Petrographic and geochemical signatures of the Upper Permian Gondwana coals: Newcastle Coal Measures, Northern Sydney Basin, Australia

Abstract

This research presents an integrated approach to organic petrographical and geochemical characteristics of the Upper Permian Newcastle Coal Measures outcropping in the northern Sydney Basin, southeast Australia. Twenty-seven coal, coaly mudstone and mudstone samples were collected and analyzed by using organic petrography (maceral composition and vitrinite reflectance) techniques, Rock–Eval pyrolysis, bitumen extraction and gas chromatography - mass spectrometry (GC–MS). The obtained results indicate that the samples exhibit promising gas-prone source rock potential and contain a predominantly terrestrial in origin organic matter, with limited contribution by marine organisms. The prevalence of terrestrial organic matter is inferred by the presence of collotelinite and sporinite macerals in most of the samples and the ternary plot of C₂₇, C₂₈ and C₂₉ regular steranes. Further, the low C₂₇/C₂₉ ratio, the cross-plots of C₂₇/(C₂₇ + C₂₉) regular steranes vs. Pr/Ph ratio, the C₂₇/C₂₉ vs. Pr/Ph and the C₂₄ tetracyclic/C₂₆ tricyclic terpane ratio point also to the terrestrial origin of the organic matter. The obtained values of vitrinite reflectance, T_max, OEP_27–31 and CPI_25–33, along the cross plots of 20S/(20S + 20R) versus ββ/(ββ + αα) suggest that the samples are immature and were subjected to a low burial depth. The examined samples indicate accumulation in a delta-plain environment of deposition, under fluctuating oxic/anoxic conditions, in a humid climatic zone that is characterized by dry periods, interrupting the longer-termed humid climatic regime. This research adds knowledge on the source rock potential in the northern Sydney Basin and on the environmental and climatic setting of a time interval just prior to the major Permian - Triassic climatic crisis.