VIMP Report 31 - Early Cretaceous Source Rock Evaluation for Oil and Gas Exploration, Victorian Otway Basin
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Product description:Download The downloadable version of this report is supplied in PDF format. Executive Summary Preliminary analyses (Mehin & Link, 1995) of Early Cretaceous source rocks in the Otway Basin had shown that thick, mature, organic-rich source rocks containing gas- and oil-prone macerals exist to the northwest of Portland, in the Tyrendarra Embayment and around the Port Campbell region. (Note that sampling opportunities for source rock evaluation are geographically and stratigraphically limited because only a small portion of the onshore Victorian Otway Basin is deeply drilled). Current studies are revealing that: Rift valley development progressed at different and uneven rates across Victoria. The basin subsided at 70 to 80 metres/million years during the period of 140 to 105 Ma. Subsidence continued after this time at 20 to 25 metres/million years with brief periods of non-sedimentation and/or erosion at around 65 Ma in the Portland region and 35Ma in the Port Campbell region. In the Ross Creek-1 area, there appear to have been two short periods of non-deposition between 112 and 110 Ma. Biomarker analyses indicate that gas-prone macerals are the most abundant and wide-spread, and that oxic, open plain and shallow lacustrine environment conditions prevailed over most of the northern areas. In contrast, deeper water reducing conditions prevailed in the southern and north-western marginal areas. Hydrocarbon progenitor materials are predominantly aromatic/asphaltic, but in some locations, such as in the Greenslopes-1 and Pretty Hill-1 areas, the organic contents contain more naphthenic and paraffinic compounds. Hydrocarbon generative potential ranges from poor to excellent, and liquid hydrocarbons are present in some of the deep shales, especially in the Casterton-1, Lindon-1 and Woolsthorpe-1 areas. Maximum heatflow occurred about "break-up time" between 105 and 90 Ma, coinciding with the first event of oil generation at the eastern and western parts of the basin. Two additional pulses of oil generation (88 to 85 Ma, and 65 to 50 Ma) occurred in the eastern part. Some of the hydrocarbons that were generated and migrated, before 65 Ma in the Portland region and 35 Ma in the Port Campbell region, were probably pooled in existing stratigraphic traps. Most structural traps were formed during the Palaeocene (~65Ma) uplift event in the Portland region, and during the Oligocene (35Ma) uplift event in the Port Campbell region. These structural traps, particularly those in the Port Campbell Embayment, have been loaded by the continuing oil/gas generation and expulsion from the Aptian-Albian source rock. In spite of the lack of state-wide sampling opportunities, analytical results suggest that the most prospective areas for oil and gas (so far as close proximity of reservoir/seal rocks to thick, rich, mature source rock is concerned) are within the Casterton and Lindon to Pine Lodge regions, in the Portland/Tyrendarra Embayment, and between Peterborough and Port Campbell along and offshore from the present Victorian coastline. Bibliographic reference Mehin, K. & Link, A.G., 1996. Early Cretaceous source rock evaluations for oil and gas exploration, Victorian Otway Basin. Victorian Initiative for Minerals and Petroleum Report 31. Department of Natural Resources and Environment. | ||||||
