Origin of organic matter input and paleoenvironmental conditions of the Late Cretaceous coal and carbonaceous shale deposits in the Canterbury Basin, New Zealand: prediction from the biomarker and carbon isotope characteristics

Late Cretaceous Pukeiwitahi Formation is a good petroleum source rock in Canterbury basin of New Zealand. The Pukeiwitahi coals and carbonaceous shales in the offshore Canterbury Basin were geochemically analysed, including total sulfur and organic carbon (TOC) contents with finding from stable carbon isotope and biomarker measurements. These geochemical results were used to provide information about inputs of organic matter and dispositional setting and their effect on the occurrences of organic matter. The Pukeiwitahi coal and shale samples are characterzied by TOC from 7.44 to 73.74%, with sulfur content between 1.07 and 3.13%, resulting in very low sulfur-to-carbon ratios of less than 0.1 and indicating well-oxygenated environmental conditions. The Pukeiwitahi coal and carbonaceous shale sequence also shows high pristane/phytane ratios and low C31R/C30 hopane and DPT/P ratios, suggesting high oxygenated fluvial to fluvio-deltaic environments. These coal and carbonaceous shale sediments are also characterzied by high values of Tm/Ts and C24Te/C26Ti ratios, along with high abundance of C29 regular sterane, indicating high accumulation of terrigenous organic matter. This finding is further corroborated by the outcome of bulk carbon isotopes of hydrocarbon fractions. Maturity biomarker-related parameters and ratios show that the Pukeiwitahi sediments reached diffrent thermal maturity levels, ranging from very early-mature and moderate mature of oil window. Thereby, the Pukeiwitahi organic-rich succession in the studied wells has not yet release significant oil amounts for an expulsion, and the more oil exploration of the Late Cretaceous Pukeiwitahi Formation is needed in the deeper sections of the Canterbury Basin,