A new bacterial origin for the hydrocarbon-bearing zones in the Goldwyer Formation on the Broome Platform, Canning Basin, WA, from Nanoscale ToF-SIMS geochemical characterization.

The organic petrology and nano-scale geochemistry of Goldwyer Formation, Canning Basin were mapped to identify the organic macerals, their probable origin and their control by lithofacies. Samples were selected from Theia-1 and Solanum-1 guided by fine-scale lithology, logs and adjacent RockEval data. Most are from the Goldwyer III mudstone in Theia-1, where the highest TOC and gas readings occur, with the rest from Goldwyer I and Goldwyer II units in Theia-1 and a few from Goldwyer I in Solanum-1. The organic macerals were identified by incident and fluorescent light microscopy and inorganic and organic grains were analysed via Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS) to obtain Mass-Spectrogram fingerprints for bituminite, graptolites, algae and possibly bacteria/archaea. The shale minerals are dominated by illite with minor quartz or carbonate, the proportions of which vary correspondingly between the argillaceous, siliceous and calcareous shales. The organics in most samples are dominated by graptolites (vitrinite equivalent macerals), either Detrovitrinite from broken graptolite fragments or Telovitrinite (elongate graptolite stipes or branches). In contrast, a limited number of samples contain a high proportion of Liptinites, mainly fluorescing Bituminite groundmass and Lamalginite (thin algal lamellae or filaments). Discrete algal bodies of G. prisca are rare and seem to be restricted to very thin beds mainly within the Goldwyer I sequence. The maceral compositions control the potential gas zones found during drilling of Theia-1. The zones dominated by graptolites have low potential for hydrocarbons. In contrast, the zones with a pervasive Bituminite groundmass (probably derived from degradation of algae by bacteria/archaea) and filamentous Lamalginite bands are lipid-rich and correspond with the main hydrocarbon-bearing zones in Theia-1. This contrasts with the perception from previous studies that the discrete G. prisca algal bodies would be the main source of hydrocarbons.