Gas chimney mapping in the Northern Terrace, Gippsland Basin, Australia
Nguyen, Quynh Thi Tu
Master of Science Degree 2009
University of Adelaide
Gippsland Basin is a world-class oil and gas province with a number of large to giant oil and gas fields producing hydrocarbons since 1969. The petroleum systems are well-understood because the Gippsland Basin is well-explored. Recently, it has been determined that the Gippsland Basin has the potential for CO2 containment and storage. Therefore, an understanding of seals, reservoirs and migration in the basin is crucial for Carbon Capture and Storage (CCS) projects.
The structure of Gippsland Basin consists of three parts: the Central Deep, the northern and southern parts. The study was conducted in the area on the Northern Terrace which is separated to Central Deep by the Rosedale Fault System and to the Northern Platform by the Lake Wellington Fault System.
Studies have been conducted to evaluate the capacity of the Lakes Entrance Formation regional seal and migration pathways of hydrocarbon within the Gippsland Basin. Previous sniffer studies carried out by AGSO in 1989 and 1991 suggested a likelihood of gas leaking near Flathead-1 on the Northern Terrace of the Gippsland Basin. Moreover, a study done by GeoScience Victoria indicates a decreasing thickness of the seal towards the Northern Platform.
Since gas chimneys indicate a gas migration pathway, their presence implies a risk of gas leakage in this area. For that reason, the gas chimney mapping was processed in order to map out the locations of gas chimneys within this area. The process used a combination of seismic attributes and neural networks. Seismic attributes are calculated at representative samples to train the neural network. The outcome of the trained neural network is affected by the selection of seismic attributes and the number of input data. This outcome is tested by increasing the number of input data and the best result is chosen to apply for the whole data volume.
On a seismic section, gas chimneys appear as vertical chaotic disturbances. However, this vertical noisy zone could be caused by other reasons. Therefore, the gas chimney map produced by the neural network training process needed to be validated. Validation of gas chimneys can be done by using direct hydrocarbon indicators (DHIs), sniffer data or displaying instantaneous amplitude on a horizontal slice.
The result of the neural network training process pointed out the boundary of gas chimneys which is consistent with hydrocarbon anomalies as indicated by the sniffer study done by AGSO. The mapped gas chimney in the study area highlighted a vertical migration pathway of gas along the Top Latrobe Unconformity into the overlying Lakes Entrance Formation.
A study of source rocks for gas generation indicates that gases in the Northern Terrace of the Gippsland Basin were probably generated from rocks of the Strzelecki Group. This result together with the mapped gas chimneys, verified that gas has been migrating vertically from the Strzelecki source rocks. This suggested that the Rosedale Fault System is a seal for hydrocarbons and CO2.
On the other hand, well completion reports from eight vertical wells in the study area indicate that the seal is able to hold a significant column of hydrocarbon. This means the Lakes Entrance Formation within this area is not breached and it actually can seal hydrocarbon. It also provided evidence for a failure of fault seal in the area near Flathead-1 in the Northern Terrace of Gippsland Basin.