New Insights on Potential Sources of Hydrocarbons in the Gippsland Basin

Abstract

To better define the hydrocarbon generation characteristics of potential source rocks in the Upper Cretaceous–Paleogene Latrobe Group of the Gippsland Basin, pyrolysis and kinetic experiments were performed on 91 shale, coal and coaly shale samples. These lithofacies contain predominantly Type III and Type II/III kerogen, with the occasional occurrence of Type II and Type I kerogen, all with capacity for oil and gas generation. The shales and coaly shales have consistently higher Hydrogen Indices (mean 300 mgHC/gTOC), than those exhibited by the coals (mean <200 mgHC/gTOC) from the Halibut Subgroup (L. balmei and F. longus biozones). Nevertheless, using open-system pyrolysis gas chromatography these coals show the potential to generate high wax paraffinic-naphthenic-aromatic (PNA) oils, as well as large volumes of gas. The shales and coaly shales also generate PNA oils with both high and low wax content. Hydrocarbons released upon pyrolysis from hydrogen-rich shales within the L. balmei and F. longus biozones produce high wax-paraffinic oils, as do those from the Golden Beach Subgroup (T. lilliei biozone), whereas shales from the stratigraphically older N. senectus and T. apoxyexinus biozones produce gas-condensate. The aliphatic- and phenolic-rich composition of the pyrolysates from the L. balmei, F. longus and T. lilliei biozones are in agreement with the waxy nature of the crude oils found in the Central Deep. However, gas-condensate and light oil, as occurs within fields on the southeastern margin, could originate from the N. senectus and T. apoxyexinus biozones. The onset of bulk petroleum generation from the Latrobe Group, predicted using open-system pyrolysis (at a constant geological heating rate of 3.3°C/Ma), ranges from 125 to 142°C, with the main phase of generation occurring from 155 to 178°C. In response to increased thermal stress under closed system pyrolysis, four shale samples from the Latrobe Group have the potential to produce variable petroleum compositions. During the early and main stages of petroleum generation, black oils with variable GORs (107–222 Sm³/Sm³) are generated from the shales. In contrast, either black oils with higher GORs, light oils, or wet gases are produced at higher levels of thermal maturation. Results based on the present data-set indicate that shales and coaly shales of the Latrobe Group, rather than coals, seem to have a more significant role in sourcing the oil accumulations in the Gippsland Basin.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015