An Investigation into the Causal Mechanisms of Hydrocarbon Seeps in Passive Margin Environments: A case study from the Yampi Shelf, Browse Basin
Thomas R. Bell - 2013
Honours Degree of Bachelor of Science (Petroleum Geology and Geophysics)
Australian School of Petroleum
The University of Adelaide
The Yampi Shelf, north-eastern Browse Basin, exhibits well documented occurrences of both active and ancient thermogenic methane seeps. Such features appear to be exceptionally uncommon in other offshore Australian basins, possibly owing to the lack of tectonism/diapirism and relatively low overall sedimentation rates typical of passive margin environments. Studying hydrocarbon seeps within this region may provide insights into the fundamental processes that cause hydrocarbon migration in similar passive margin environments. A detailed structural analysis relating fault zones potentially reactivated by far field stresses to known occurrences of hydrocarbon chimneys in the Yampi Shelf is yet to be conducted. This study aims to assess the contribution of recently (Neogene) reactivated faults to migration pathways for hydrocarbons within the Yampi Shelf region.
Stress simulation modelling was performed on a fault model built from 3D seismic data interpretation over a seepage prone region of the Yampi Shelf. The results show that north-east trending faults are preferentially oriented to be reactivated under the current day strike-slip stress regime with average slip tendencies of ~0.7. Fault planes appear to be associated with some of the identified hydrocarbon related diagenetic zones (HRDZs), suggesting they are acting as effective seepage conduits. Shale gouge analysis showed that while faults in the study area have high sealing potential (of up to 50% shale gouge ratio), the enhanced structural permeability as a result of the current dilation of fault damage zones is sufficient to exceed these fluid flow impediments.
The thinning of the regional sealing unit is observed to be the principle control in relation to leakage, allowing significant migration within the study area. This is observed to occur where the seal thins to under 50 m, or where it directly onlaps onto the basement Kimberley Block. The small observed fault throws, which is close to the seismic resolution, suggest that the thinning of the seal (< 50 m) above topographic highs may be critical to enabling faults to act as effective fluid conduits. This study also documents an isolated case where intersecting fault planes (both currently reactivated) coincide with vertical hydrocarbon migration to shallow stratigraphic levels. This is observed to occur in the deepest section of the study area, where the seal is thick (~80 m), and leakage is not expected.
Other geological factors identified as playing an ancillary role in promoting seepage include (i) active charge and migration of hydrocarbons occurring currently in the Yampi Shelf; (ii) the presence of a thick carbonate units resulting in reservoir overpressuring, and (iii) a Miocene tilting event that shifted the current day migration route eastwards, towards the Yampi Shelf margin. The combination of these geological factors may be the fundamental cause of the prolific seepage occurring in the Yampi Shelf, and may also explain the lack of similar seepage events in other Australian passive margin basins that do not exhibit such features.
The seepage model developed for the Yampi Shelf may be applicable to other analogous basins. This model may therefore serve as a reference to understanding the possible mechanisms that are contributing to hydrocarbon seeps in similar offshore passive margin basins.