalous structures suggest that they are not a result of sediment thickness, variation in lithology or hydrothermal venting from intrusives. Of the five possible hypotheses for the formation of the anomalous structures the evidence supported three. The first hypothesis was that the chaotic reflector zones in the seismic section are chimneys and the circular depressions are pockmarks. The second hypothesis was that the circular depressions are sinkholes and that the chaotic reflector zones below are a shadow zone or similar artefact in the data. The third hypothesis was that a combination of the mechanism in both hypothesis one and two have lead to the formation of the structures. Unfortunately, there is insufficient evidence to distinguish between the hypotheses.
The implications for prospectivity depend heavily on the causal mechanism of the anomalous structures. If the causal mechanism is carbonate dissolution or cover and collapse leading to the formation of a sinkhole then the effect on the reservoir and charge is probably negligible, providing that the chaotic reflector zones in the seismic are artefacts. However, if the anomalous structures are the result of gas escape then this could have numerous implications including traps not being full, seals being breached and hydrocarbon lost or found at shallower depths, an indication that the seal is not very effective and cannot hold large columns of hydrocarbon, or that there may be a very rich source rock.