2018 Release Areas

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Acreage Release 2018: Browse Basin

The Browse Basin is a Paleozoic to Cenozoic extensional basin situated on Australia’s North West Shelf. It covers an area of approximately 140 000 km2 and contains a Paleozoic, Mesozoic and Cenozoic sedimentary succession in excess of 15 000 m that hosts significant hydrocarbon discoveries. Parts of the basin are being actively explored, and several large gas accumulations are currently under development for liquefied natural gas (LNG) and condensate production. Petroleum resources are estimated at 33 MMbbl (5.2 GL) of crude oil, 1214 MMbbl (193 GL) of condensate, 404 MMbbl (64 GL) of LPG and 41.3 Tcf (1169 Bcm) of gas (Geoscience Australia, 2018).

The 2018 Acreage Release offers opportunities in the lightly explored southern and central parts of the basin (Figure 1). Release Areas W18-2 and W18-3 are surrounded by gas fields and close to the recently completed Ichthys pipeline (Figure 2).

Release Area W18-2, Caswell Sub-basin, Browse Basin, Western Australia

Highlights

Bids close 21 March 2019

  • Adjacent to the Ichthys and Prelude gas fields
  • Close to the Ichthys pipeline
  • Shallow water depths, 90–270 m
  • Access to oil and gas migration pathways in the Cretaceous and Jurassic stratigraphic and structural traps
  • Hydrocarbon shows at Yampi 1, Yampi 2 and Schooner 1
  • Release Area fully covered by 2D MSS and 3D MC3D MSS
  • Further guidance available, refer to 2018 Special Notices

Release Area W18-2, consisting of 32 full graticular blocks, is located on the southeastern flank of the Caswell Sub-basin (Figure 3). The Release Area lies in water depths of 90–270 m. The Burnside gas field may extend in the northern part of the Release Area (Figure 1; Field outlines are provided by Encom GPinfo, a Pitney Bowes Software (PBS) Pty Ltd product. Whilst all care is taken in the compilation of the field outlines by PBS, no warranty is provided regarding the accuracy of completeness of the information, and it is the responsibility of the Customer to ensure, by independent means, that those parts of the information used by it are correct before any reliance is placed on them. Outlines as at April 2018).

Three exploration wells are located in Release Area W18-2 (Table 1). They all encountered hydrocarbons (Yampi 1, Yampi 2 and Schooner 1; Figures 4 and 5) confirming that the Release Area has received charge from Cretaceous and Jurassic source rocks. Exploration targeted Oxfordian Montara Formation equivalent sandstones reservoired within a combined stratigraphic and structural trap in a Permo-Triassic fault-controlled anticline at Yampi 1 and Yampi 2, and Bathonian-Callovian Plover Formation sands at Yampi 2. Schooner 1 targeted a stratigraphic prospect in the Lower Cretaceous Brewster Sand of the Upper Vulcan Formation and tested for deeper additional Vulcan Formation sandstones and Plover Formation sandstones. The Yampi 1 and Yampi 2 wells encountered poor reservoir quality within Oxfordian and Bathonian–Callovian sandstones (Figures 5, 6 and 7). Low reservoir porosity was also observed deeper at Yampi 1 within the Jurassic, Triassic and Permian sections due to cementation. Schooner 1 tested the extent of the Brewster Member that is the main reservoir of the adjacent Burnside gas field. The well reached altered volcanics in the Plover Formation and encountered minor gas within the Echuca Shoals and Plover formations (Hunt and Schlumberger Geoservices, 2014). Other potential exploration targets may be structural and stratigraphic traps (Rollet et al, 2016a): within the Permian–Triassic basin margin plays; within the basal K10 sandstones associated with third-order clinoform topset (e.g. Yampi 1) sealed by K20–K30 supersequences (Echuca Shoals Formation) that may receive charge from the J10–K10 source rocks (Plover and Vulcan formations); and within the Lower Cretaceous K20–K40 clinoform topsets sands sealed by intraformational and top regional seals that may receive charge from the K20–K30 source rocks (Echuca Shoals Formation).

Further exploration success in this Release Area will hinge on the detailed mapping of existing 3D seismic data (Figure 8) to better understand reservoir distribution and quality, trap geometry, fluid migration pathways and fault seal integrity in the Cretaceous, Jurassic and older successions.

Release area W18-3, Caswell Sub-basin, Browse Basin, Western Australia

Highlights

Bids close 21 March 2019

  • Surrounded by gas fields of the Brecknock-Scott Reef Trend and central Caswell Sub-Basin
  • Oil discovery at Caswell
  • Close to the Ichthys pipeline
  • Shallow to medium water depths, 90–490 m
  • Access to oil and gas migration pathways in Cretaceous, Jurassic, Triassic and Permian stratigraphic and structural traps
  • Potential for younger Eocene and Miocene stratigraphic pinch out plays

Release Area W18-3 is located in the southern and central parts of the Caswell Sub-basin (Figure 3), extending into the Barcoo Sub-basin in the southwest. The Release Area lies in water depths of 90–450 m deepening outboard. Release Area W18-3 consists of 113 full graticular blocks and 5 part graticular blocks.

Although this Release Area is surrounded by major gas fields (Calliance, Brecknock, Torosa, Poseidon, Crown/Proteus, Lasseter and Burnside), it only contains five exploration wells (Table 1). Exploration has largely focussed on sandstone targets within the Campanian and Barremian submarine fans and Upper Jurassic and Lower-Middle Jurassic sands on the Caswell northeast trending fault controlled anticline (Figures 6 and 9). Oil was discovered in an Aptian sand at Caswell 1 (Figure 5) and in a Lower-Middle Campanian submarine fan at Caswell 2 and 2ST1. Drilling difficulties prevented the well reaching the Jurassic objectives in Caswell 1 and extremely low reservoir porosity (2-6%) and lack of permeability have been suggested as the main reasons for failure for the Jurassic targets in Caswell 2. The geochemical correlation of oils in Caswell 1 and Caswell 2 to those from the Echuca Shoals Formation confirms that Lower Cretaceous source rocks have generated in the area. Minor gas shows were also encountered within claystones of the Barremian Echuca Shoals Formation at Caswell 1 and Walkley 1. Failure at Walkley 1 is thought to result from the absence of Barremian reservoir and from the lack of structural integrity and seal for the Campanian reservoir targets. A potential accumulation based on log analysis was interpreted at Marabou 1 in Upper Cretaceous “ponded” turbidites (Benson et al, 2004), however the presence of movable oil and gas could not be confirmed in the absence of neutron and density logs (Rollet et al, 2016a). Lack of adequate stratigraphic seal at this location may be a cause of failure. The dry well Firetail 1 is interpreted to have failed due to inadequate source rock and/or lack of migration pathway. Potential upper Eocene and Miocene stratigraphic pinch-out plays have been identified in the central part of the Acreage Release Area (ConocoPhillips, 2011a).The presence of shallow gas (Woodside, 2012a) in the upper Cenozoic indicate fluid migration pathways towards shallow reservoirs.

Despite well control 5–15 km to the south, at Carbine 1 and Eupheme 1 (Figure 1), and some 3D seismic data coverage over the area (Figure 8), complex faulting and poor quality regional seismic prevent correlation of Jurassic and deeper mapped horizons to other distant wells. Further exploration in this area will help determine the hydrocarbon potential of the southwestern part of the Caswell Sub-basin below the main Jurassic break-up unconformity (Callovian Unconformity).