Investigation of biomarkers in source rocks and crude oils of the Gippsland Basin, offshore Australia

Gippsland Basin is the largest hydrocarbon-bearing province in Australia, and has continuously contributed to the Australian energy market since the 1960s. Geochemical and geological research on the petroleum system of the Gippsland Basin has gained much attention, but few systematic biomarker studies on its source rock units have been publicly reported. This thesis examines basic geochemical characteristics of the Upper Cretaceous to Eocene Latrobe Group source rocks and three oil families from the Gippsland Basin. Two new geochemical parameters to evaluate the source of organic matter have been developed, and some new potential biomarkers are proposed for aiding oil-oil and oil-source correlations in the basin.

The 48 rock samples examined include shales, coaly shales and coals deriving from the Nothofagites asperus (N. asperus), Proteacidites asperopolus (P. asperopolus), Malvacipollis diversus (M. diversus), Lygistepollenites balmei (L. balmei), Forcipites longus (F. longus), and Tricolporites lilliei (T. lilliei) palynological biozones (grouped by increasing age). These source rocks are immature to mature, and were all deposited under sub-oxic to oxic conditions in fresh water with low salinity, possibly in a lacustrine depositional environment. Their organic matter inputs are from plankton/algae and higher land plants, and are dominated by organic matter with a terrigenous origin. Similar to the source rocks, the three oil families analysed (27 normal crude oils and 2 biodegraded oils) are derived from a similar depositional environment (oxidising and fresh water) with a mixed source from both angiosperms and gymnosperms. The GA1 oils are in the thermally mature stage and have a narrow maturity range, while the GA2 and GB oils are mixtures of immature/less mature and mature oils, and have a relatively wider maturity range. Based on source-related biomarkers and polycyclic aromatic hydrocarbons (e.g. oleanane-/ursane-type triterpanes), it is inferred that higher-plant derived polycyclic aromatic hydrocarbons are potential indicators for oil-oil correlation when regular biomarkers are not efficient.

Vegetation in the Upper Cretaceous was dominated by Podocarpaceae (Lagarostrobus, Dacrydium, Dacrycarpus, Podocarpus, Microcachrys), Araucariaceae (e.g. Agathis and Araucaria), Cupressaceae, several Proteaceae (e.g. Macadamia, Knightia and Grevillea), with minor ancestral Nothofagus, flex (Aquifoliaceae ), Gunnera, Epacridaceae and probable Trimeniaceae. Angiosperms ( e.g. Euphorbiaceae, Proteaceae, Casuarinaceae, and Araucariaceae ) gradually increase in both diversity of species and absolute abundance from the Late Cretaceous to the Eocene, including the first appearance of Nypa and Tiliaceae in the M. diversus biozone. Analyses of the oleanane index, higher plant parameter, higher plant index, higher plant fingerprint, and various aliphatic and aromatic angiosperm/gymnosperm indices indicate that the Upper Cretaceous-Eocene climate within the Gippsland Basin varied from warm (T. lilliei biozone) - cool (upper F. longus biozone) - warm (L. balmei biozone) - hot (M. diversus and P. asperopolus biozones) - warm and cool (N. asperus biozone). These changes in inferred climate are consistent with global climate changes indicated by the southern hemisphere deep-sea oxygen isotope record. Compared to the Taranaki Basin, a same-age analogue in New Zealand, the Gippsland Basin in the Late Cretaceous seems to have had a smaller gymnosperm-dominated rainforest and a less cool climate, whereas the basins had a similar Paleogene climate.