Basin scale evaluation: Organic geochemical characteristics of northern Perth Basin (Early Triassic)

Summary Biomarker analysis has been conducted on rock samples of the Late Permian to Early Triassic Kockatea Shale containing microbial mats in the northern Perth Basin. The northern wells of the northern Perth Basin exhibit darker shale horizons and contain microbial mats. These horizons contain abundant carotenoid biomarkers attributed to PZE and microbial mats. Such biomarkers are however, not identified in the southern wells of the northern Perth basin which are deposited under more oxic conditions in a tidal regime. In the northern wells, there are fluctuations of PZE conditions intruded by oxic conditions, caused by fluctuations of the chemocline. Elevated 2α-methylhopanes index and higher renieratane and renierapurpurane/isorenieratane ratio are identified in the microbial mats formed under oxic conditions. In addition, the higher proportion of both okenane against chlorobactane and isorenieratane and C33 n-alkylcyclohexane ratio (n-C33 ACH ratio) are identified in the microbial mat facies. The combination of organic geochemistry (biomarker analysis) and geology (core observation, well analysis and seismic interpretation) show a strong relationship between basin morphology, paleogeography and paleoenvironmental conditions. The southern wells experienced higher influence of fluvial input (oxic conditions). The northern wells, which are far north of the fluvial input show evidence for PZE conditions.