Samples of the purported Imburu source rocks, two sedimentary sequences, oil/gas-bearing reservoirs, and oils have been investigated by means of petroleum geochemistry methodologies, aiming to better understand the hydrocarbon sources and potentials, their relations with depositional environments, and petroleum occurrences in the central Papuan Basin, as well as the geochemical behaviour of aromatic hydrocarbons.
The distribution of aromatic hydrocarbons reflects the result of comprehensive geological constraints. Increasing maturity may reduce but not eliminate the source effects. Taking maturity effect into account is essential in the investigation and use of aromatics as origin indicators. The Master-Line approach has been introduced to allow differentiation between source and maturation effects on aromatic origin indicators showing varying degrees of maturity-dependence, and thus a further source characterisation of samples of different maturity. This scheme should throw light on a greater use of the proposed aromatic origin indicators.
Using the approach to assist terrestrial input assessments, the relative abundances of numerous land plant indicators show: (1) a consistent decrease with distance from the terrestrial sources among Imburu sediment samples covering a large part of the basin, reflecting the N/NW directed terrestrial input from the palaeo-highs such as the Oriomo High and the Australian continent; (2) a relative increase down the two sedimentary sequences, reflecting swamp/deltaic deposition with substantial land plant input in the Early-Middle Jurassic time and the depositional environment shift with progressing marine transgression. Terrigenous organic matter input impacts on the source rock quality and hydrocarbon potential, and provides additional precursor sources for some specific naphthalene and phenanthrene isomers. In contrast, the Lower Palaeozoic-derived marine oils of the Tarim Basin, China, that predate the emergence of vascular plants demonstrate a strong lithofacies control on the distribution of isomeric naphthalenes and phenanthrenes, and related maturity indices, which documents the long disputed oil contribution from the euxinic basinal-shales. Comprehensive evaluation of aromatic maturity indices and their application allow the maturity calibration of reservoir extracts and detailed cross-examination of aromatic geochemical behaviours. The 4-methyldibenzothiophene to 1-methyldibenzothiophene ratio (MDR) is an effective maturity indicator in wide thermodynamic ranges. The methyldibenzothiophene to dibenzothiophene ratios are not the universal maturity tools. Source effects remain influential on naphthalenes and phenanthrenes maturity parameters particularly in the early maturity stage.
Source rock formation and distribution is largely controlled by the individual tectonic and sedimentary units with varying depositional conditions. As shown by numerous origin indicators, both vertical and lateral facies variations and associated hydrocarbon potential changes are evident in the basin. The Imburu and Koi-iange Formations, deposited with the Late Jurassic maximum marine-transgression, are the most likely source intervals containing oil-prone kerogen, particularly further to the north/northwest parts of the basin where the larger scale Jurassic sedimentary depressions were developed with deeper-marine environments. Nevertheless, the lower maturity levels of Imburu source interval may discount the petroleum potential in the Foreland parts.
The Omati Trough, largely a Tertiary depocenter, is primarily gas-prone. There, the lower levels of TOC% and maturity may constitute a disadvantage for sizable oil generation and migration from Imburu and Koi-Iange source intervals. Favourable Rock-Eval data around the Bosavi Arch area may likely be the result of contamination from earlier migrated hydrocarbons. A wide extent of Triassic lacustrine source rocks in this area is also likely.
Reservoir extracts have been employed as the proxy for gas-source correlation of Iehi/Barikewa dry gas. Source attributes of Iehi-1 Toro reservoir extracts, reflecting predominately overmature migrated hydrocarbons, and more oxic and dynamic deltaic/swamp environments with extremely abundant land plant input, support a dry-gas origin from the lower units of Jurassic coal-bearing source intervals. These are the Magobu and Barikewa Formations dominated by highly mature gas-prone Type III and oxidized Type IV kerogens. These strongly suggest different hydrocarbon sources for Barikewa-Iehi dry gas and SE Gobe oils.
The newly derived generation/accumulation model from a wide range of geochemical data supports petroleum deposition along the Papuan Fold Belt by mixed oil and gas related to different sedimentary facies, and depositionally controlled by the KuborMendi and Om-Muller Jurassic depocenters. The area along the Bosavi Lineament may represent a Jurassic depositional-high between the depressions, away from which, the oil and gas have a stronger marine source influence. The crude oils are mainly derived from the Jurassic source rocks marking the shelf slope facies. The co-reservoired natural gases suggest a substantial gas input from the basinal facies further to the north, reflecting relatively more marine-influence, high maturity and cracking-genesis attributes. The basinal facies of Jurassic source rocks may have only contributed highly mature gas-condensate to the current deposits, however, implying a loss of the earlier-generated black oils. The specific gas/oil ratios are controlled by numerous factors. The compositional data best describes the gas/oil ratios and support a short time residency of oil/gas in the reservoir. This new scenario illustrates the petroleum occurrences in the Papuan Basin and supplements previous understandings.