Biomarker signatures of Upper Cretaceous Latrobe Group hydrocarbon source rocks, Gippsland Basin, Australia: distribution and palaeoenvironment significance of aliphatic hydrocarbons

Upper Cretaceous (Maastrichtian and Campanian) hydrocarbon source rocks from the Latrobe Group, Gippsland Basin (Australia) have been analysed using gas chromatography–mass spectrometry (GC–MS) in order to understand their geochemical characteristics and to reconstruct palaeovegetation and palaeoclimate changes. n-Alkanes ranging from C₁₀ to C₃₇ dominate the aliphatic hydrocarbons in the rock extracts. Both the carbon preference index and the odd-to-even predominance values of n-alkanes are higher than 1.0, suggesting an input from terrigenous higher plants. Low wax indices (mostly < 1.0) and a predominance of C₂₉ over C₂₇ steranes corroborate a major contribution from higher plants. High pristane/phytane (Pr/Ph) ratios (>3.0), a low gammacerane index (<0.3) and the Pr/n-C₁₇ versus Ph/n-C₁₈ cross-plot indicate that the Upper Cretaceous hydrocarbon source rocks were probably deposited in an oxidising environment with fresh water. Thermal maturity parameters such as sterane isomerisation ratios, C₃₁ 22S/(22S + 22R) homohopane, C₃₀ αβ/(αβ + βα) hopane and 16(H) β/(α + β) phyllocladane, together with T_max data and the presence of C₂₇ 17β(H)-22,29,30-trisnorhopane (βTm), suggest that most of samples have a relatively low maturity in the early to mid-oil generation window. Angiosperm- and gymnosperm-derived biomarkers, including labdane, pimarane, isopimarane, phyllocladane, abietane, rimuane, ent-beyerane, ent-kaurane, oleanane, 10β(H)-des-A-oleanane, 10β(H)-des-A-lupane, and 10β(H)-des-A-ursane, suggest that palaeovegetation during the Upper Cretaceous was characterised by both gymnosperms (e.g. Araucariaceae and Podocarpaceae) and angiosperms (e.g. Nothofagus), but was dominated by gymnosperms. Compared with samples from the Campanian, there was a relatively lesser contribution from angiosperms during the Maastrichtian. This shows that the palaeoclimate was becoming cooler from the Campanian to the Maastrichtian, which is consistent with previous palynology evidence and sea-surface temperature estimates.