Areas W07-8, W07-9, W07-10 and W07-11
Yampi-Leveque Shelf, 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
Two wells, Gwydion 1 (1995) and Caspar 1, 1A (1998) have been drilled in Area W07-8. No wells have been drilled in areas W07-9 and W07-10, but Adele Island 1 (1982) was drilled on Adele Island, which is situated on the southern edge of Area W07-9. Several wells, Leveque 1 (1970), Psepotus 1 (1998) and Carbine 1 (2001) have been drilled in Area W07-11.
Gwydion 1 (1995), located in Area W07-8, tested a series of stacked seismic amplitude anomalies within Early Cretaceous (Barremian–Albian) shallow marine sandstones (lower Heywood or Echuca Shoals Formation) draped over a prominent basement high. This well discovered three gas-bearing, and one oil and gas-bearing, zones, and validated a new Early Cretaceous basement drape play on the Yampi Shelf that was dependent on long distance oil migration from the Browse Basin depocentre (this play was subsequently extended by the Cornea discovery wells). The lowermost hydrocarbon zone in Gwydion 1 is Hauterivian–Barremian in age and consists of 9.5 m net oil-filled quartz sandstone (30°API oil) overlain by 12.6 m of net gas-filled glauconitic sand (Spry and Ward, 1997). The three overlying hydrocarbon zones consist of Barremian–Albian glauconitic sandstone.
Caspar 1A (1998) was drilled to test a small drape closure structure within sandstones of the Echuca Shoals Formation, overlying basement. A 5 m gas column was discovered on low porosity sandstones, with traces of mildly biodegraded oil, believed to represent an earlier hydrocarbon charge also present. No hydrocarbons were encountered in other suitable reservoir facies of the Echuca Shoals Formation in the well, indicating that migration across the Yampi Shelf is potentially complex and possibly restricted to specific carrier beds.
Adele Island 1 (1982) was drilled to test a possible closed structure on the northern tip of Adele Island. No significant hydrocarbon shows were encountered, with methane readings remaining below 1000 ppm. Over 600 m of excellent reservoir rocks were encountered beneath Tertiary limestones, but no suitable seal rocks were found in the well.
Psepotus 1 (1998), is located in Area W07-11. The primary target was Early Cretaceous sands in a subtle drape anticline above a basement high. Late Jurassic sands that were predicted to onlap the basement formed a secondary target. A seismic amplitude anomaly is present over the anticline with an area exceeding mapped closure, thereby suggesting a stratigraphic component to the trap (Woodside, 1998). The well intersected a 10 m gross gas column in the target M. Australis sandstone, and the gas/water contact at 846.5 m indicates that the structure is full to spill point, and that an effective top seal is present. The M. Australis sandstone has an average log porosity of 35%, and the average gas saturation was estimated to be at least 60%. A modular dynamic test (MDT) at 837 m recovered 0.35 CF gas, and 65 mL muddy filtrate. Fluid inclusion analysis of the M. Australis sands over the depth range 840–852 m had very low Gas-to-Oil (GOI™) values (0–0.1%; CSIRO Petroleum Report No. 98-004 in Woodside Petroleum, 1998) and provides no evidence of significant oil migration or accumulation. The predicted Late Jurassic reservoir sands were not present in the well, but fractured basement metasediments with a measured porosity of 16–30% were identified as a potential future reservoir play.
Carbine 1 (2001) was drilled on the southern inner margin of the Caswell Sub-basin to test a mid-Campanian ponded turbidite stratigraphic play. The well encountered 77 m of well-sorted, medium grained turbidite sandstone with excellent reservoir characteristics, and this reservoir section is overlain by 150 m of marl with good seal potential (Bensen et al, 2004). There were no hydrocarbon indications on drilling or from wireline logging. This failure is attributed to lack of either significant charge or up-dip seal.
Lombardina 1 (1974), located near the western boundary of Area W07-11, was drilled to test a major Late Miocene reactivation structure on the down-thrown side of a regional down-to-basin fault near the margin of the Leveque Shelf and Barcoo Sub-basin. Arquebus 1 (1991) and Sheherazade 1 (1993) were subsequently drilled on the same structure. Lombardina 1 penetrated a Early Jurassic–Cenozoic section, including about 120 m of Middle Jurassic basic volcanics, and was terminated at 2855 mRT. Ditch gas readings were low throughout Lombardina 1, but many of the sidewall cores within the depth range 2045–2570 m (Early Cretaceous–Middle Jurassic) exhibited weak fluorescence cut, and claystone sidewall cores from 2522.5, 2551 and 2570 m showed instantaneous cut. Log evaluation over the depth range 2432.5–2577 m showed zones of up to 30% hydrocarbon saturation; regarded as residual hydrocarbons only. A Formation Interval Test (FIT-1C) was run at 2511.5 m within a sandstone zone with a log porosity of 15% and 70% water saturation. The test recovered 0.1 CF gas and 0.112 L of water in a 22.2 L chamber, indicating that the reservoir is tight or dry. No other formation tests were carried out in the well.
Haston and Farrelly (1993) considered that Lombardina 1 was positioned in a structurally complex transfer zone between two fault segments of the Lombardina wrench anticline structure, and was probably not a definitive test of the structure’s prospectivity. They noted that the Jurassic sandstones had poor overall porosity (5–9%), with much of the primary pore space being occupied by diagenetic minerals, together with mineral filled fractures, and that evidence of cataclastic deformation was present throughout the well.
Relevant Wells Listing
| Well | Operator | Year | Total Depth (m) | Hydrocarbons |
|---|---|---|---|---|
| Adel Island 1 | Oberon Oil | 1983 | 789 | Gas indications |
| Carbine 1 | Santos Limited | 2001 | 1561 | No shows |
| Caspar 1 | BHP Petroleum Pty Ltd | 1998 | 392 | No shows |
| Caspar 1A | BHP Petroleum Pty Ltd | 1998 | 1099 | Potential oil zone, oil indications |
| Gwydion 1 | BHP Petroleum Pty Ltd | 1995 | 876 | Proven oil & gas zones |
| Leveque 1 | B.O.C. of Australia Ltd | 1970 | 900 | Gas indications |
| Lombardina 1 | Woodside/Burmah Oil NL | 1974 | 2855 | Oil and Gas indications |
| Psepotus 1 | Woodside Offshore Petroleum Pty. Ltd. | 1998 | 1118 | Potential oil zone, oil indications |