Areas W07-5, W07-6 and W07-7
Caswell Sub-basin, Browse Basin
Exploration History
Browse Basin Exploration Overview
Five large undeveloped gas fields have been discovered in the Browse Basin; Scott Reef (Torosa), Brecknock, Brecknock South (Calliance), Brewster (Ichthys) and Crux, together with a number of other gas accumulations (Adele, Argus, Arquebus, Caspar, Echuca Shoals and Psepotus). Small oil accumulations have been discovered at Caswell 2 and Cornea 1; oil and gas accumulations were encountered at Gwydion 1, Cornea South 2, Focus 1 and Sparkle 1, and minor oil shows have been found in several other wells.
The first well drilled in the Browse Basin was Leveque 1 (1970), which was a stratigraphic test of the sedimentary succession on the Leveque Shelf. This was followed by the discovery of gas at Scott Reef 1 in 1971. Scott Reef 1 intersected a thick sequence of gas-bearing reservoirs within Early–Middle Jurassic (Plover Formation) sandstones, together with sandy dolostones of Late Triassic–Jurassic age on the southern culmination of a faulted anticline located on the Buffon–Scott Reef–Brecknock anticlinal trend. Gas flows of 278,000–515,000 m3/day were recorded from drill stem tests (DSTs), and were accompanied by 49–54° API gravity condensate (Willis, 1988). Two appraisal wells (Scott Reef 2A in 1977, and North Scott Reef 1 in 1982) were drilled to further delineate the extent of the field (Bint, 1988). North Scott Reef 1 recorded a maximum gas flow rate of 1,275,000 m3/day from a DST (Willis, 1988).
In 1979, Brecknock 1 tested a broad anticlinal feature 40 km southwest of Scott Reef. The well penetrated 68.3 m of net gas sand in Early to Middle Jurassic sediments, of similar age to the reservoir section at Scott Reef (Bint, 1988).
Other significant discoveries during the early 1980’s include Brewster 1A (1980), Caswell 2 and Echuca Shoals 1 (both 1983). Late Jurassic to Early Cretaceous gas-bearing sands were interpreted in Brewster 1A. Caswell 2 encountered numerous minor oil shows and high gas readings within the Late Jurassic and Early Cretaceous sediments, and it recovered oil from a thin Late Cretaceous (Campanian) sandstone. Echuca Shoals 1 discovered gas in two separate reservoirs of Late Jurassic (Tithonian) to Early Cretaceous (Berriasian) age (Willis, 1988).
Between 1984 and 1994 exploration was focussed largely in the northern Caswell Sub-basin (Gryphaea 1, Asterias 1, Discorbis 1 and Kalyptea 1, ST1), and along the basin margin faults of the Leveque Shelf (Trochus 1, Arquebus 1 and Sheherazade 1) and Prudhoe Terrace (Copernicus 1 and Yampi 2). Many of the wells reported minor hydrocarbon shows from Late Jurassic or Early Cretaceous reservoirs (Maung et al, 1994). Definitive evidence of the oil generative potential of the basin was demonstrated by the Gwydion 1 oil and gas discovery in 1995, and the Cornea 1 oil discovery in 1997, both located on the Yampi Shelf.
Gwydion 1 discovered three gas-bearing zones and one oil/gas-bearing zone in Barremian to Albian shallow marine sandstones draped over a prominent basement high (Spry and Ward, 1997). The Cornea 1, 1B and 2 wells encountered a 25 m gas column overlying an 18 m oil column in the base Albian reservoir sequence (Ingram et al, 2000), and was the first potentially commercial oil discovery in what was previously considered to be a gas-prone basin (Stein et al, 1998). In 1998, Adele 1 discovered gas with oil shows in Middle Jurassic (Plover Formation) sands, and Psepotus 1 and Caspar 1A discovered small gas accumulations within Early Cretaceous sands on the Leveque Shelf and Yampi Shelf, respectively.
Drilling in 2000 resulted in the discovery of several major gas accumulations in the Browse Basin, as well as the extension of previous recognised gas provinces. Brecknock South 1, located on the Scott Reef–Brecknock-trend some 19 km south of Brecknock 1, intersected a 134 m gross gas column in good quality reservoir sandstones of the Middle Jurassic Plover Formation (King, 2001). To the north, on the same structural trend, Argus 1 encountered a gas column in excess of 240 m in Oxfordian sandstones (Keall and Smith, 2004). Significant gas accumulations were encountered in the central Caswell Sub-basin in 2000 with the drilling of Titanichthys 1, Gorgonichthys 1 and Dinichthys 1.
