Source rock characteristics and 1D basin modeling of the lower Cretaceous Latrobe Group, Gippsland Basin, Australia

Abstract

The aim of this study is to perform source rock characterization of the Lower Cretaceous Latrobe Group’s formations in the Gippsland Basin. A total of 138 source rock samples with coal and shale lithologies were selected from three formations represented by Chimaera, Curlip and Kipper Shale formations. The geochemical analyses consisting Rock-Eval pyrolysis, total organic carbon (TOC), vitrinite reflectance (% VR), maceral composition, and biomarkers were used for source rock characterization of the studied formations. It can be interpreted that the formations have poor to very good TOC (average of 2.32 wt %) indicating good organic abundance, while the coal samples from Chimaera Formation have higher organic abundance with excellent TOC (average of 45.25 wt %). These indicate good hydrocarbon generative potential. The formations mostly generate type II/III (mixed) to type III (gas prone) kerogens. Maturity parameters including production index (PI), temperature of maximum pyrolysis (Tmax), % VR and carbon preference index (CPI) indicated that the source rocks are immature to late mature (oil to oil/gas windows). Paleodepositional environment of source rocks is evaluated with biomarker data – normal alkane distribution, pristane/phytane (Pr/Ph) and isoprenoid/normal-alkane ratios. The results show that the source rocks are dominantly terrestrial, deposited in oxic setting. The rock samples did not show any sign of hydrocarbon migration, but experienced biodegradation. However, biomarker analyses were only done on the Chimaera Formation and the results do not represent the whole source rocks in this study. 1D basin modeling was done for Kipper wells. The results show that in Kipper-1, the oil and gas generations started in Kipper Shale at depths 1900 m about 24 Ma, and 2510 m about 15.4 Ma, respectively. In Kipper-2, oil and gas generations started in Chimaera Formation at depths 2540 m about 8 Ma, and 2580 m about 2.5 Ma, respectively. Transformation ratio (TR) plots show that the oil and gas generations have not reached the peak. Petroleum system event (PSE) chart suggested that the main hydrocarbon generation of the Latrobe Group started 26 Ma and is simultaneously being preserved in the Gippsland Basin. This study hopes to give insight of the burial history concept, hydrocarbon generation, and give support in the development of the hydrocarbon fields in the Gippsland Basin.