OilVoice - AWE Announces T/44P Block Farm-in Opportunity in Bass Basin, Offshore Australia

AWE Announces T/44P Block Farm-in Opportunity in Bass Basin, Offshore Australia

Thursday, July 29, 2010

• Ready to drill prospect defined by 3D seismic and de-risked by convincing AVO anomaly - T44/P Bass Basin offshore Australia

Project Overview:
T/44P block is located in Bass Basin (offshore south-eastern Australia). Bass Bain is one of a series of sedimentary basins that were formed in response to rifting during the Late Jurassic to Early Cretaceous between Australia and represents a failed arm of the Southern Margin Rift System.

The block has been lightly explored previously. No wells have been drilled within T/44P. The nearest wells to the permit are White Ibis 1, Bass 3 and Trefoil 1 (to the east southeast, within T/18P) and Koorkah 1 (to the north in currently vacant acreage). White Ibis and Trefoil are gas fields.

Source
The largely terrestrial setting of the Bass Basin has meant that the principal potential source rocks in the Bass Basin are interbedded coals and fluviodeltaic and lacustrine shales of the Eastern View Coal Measures.

Geochemical analyses indicate that these source rocks have generated oil and gas, with terrestrial organic matter within the Late Cretaceous to Eocene EVCM being the dominant source for the oils.

The oil is typically paraffinic with oil in shallower reservoirs undergoing significant alteration by biodegradation. Some gas accumulations in the Bass Basin contain CO2 content which is interpreted to be sourced from localised igneous intrusions. The hydrocarbon gas is interpreted to be generated from a similar source but over a wider maturity range to the oil.

Reservoir
Proven reservoir rocks within the Bass Basin mainly occur within the Maastrichtian, Paleocene and Lower Eocene and are predominantly fluviodeltaic sandstones. Facies analysis and correlation of cores with wireline logs suggests that the potential reservoir sandstones were deposited in a range of depositional environments from terrestrial; paralic to shallow marine, with the lateral continuity of these facies expected to vary widely.

Coarse-grained facies in the Bass Basin include the fluvial channels, some crevasse-splays and coarser-grained elements of lacustrine/lagoonal or foreshore facies. The coarser-grained, arkosic or more quartzose rocks, particularly clean with moderate to good sorting, have the best chance of preserving good reservoir quality even where deeply buried. Of these reservoirs, channels have the highest ranking, followed by the shoreface and then foreshore facies.

Reservoir quality is attributed to textural maturity, grainsize, diagenesis, degree of marine influence, depth of burial and heat flow within the basin. Silt-free sediments are more likely to develop and retain good reservoir properties. Marine influence during sedimentation removed silt and provided carbonate that acted as an early cement until later removed through dissolution processes. Raised temperatures facilitated the growth of illite which damaged permeability.

The primary relationship between reservoir quality and depositional facies is interpreted to be linked to coarse grain size associated with specific facies, with only sandstones of medium grain size, or larger, likely to develop and retain high quality reservoir properties. A secondary link is defined as the dissolution of early siderite cementation associated with organic-rich facies (Blevin, 2003). Early cementation of the siderite helped preserve porosity and permeability during periods of compaction and deformation, and subsequent dissolution resulted in good secondary intergranular porosity. Dissolution porosity is the main form of porosity seen at depth, and there is a general decrease in porosity with increasing depth

Seals
The vertical sealing capacity of multiple intra-formational seals within the Paleocene aged part of the EVCM has been demonstrated by the stacked gas accumulations at the Trefoil, Yolla and White Ibis fields. Another regional sealing facies is the Demons Bluff Shale which provided top seal for the top EVCM gas and oil accumulation at Yolla. The highest seal capacity facies (>700m oil columns) occur within lower shoreface, lacustrine, lagoonal, abandoned channel fill and interdistributary bay mudstones and silty mudstones of latest Paleocene to Early Eocene age, and are interpreted to be laterally extensive (Blevin, 2003). Coastal plain, fluvial overbank and back barrier berm deposits are the least significant sealing lithologies, supporting <200m oil columns.

Fault seal is a minor risk in the basin, because fault seal is demonstrated to be working in the White Ibis and Yolla discoveries which are fault dependant structures, even though a high net sand section is present at White Ibis.

Traps
Wells drilled to date in the Bass Basin have tested a number of different trap styles including:
1) Onlap or drape over the crests of intra-basin fault blocks
2) Miocene and younger inversion structures
3) Eocene anticlines within the hanging wall of half graben fault blocks
4) Low relief fault blocks
5) Footwall fault blocks along basin margin with associated onlap
6) 4-way dip anticlinal closures

Prospects
The Silvereye 3D Seismic Survey addressed the following key prospects:

Silverey. The Silvereye Prospect is located approximately 18 km northwest of the White Ibis 1 well, in water depths of approximately 55 m. The feature is interpreted as a fault dependent anticlinal closure located on a larger northwest-southeast high trend.

Closure is mapped at the Top 2973 seismic horizon and the Upper Cretaceous
Unconformity seismic horizon. At the Top 2973 seismic horizon, three culminations are mapped over the greater Silvereye structure.

The Silvereye 3D seismic data has better defined the size and closure area for the
feature with a greatly enhanced confidence in fault mapping.

Golden Whistler
The Golden Whistler Prospect is located approximately 9 km west of White Ibis 1 in water depths of approximately 58 m. Fault dependent closure is present at the Golden Whistler structure at both the Top 2973 seismic horizon and the Upper Cretaceous Unconformity seismic horizon.

The Silvereye 3D seismic data has better defined the size and closure area for the feature with a greatly enhanced confidence in fault mapping. The Golden Whistler structure is considered small and has a charge risk, because it is partially shielded from the Yolla Trough by the White Ibis structure.