Dr Sandra Rodrigues1, Professor Suzanne Golding1, Professor Joan Esterle1, Professor Joao Mendonca Filho2, Professor Deolinda Flores3
1The University Of Queensland, St Lucia, Australia, 2Universidade Federal do Rio de Janeiro, LAFO, Rio de Janeiro, Brazil, 3Universidade do Porto, Porto, Portugal
This research provides new insights into the petrology of the organo-facies of the Cretaceous marine Toolebuc Formation, Eromanga Basin. One well from the deep section of the Toolebuc Formation in the most prospective southern part of the basin in Queensland was used for this study. The core section is a 20 m thick carbonate-mud shale with poor expression of the conquinite section often associated with this formation at other locations in the basin. Around 26 polished blocks cut perpendicular to the stratification were used for maceral analysis and 7 slides were prepared for palynofacies analysis. Telalginite represents the structured components. In the samples studied, Tasmanite (green algae) is easily recognised in most of the samples. Its original spherical cyst (phycoma) is flattened along the bedding plane during sediment compaction. Other alginite are present but their identification is difficult. However, the palynofacies showed the presence of dinoflagellates and acritarchs, which may correspond to some of the alginite observed in the petrographic blocks. Furthermore, organic matter with a concentric arrangement of walls is also observed. It has been described in the literature as “organic matter of unknown affinity” and sometimes sporangia (term associated with terrestrial material). However, given the good preservation of this material in this well, this component actually suggests different stages of a life cycle, possible from the Tasmanite. Other organic components include lamalginite and liptodetrinite, probably derived from the algae materials. Bituminite was identified as one of the major organic components in the petrographic blocks; it presents very low fluorescence and is often micrinitized, and corresponds to the amorphous organic matter in the palynofacies slides. In part, bituminite could be derived from the algal components, however, no direct association between the two components was found. The micrinitization of the bituminite indicates that hydrocarbons were generated from this maceral. Other petrographic evidence for the production of hydrocarbons is the occurrence of solid bitumen. Although not often described in the literature, solid bitumen occurs filling the cavities of the foraminifera fossils as well as in the rock matrix of the Toolebuc samples. The origin of these solid bitumens might be different as they have distinct optical properties. Live oil was also observed in some samples during microscopic analysis. There is no direct petrographic evidence of hydrocarbon migration in the samples studied, as solid bitumen was not observed in the fractures, therefore, it might be assumed that hydrocarbons were produced within the Toolebuc Formation and not migrated from the underlying formation.
Sandra Rodrigues is an Organic Petrologist working at The University of Queensland for the past seven years. She has a vast experience in petrographic characterisation of coal and other organic matter dispersed in sediments.