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A high resolution sequence stratigraphic study has been undertaken on the three wells in the Houtman Sub-basin, offshore North Perth Basin: Gun Island 1 (1968), Houtman 1 (1978) and Charon 1 (2008). The study focussed on the late Jurassic Yarragadee Formation, mid Jurassic Cadda Formation and early Jurassic Cattamarra Coal Measures. Log character (particularly gamma ray and sonic), cuttings, sidewall core and conventional core lithologies (including sedimentary structures) and palynological data were used to identify paleoenvironments. Stacking patterns of the interpreted environments were used to define systems tracts and then sequences. New palynological data have been collected by Geoscience Australia for Gun Island 1 and the palynology for all wells has been reviewed from Well Completion Reports and slides from intervals in each well. A number of transgressive systems tracts within the dominantly continental Yarragadee Formation and Cattamarra Coal Measures record small marine incursions into the Houtman Sub-basin. Within these units, the shallow marine intervals switch rapidly with non-marine intervals suggesting a more dynamic environment existed in the Houtman Sub-basin during the Jurassic than previously thought. These marine incursions are not evident in the Yarragadee Formation in Charon 1, indicating a lack of accommodation space or proximal sediment input into the north during the mid-late Jurassic. This has significant implications for reservoir and seal facies of potential Mesozoic petroleum systems in the Houtman Sub-basin.

Presentation delivered on 9 March 2012 by Marita Bradshaw.

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Introduction This National Carbon Infrastructure Plan study assesses the suitability of the Vlaming Sub-basin for CO2 storage. The Vlaming Sub-basin is a Mesozoic depocentre within the offshore southern Perth Basin, Western Australia (Figure 1). It is around 23,000 km2 and contains up to 14 km of sediments. The Early Cretaceous Gage Sandstone was deposited in paleo-topographic lows of the Valanginian breakup unconformity and is overlain by the South Perth Shale regional seal. Together, these formations are the most prospective reservoir/seal pair for CO2 storage. The Gage Sandstone reservoir has porosities of 23-30% and permeabilities of 200-1800 mD. It lies mostly from 1000 - 3000 m below the seafloor, which is suitable for injection of supercritical CO2 and makes it an attractive target as a long-term storage reservoir. Methods & datasets To characterise the Gage reservoir, a detailed sequence stratigraphic analysis was conducted integrating 2D seismic interpretation, well log analysis and new biostratigraphic data (MacPhail, 2012). Paleogeographic reconstructions of components of the Gage Lowstand Systems Tract (LST) are based on seismic facies mapping, and well log and seismic interpretations. Results The Gage reservoir is a low stand systems tract that largely coincides with the Gage Sandstone and is defined by the presence of the lower G. mutabilis dinoflagellate zone. A palynological review of 6 wells led to a significant revision, at the local scale, of the Valanginian Unconformity and the extent of the G. mutabilis dinoflagellate zones (MacPhail, 2012). G. mutabilis dinoflagellates were originally deposited in lagoonal (or similar) environments and were subsequently redeposited in a restricted marine environment via mass transport flows. Mapping of the shelf break indicates that the Gage LST was deposited in water depths of >400 m. Intersected in 8 wells, the Gage LST forms part of a sand-rich submarine fan system (Figure 2) that includes channelized turbidites, low stand fan deposits, debris flows (Table 1). This interpretation is broadly consistent with Spring & Newell (1993) and Causebrook (2006). The Gage LST is thickest (up to 360 m) at the mouth of large canyons adjacent to the Badaminna Fault Zone (BFZ) and on the undulating basin plain west of Warnbro 1 (Figure 1). Paleogeographic maps depict the evolution of the submarine fan system (Figure 3). Sediment transport directions feeding the Gage LST are complex. Unit A is sourced from the northern canyon (Figure 3a). Subsequently, Unit B (Figure 3b) derived sediment from multiple directions including incised canyons adjacent to BFZ and E-W oriented canyons eroding into the Badaminna high. These coalesce on an undulating basin plain west of Warnbro 1. Minor additional input for the uppermost Unit C (Figure 3c) is derived from sources near Challenger 1. Summary 1: The Gage LST is an Early Cretaceous submarine fan system that began deposition during the G. mutabilis dinoflagellate zone. It ranges from confined canyon fill to outer fan deposits on an undulating basin plain. 2: The 3 units within the Gage LST show multidirectional sediment sources. The dominant supply is via large canyons running north-south adjacent to the Badaminna Fault Zone. 3: Seismic facies interpretations and palaeogeographic mapping show that the best quality reservoirs for potential CO2 storage are located in the outer fan (Unit C sub-unit 3) and the mounded canyon fill (Unit A). These are more likely to be laterally connected. 4: The defined units and palaeogeographic maps will be used in a regional reservoir model to estimate the storage capacity of the Gage LST reservoir.

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In 2011 as part of the National CO2 Infrastructure Plan (NCIP), Geoscience Australia started a three year project to provide new pre-competitive data and a more detailed assessment of the Vlaming Sub-basin prospectivity for the storage of CO2. Initial assessment by Causebrook 2006 of this basin identified Gage Sandstone and South Perth Shale (SPS) formations as the main reservoir/seal pair suitable for long-term storage of CO2. SPS is a thick (1900 m) deltaic succession with highly variable lithologies. It was estimated that the SPS is capable of holding a column of CO2 of up to 663m based on 6 MICP tests (Causebrook, 2006). The current study found that sealing capacity of the SPS varies considerably across the basin depending on what part of the SPS Supersequence is present at that location. Applying a sequence-stratigraphic approach, the distribution of mudstone facies within the SPS Supersequence, was mapped across the basin. This facies is the effective sub-regional seal of the SPS. Analysis of the spatial distribution and thickness of the effective seal is used for characterisation of the containment potential in the Vlaming Sub-basin CO2 storage assessment.

No abstract available

The area northwest of Elura mine, northwest of Cobar, has diverse surficial materials reflecting a complex landscape history. This paper examines some of this history and its possible effects on surface geochemical sampling programs.

The Early Cretaceous South Perth Shale has been previously identified as the regional seal in the offshore Vlaming Sub-basin. The South Perth Shale is a deltaic succession, which unfilled a large palaeotopographic low in the Early Cretaceous through a series of transgressive and regressive events. The new study undertaken at Geoscience Australia has shown that the seal quality varies greatly throughout the basin and at places has very poor sealing properties. A re-evaluation of the regional seal based on seismic mapping determined the extent of the pro-delta shale facies within the South Perth Shale succession, which are shown to provide effective sealing capacity. New sequence stratigraphic interpretation, seismic facies mapping, new and revised biostratigraphic data and well log analysis were used to produce palaeogeographic reconstructions which document the distribution of depositional facies within the South Perth Shale Formation and reveal evolution of the Early Cretaceous deltas. Our study documents spatial variations in the seal quality and re-defines the extent and thickness of the regional seal in the central Vlaming Sub-basin. It provides an explanation for the lack of exploration success at some structural closures and constraints for possible location of the valid plays.