Crux 1 was drilled in 2000 in the northeastern part of the Heywood Graben, and encountered a 280 m gross gas column in the Late Triassic to Early Jurassic Nome Formation (Kaoru et al, 2004).
In 2001–2002, exploration of Early Cretaceous lowstand fans and ponded turbidite oil targets within the Caswell Sub-basin was unsuccessful (Carbine 1, Firetail 1 and Marabou 1). In 2002–2003 Maginnis 1, 1A tested the hydrocarbon potential of the deep-water Seringapatam Sub-basin, but no hydrocarbons were encountered.
Appraisal drilling of the Brewster (Ichthys) field was completed in 2003–04 (Ichthys 1, 1A, Ichthys 2, 2A, ST2 and Ichthys Deep 1). Gas is primarily reservoired within the Brewster Member of the upper Vulcan Formation and within the Plover Formation (Ban and Pitt, 2006). In addition, these authors report that some gas also occurs within Callovian sands, named as the Ichthys Formation, and in the basal Oxfordian sands of the lower Vulcan Formation. Exploration is continuing in this region with Prelude 1, 1A being drilled in December 2006–January 2007.
Evaluation of the gas accumulations along the Scott Reef–Brecknock anticlinal trend is continuing since the drilling of the appraisal wells Torosa 1, 2 and 3, Brecknock 2 and 3, and Calliance 1.
Appraisal drilling of the Crux field is continuing with the drilling of Crux 2 (December 2006–January 2007).
Relevant wells
Multiple wells, Caswell 1 (1978), Caswell 2, ST1, ST1 (1983), Walkley 1 (1993) and Marabou 1, ST1 (2001) have been drilled in Area W07-7. A number of wells have been drilled in permits adjacent to areas W07-5 (Argus 1, 2000) and W07-6 (Basset 1, 1A).
Caswell 1 (1978) targeted Jurassic sandstones in a northeast–southwest-trending anticlinal feature, but bottomed in Early Cretaceous claystones at 4,097 m due to drilling difficulties. The well encountered oil and gas shows within Albian sandstones in the depth range 3,606–3,611 m. Four sidewall cores within this section recovered claystones, siltstones and limestones with a sample of oil with an API gravity of 46° being extracted from one of these sidewall cores. Repeat formation tests (RFTs) confirmed over-pressuring in the Early Cretaceous claystones, but provided little information on the occurrence of hydrocarbons.
Caswell 2, ST1, ST1 (1983) was drilled to follow-up the shows encountered in Caswell 1 and to test Late Jurassic and Early–Middle Jurassic sandstones in a drape anticline. The well discovered a thin oil accumulation within Campanian silty sandstones at 3,264.7–3,268.7 m. A RFT fluid sample taken at 3,265.5 m in the first sidetrack recovered 0.8 L of oil with an API gravity of 47° and 21.2 CF gas. No significant hydrocarbons were recorded in the overlying clean Campanian sandstone between 3,009 and 3,221.5 m, but numerous minor oil shows and high gas readings were encountered below 3,680 m in claystones and low-porosity sandstones in the Early Cretaceous and Late–Middle Jurassic section. The well bottomed in Middle Jurassic sediments of the Plover Formation at 5,000 m.
Walkley 1 (1993) targeted two submarine fan sandstones of Campanian (primary objective) and Barremian age within a 4-way dip mounded structural/stratigraphic play. Caswell 1 and 2 were previously drilled on the flank of this structure. Walkley 1 intersected Campanian reservoir sands (Puffin Sandstone, 2,899–3,062 m) that had no significant hydrocarbon shows, and failed to encounter the deeper prognosed Barremian sands. High gas readings were recorded in Aptian radiolarite and peaked within the Barremian claystones (3,732 m to TD at 3,950 m).
