Fluid inclusion evidence for an early, marine-sourced oil charge prior to gas-condensate migration, Bayu-1, Timor Sea, Australia

Abstract

The distribution of oil-bearing fluid inclusions (FI) in currently gas-bearing Jurassic reservoir sandstones from Bayu-1 (Northern Bonaparte Basin, Timor Sea) is consistent with the Bayu gas-condensate field originally containing a palaeo-oil column beneath a thick palaeo-gas cap. In order to assess the origin of the oil trapped in the FIs and its relationship, if any, to the gas condensate, a detailed molecular geochemical study was carried out on a FI oil extract and a condensate sample recovered from a similar interval by Modular Formation Dynamics Tester (MDT). Compared to the condensate, the FI oil was generated from a more marine-influenced, less clay-rich source rock or source facies, which was deposited in a less oxic environment with greater eukaryotic input. The source rock of the condensate was more terrigenous and had greater microbial input. The Bayu FI oil contains a greater amount of C 28 and C 29 tricyclic terpanes than the Bayu condensate, and particularly compared to the Elang/Plover sourced oils from further to the northwest (e.g. Corallina and Laminaria), which are more terrestrially-dominated. The Bayu condensate has previously been attributed to either the marine Cretaceous Echuca Shoals Formation, or mixed sourcing from the less terrestrially-influenced facies of the Jurassic Elang and Plover formations, together with the marine Flamingo Group. Analysis of the FI oil confirms a more marine-influenced source facies of the palaeo-oil, with the Echuca Shoals Formation being the most likely source based on oil–oil and oil–source correlations. The FI oil appears to represent a marine source end-member and it is likely that mixing of this oil (sourced from the Echuca Shoals Formation) with hydrocarbons sourced from the more terrestrially dominated Plover/Elang source facies could account for the intermediate composition of the currently reservoired condensate. A discrete ‘Flamingo Group’ is not required and this oil family may not be present in the Bayu area. The differences are nevertheless subtle and a contribution from the Flamingo Group cannot be completely discounted. The FI oil has a mid-oil window maturity (∼0.75% vitrinite reflectance equivalent, VRE), whereas the currently reservoired condensate has a higher maturity (∼0.9% VRE). These maturity data are consistent with early expulsion from the more labile, marine-derived organic matter in the Echuca Shoals Formation, followed by expulsion of large amounts of condensate from the more terrestrially-dominated Elang and Plover formations. Three possible transition mechanisms from gas over oil to condensate are consistent with the FI petrographical and geochemical data. The first charge may have (1) been lost by breaching of the seal, (2) been displaced by the condensate, or (3) been partly dissolved in the later condensate charge. A combination of factors 2 and 3 is considered most likely, but further investigation is required to assess these options. The FI oil at Bayu-1 predominantly represents a different hydrocarbon charge compared to the condensate liquid, and so by analogy the large residual oil columns that are observed elsewhere in the Northern Bonaparte Basin are unlikely to be due to water-washing of a pre-existing gas-condensate column similar to Bayu-Undan.