A Seismic Sequence Stratigraphic Model of Miocene Reef Systems, their Depositional Environment and Implications for Shallow Drilling Hazards in the Browse Basin, North West Shelf, Australia
Digance, Carissa J.
Geoscience Honours Degree, 2012
University of Adelaide
The Miocene carbonates of the Oliver Formation cover a large area of the Browse Basin (North West Shelf, Australia) and lie above significant hydrocarbon reservoirs, including the Torosa, Calliance and Ichthys Fields. The high porosity and permeability values associated with carbonate reef systems make these shallow, uncharged carbonates a significant drilling hazard. Thus, the Oliver Formation’s buried reef complexes represent a significant shallow drilling hazard above Browse Basin clastic sediments. This study aims to provide an understanding of the seismic architecture and depositional environment of the carbonate reefs in the Browse Basin with the use of high quality 3D seismic data, and thus to identify areas which may pose serious drilling hazards.
Four reef zones were identified for further study. Reef Zone 1 is located in the inner to middle shelf region and Reef Zones 2, 3 and 4 are located on the north-western boundary of the outer shelf environment. With the use of coherency and amplitude extractions along several horizons and proportional slices, and comparison with modern day analogues, a regional depositional environment was interpreted. The combination of these reef environments give evidence of the evolution from a carbonate ramp to a rimmed carbonate environment.
A period of Mid to Late Miocene inversion along with faulting caused by the collision of the Australian and Eurasian plates has had a significant impact on relative sea-level in the basin, and thus the carbonate reef evolution. Rising global eustatic sea-levels in the Lower Miocene were interrupted by a fall in sea-level in the Mid Miocene that caused subaerial exposure of the inner to middle shelf in the Browse Basin. A rapid increase in global sea-levels following this exposure event caused the drowning of most of the reef systems in the basin. It has been a common occurrence in the Browse Basin to relate the reef growths to the TB1, TB2 and TB3 3rd order eustatic sea-level cycles. Reef growth was greatest during highstand and was amplified by the ‘Miocene Oscillation’.
Reef environments that were identified in the outer shelf environment appear to be a continuation of the reef trend identified in the Carnarvon Basin’s Rowley Shoals. These reef features have well-defined structures that are readily identified in coherency slices and thus easily avoided when drilling. The inner to middle shelf region has a much larger area dominated by shallow patch and platform reef systems. This environment represents the carbonate factory of the Browse Basin. Due to its shallow water-depths, in periods of low relative sea-level this shelf environment is prone to subaerial exposure. Palaeochannels were identified in the inner to middle and outer shelf environments and have been taken as evidence of subaerial exposure of the shelf environments during lowstands. However, it is the inner to middle shelf environment that is exposed for longer periods of time and is therefore more likely to have an abundance of karst features which can increase porosity and permeability. Therefore, the inner to middle shelf environment poses more of a drilling risk due to the loss of drilling fluids and lost circulation material in high porosity intervals.