Organic geochemical characteristics and paleodepositional conditions of an Upper Carboniferous mud-rich succession (Yagon Siltstone): Myall Trough, southeast Australia

This research evaluates the organic geochemical characteristics of a Carboniferous mud-rich sedimentary succession (Yagon Siltstone) that outcrops in the Myall Trough, a generally regarded fore-arc basin from southeast Australia. Twenty eight mudstone (28) samples were tested by using standard organic geochemistry methods (Rock-Eval VI-TOC, bitumen extraction and fractionation), as well as GC-MS analysis of the hydrocarbons fraction to define the origin, type and maturation level of organic material, and the environment conditions during deposition. The data indicate that the studied samples have poor to fair, gas-prone source rock potential. A marine environment of deposition, with organic material of mixed aquatic and terrigenous origin is envisaged. The marine setting is indicated by the C26/C25 tricyclic terpanes and C23 tricyclic/C30 hopane ratios and the C31R homohopane/C30 hopane versus Pr/Ph cross plot. The greater contribution of terrestrial organic matter is indicated by the Rock-Eval VI data, predominance of higher n-alkanes over short-chain n-alkanes, n-C19/n-C31 and n-C24+/n-C24- ratios, OEP25-33, CPI25-33, ACL25-33, and TAR values, relative abundance of C27-C28-C29 regular steranes and the C27/(C27 + C29) and C27/C29 versus Pr/Ph cross plots, and C24 tetracyclic/C26 tricyclic terpane versus C26/C25 and C23/C22 tricyclic terpane cross plots. Suboxic to anoxic bottom water conditions are supported by the Tm/Ts, C29Ts/C29 hopane ratios and the pristane/n-Cl7 versus phytane/n-Cl8 cross plot. The samples studied are mature and have experienced high temperature during burial (over 100 °C), as assessed from Tmax, OEP25-33, and CPI25-33 values, and the cross plots of 20S/(20S + 20R) versus ββ/(ββ + αα) and C30-moretane/hopane versus Tm/Ts.