Marabou 1, ST1 (2001) was drilled in the central part of the Caswell Sub-basin to test late Campanian ponded turbidites similar to those tested at Caswell°2. Log interpretation suggested a 22 m gas column within a small four-way dip closure (Benson et al, 2004). These authors concluded that the probable occurrence of trapped hydrocarbons at Marabou 1 indicates that the vertical migration from the Early Cretaceous (Echuca Shoals Formation) source rocks is probably effective, but that up-dip seal is lacking at this location (Benson et al, 2004).
Firetail 1 (2002) was drilled immediately to the west of Area W07-7. The well targeted Campanian turbidite sands of the Puffin Sandstone in a four-way dip closure, similar to that tested at Caswell 1 and 2, Walkley 1 and Marabou 1 (Woodside Petroleum, 2002). The well was plugged and abandoned as a dry hole (Benson et al, 2004).
Argus 1 (2000) encountered a gas column in excess of 240 m in Late Jurassic (Oxfordian) sediments, and oil was recovered from the drilling mud and sidewall cores of Paleocene carbonates. The Argus structure consists of a complex east–west-oriented horst that has undergone several phases of faulting, and the source of the gas is interpreted to be deeply buried Triassic and Jurassic sediments. The oil is interpreted to be sourced from Late Jurassic–Early Cretaceous marine shales, and contained in micro-porosity, possibly linked by a network of fractures that developed over the crest of the deeper Argus structure (Keall and Smith, 2004).
Bassett 1, 1A (1978) tested Cretaceous (Maastrichtian–Campanian) sands on a seismically defined faulted anticline in the Central Browse Basin, but only minor amounts of gas were encountered, and it was subsequently plugged and abandoned.
Relevant Wells Listing
| Well | Operator | Year | Total Depth (m) | Hydrocarbons |
|---|---|---|---|---|
| Bassett 1 | Woodside Petroleum | 1978 | 949 | No shows |
| Bassett 1A | Woodside Petroleum | 1978 | 2706 | Gas indications |
| Brecknock 1 | Woodside Petroleum Development Pty Ltd | 1979 | 4300 | Potential oil zone, oil indications |
| Brecknock 2 | Woodside Energy Ltd | 2005 | 3872 | Proven gas zone |
| Brecknock 3 | Woodside Energy Ltd | 2006 | N/A | Gas indications |
| Brecknock South 1 | Woodside Energy Ltd | 2000 | 4008 | Proven gas zone, oil indications |
| Calliance 1 | Woodside Energy Ltd | 2005 | 4178 | Proven gas zone |
| Caswell 1 | Woodside Petroleum Development Pty Ltd | 1978 | 4097 | Oil recovered, gas indication |
| Caswell 2 | Woodside Offshore Petroleum Pty Ltd | 1983 | 3396 | Oil recovered, gas indication |
| Caswell 2 ST1 | Woodside Offshore Petroleum Pty. Ltd. | 1983 | 4900 | Oil recovered, gas indication |
| Caswell 2 ST1 ST1 | Woodside Offshore Petroleum Pty. Ltd. | 1983 | 5000 | Oil & gas indications |
| Firetail 1 | Woodside Energy Ltd | 2002 | 3355 | Gas indications |
| Gorgonichthys 1 | Inpex Browse, Ltd. | 2000 | 4767 | Proven gas zone, oil indications |
| Ichthys 1A | Inpex Browse Ltd | 2003 | 4826 | Proven gas zone, oil indications |
| Ichthys 2A ST2 | Inpex Browse Ltd | 2003 | 4752 | Potential oil zone, oil indications |
| Ichthys Deep 1 | Inpex Browse Ltd | 2003 | 4956 | Potential oil zone, oil indications |
| Kaleidoscope 1 | Coveyork Pty Ltd | 2001 | 4437 | Gas indications |
| Maginnis 1A ST2 | BHP Billiton Petroleum Pty. Ltd. | 2003 | 4642.5 | No shows |
| Marabou 1 | Santos Ltd | 2001 | 2863 | No shows |
| Marabou 1 ST1 | Santos Limited | 2001 | 3729 | No shows |
| Phrixus 1 | BHP Petroleum Pty. Ltd. | 2001 | 3874 | Oil indication |
| Toroas 1 | Woodside Energy Ltd | 2006 | N/A | Not available |
| Toroas 2 | Woodside Energy Ltd | 2006 | Drilling at time of publication | |
| Walkley 1 | Ampolex Limited | 1993 | 3950 | Oil & gas indications |