Abstract:

The argillutite with significant marine planktonic debris which mainly deposited in restricted sea during the continental margin rifting stage is likely the potential important oil source rock of the Bonaparte basin, north-western Australia. Comprehensive analyses on the tectonic geology settings, depositional filling characteristics and organic geochemistry characteristics showed that: The restricted sea shale developed during the decline rifting stage of the continental margin rifting stage in Bonaparte basin. Not only did the episodic rifting of the basin and differential subsidence of contemporaneous faults provide the high accommodation space and the undercompensation depositional environment, but also formed the depositional pattern of “big sag surrounding by little uplifts” which is more benefit for the development of relative-permanent semi-enclosed restricted sea and the propagation of marine planktonic algae. The provenance systems were consisted of the fan delta and the submarine fan, accompanying the significant terrigenous organic matter input to keep the balance of the organic matter supplying with the undercompensation depositional situation. The restricted sea shale deposited in slight oxidation marine depositional environment with large scale restricted sea and minor terrestrial organic matter input, and were mainly the margin mudstone with rich terrigenous organic matter and marine planktonic debris or the latter is higher. The sediment has a moderate thickness and restricted distribution. Geochemical characteristics show relatively high organic matter abundance, dominantly mixed kerogen type and low maturity. The gas chromatograms display front end biased distributions, and are characterized by relatively high (nC₂₁+nC₂₂)/(nC₂₈+nC₂₉) and Pr/Ph ratios. Biomarker fingerprints are characterized by low C₁₉ tricyclic terpane and C₂₄ tetracyclic terpane, high pregnane and C₂₇/C₂₉ regular steranes.

Key words: geochemistry characteristics, development model, restricted sea shale, continental margin rifting stage, Bonaparte basin, Australia