The Unconventional Hydrocarbon Potential of the Otway Basin
David W. Lemon - 2012
Honours Degree of Bachelor of Science (Petroleum Geology & Geophysics)
Australian School of Petroleum
The University of Adelaide
Unconventional hydrocarbons are becoming increasingly important in meeting global energy demands. Since shale gas plays in the United States of America have shown to be economical the investigation of possible shale plays and other unconventional plays has spread across the world. Producing conventional hydrocarbons basins are being re-examined in light of these new plays. This study re-examines the onshore Otway Basin in Western Victoria and Eastern South Australia in regards to unconventional hydrocarbons plays. The plays included in the investigation were shale gas, coal seam gas and continuous tight gas.
The project started at a basin wide scale reviewing the established literature from conventional studies to identify possible unconventional plays in specific formations in relation to known source intervals. From this information broad fairway zones were identified and potential plays examined by comparing historical drilling data to a set a favourable log characteristics. Gas in place volumes were calculated based upon a seismic interpretation conducted in the South Australian section of the Penola Trough.
Potential shale gas plays were identified in the Laira and Casterton Formations within the onshore troughs. High gas signatures were observed within the Laira Shale interval in Glenaire 1 associated with coincident conducive log responses. Gas production and increased total organic carbon results from the Casterton Formation in Digby 1 and conventional play source analysis indicated a potential shale gas play.
A key risk associated with both plays is the total organic carbon content of the formations. Modelled over limited intersection a low net-to-gross value and average total organic carbon were calculated. Thermal maturity is a limiting factor for the Laira shale gas play which severely reduces the formation volume below the derived mature depth. Shale composition was also tested using limited cuttings samples and suggested an average 40-55% clay composition. Gas in place values of 2.32 TcF for the Laira Formation and 23.8 TcF for the Casterton Formation shale gas plays were calculated within the seismic interpretation zone in the Penola Trough.
A potential continuous tight gas play was also identified in the base Eumeralla Formation associated with multiple thin coal seams. Immature in the seismic interpretation area the key risks were identified as the relative net to gross of potential reservoir, source and sealing formation. The reservoir properties of potential reservoir were not identified and remain a risk factor.
A potential coal seam gas play was discounted in the Eastern Torquay sub basin due to a lack of thermal maturity, coal thickness and aquifer isolation.