The Birkhead Formation: Reservoir Characterisation of the Gidgealpa South Dome and Sequence Stratigraphy of the Eromanga Basin, Australia.
Doctor of Philosophy
University of South Australia
The Gidgealpa south dome structure in the Eromanga Basin, central Australia is a producing oil and gas field with hydrocarbons sourced from number of sub-surface horizons within the Permian Cooper Basin and Mesozoic Eromanga Basin. Within the Jurassic Birkhead Formation this field has currently reached the end of its primary production and is now moving into the secondary production phase.
Regionally, the Birkhead Formation acts as a seal and the underlying Hutton Sandstone is a major producing reservoir within the Eromanga Basin. The change from reservoir to seal is sharp to transitional across the basin and is accompanied by a change in depositional environment from braided fluvial to fluvial-deltaic and deltaic-lacustrine facies. From Neodymium model age dating and dipmeter interpretation, the decrease in depositional energy is associated with the presence of a significant provenance change from quartzose, craton-derived sediment to lithic rich, volcanic-arc derived sediment.
In sequence stratigraphic terms, the transition from a braided fluvial to a fluvial deltaic depositional environment is separated by a regional sequence boundary within the Hutton Sandstone. Overlying the erosional break are lowstand systems tract sediments, with the beginning of the Birkhead Formation marked by a transgressive surface and decrease in depositional energy. Within the Birkhead Formation the base level continued to rise until the entire Eromanga Basin had been flooded, producing a maximum flooding surface.
The Birkhead Formation at the Gidgealpa south dome has been interpreted as a fluvial-deltaic to deltaic lacustrine depositional environment from facies analysis and embedded Markov chain analysis. These facies include channel fill, abandoned channel, interdistributary bay, crevasse splay, delta mouth bar and lacustrine environments. At the Gidgealpa south dome the progradation and subsequent retrogradation of a delta sequence into a lacustrine environment has been envisaged as the facies model. The Hutton Sandstone basically consists of channel and abandoned channel fill facies of a braided fluvial sequence.
The delta complex consists of a number of isolated and partly isolated sand bodies that represent separate delta lobes. The isolated nature of the sand packages has caused reservoir compartmentalisation. Petrology analysis indicates that sediments of reservoir quality are isolated to the high energy, distributary channels. Sediment composition also has a dramatic effect on quality with the volcanic arc derived sediments having no, or very poor reservoir quality. The loss in reservoir quality in the volcanic arc derived sediments is due to the abundance of lithics and authigenic cements identified from SEM and XRD analysis that formed from the alteration of labile grains reducing porosity and permeability. Permeability analysis from probe permeability and core plug data indicates that channel fill, over-bank and floodplain facies have good, intermediate and poor reservoir characteristics respectively. Results from core analysis also follow this trend.
The reservoir facies identified from petrology and core analysis was further characterised buy using mercury injection and petrophysics. The analysis allowed for the identification of flow units within the Birkhead Formation reservoir. Reserve estimation and a secondary oil recovery strategy has been determined from the reservoir characteristics.