29 th International Meeting on Organic Geochemistry (IMOG) 1–6 September 2019, Gothenburg, Sweden DISEQUILIBRIUM BETWEEN RELATIVE ABUNDANCES OF DIAHOPANES AND DIASTERANES SUGGESTS DIFFERENT FORMATION MECHANISMS S. C. George 1 , S. Baydjanova 1 , L. Jiang 1 , B.J. Manda 1,2 1 Macquarie University, Sydney, Australia 2 Geological Survey Department of Malawi, Zomba, Malawi Diasteranes and 17α(H)-diahopanes are rearranged biomarkers that have been associated with clay-rich, acidic, oxic conditions during early diagenesis, which promote clay catalysis and the biomarker rearrangement reactions (Moldowan et al., 1986, 1991). Additionally, high thermal maturity may enhance the relative abundance of both diasteranes and diahopanes. Certainly, some samples show co-variation of diasteranes and diahopanes, for example the Jurassic- sourced oils in the foldbelt of Papua New Guinea (PNG; Moldowan et al., 1991; George et al., 1997a). Additionally, the 18α(H)-neohopanes (Ts and C29Ts) and other rearranged hopanes including an early eluting series may co-vary with diahopanes (Jiang et al., 2018). Here the occurrence of the co-variation between diasteranes and diahopanes, and especially deviations from it, are assessed for a dataset of oils and source rocks from Australia, PNG and China. There is a clear, positive relationship between Ts/(Ts+Tm) and C29Ts/(C29Ts+C29 αβ hopane) for the sample set (Fig. 1a). Some samples have lower C29Ts/(C29Ts+C29 αβ hopane) ratios than would be expected from their Ts/(Ts+Tm), so fall below the main trend, including some fluid inclusion oils from the Vulcan Sub-basin. There is also a mostly linear relationship between C30*/(C30*+C30 αβ hopane) and Ts/(Ts+Tm), particularly as defined by a set of oils from the Songliao Basin in China (Jiang et al., 2018; Fig. 1b). A set of outcrop samples from the Permian Pebbley Beach and Snapper Point formations in the Sydney Basin have higher diahopane contents than would be predicted from Ts/(Ts+Tm), based on the relationship between these parameters for the other samples. In general, samples tend to group geographically and by age on these cross-plots. The Permian Wandrawandian Siltstone at outcrop in the Sydney Basin contains biomarkers that are dominated by unusually high amounts of diahopanes and other rearranged hopanes (Fig. 1c). One sample (UL2) has 7x more C30 diahopane (C30*) than C30 αβ hopane, one of the highest diahopane contents ever reported. There is large variation in C30*/(C30*+C30 αβ hopane) (0.69–0.87) for the Wandrawandian Siltstone at outcrop. However, diasterane contents of the samples from this formation are moderate (Fig. 1c). Samples from the same Permian formations but from a borehole (DDH01) on the other side of the Sydney Basin near Wingello contain very low diahopane contents, but moderate to high diasterane contents. Unusual characteristics of the Wandrawandian Siltstone include significant slumping and soft sediment deformation (Fig. 1d), and deposition under very cold conditions when Australia was close to the South Pole. The occurrence of the slumps on the continental slope led to significant sediment overturn, which may have resulted in enhanced diagenetic and catalytic rearrangement reactions forming the diahopanes. Alternatively, the slumping and very cold conditions could have led to the sediment hosting an unusual array of microbial life that caused elevated amounts of these compounds. The disequilibrium between the relative abundances of diahopanes and diasteranes suggests different formation mechanisms for these biomarkers, and indeed this is apparent from the lack of correlation between the C30*/(C30*+C30 αβ hopane) and the diasterane/(diasterane+sterane) ratios for the whole dataset (Fig. 1c). One hypothesis is that the catalytic reaction to form diasteranes occurred early during diagenesis, and that this was complete by the time of slumping of the Wandrawandian Siltstone. Future work will test the relationship of clay mineralogy and organic matter distribution on diahopane abundance.