PETROLEUM GEOLOGICAL SUMMARY RELEASE AREA W11-15, EXMOUTH SUB-BASIN, NORTHERN CARNARVON BASIN WESTERN AUSTRALIA Bids Close – 13 October 2011 Adjacent to Australia’s major new oil producing province Proven Locker/Mungaroo–Mungaroo/Barrow and Dingo–Barrow petroleum systems Barrow Group and Tithonian canyon plays within the Release Area Proven Triassic fault block gas play in adjacent acreage Complete 3D seismic coverage Special Notices apply, refer to Guidance Notes 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 1 of 24 LOCATION Release Area W11-15 is located in the northern Exmouth Sub-basin, approximately 85-120 km offshore from the township of Onslow (Figure 1). Water depths vary from 200 to 600 m. Release Area W11-15 comprises 9 graticular blocks with a total area of 720 km 2. The graticular block map for the Release Area is shown in Figure 2. Release Area W11-15 is located about 20 km northeast of the new oil province discovered in the Exmouth Sub-basin. Oil production started from the Woodside operated Enfield field in 2006 using a floating production, storage and offloading system, the Nganhurra FPSO. Production commenced in 2007 from the BHP Billiton operated Stybarrow development (Stybarrow FPSO) and in 2008 from the Woodside operated Vincent project (Maersk Ngujima-Yin FPSO). More recently, two new oil projects from this province started production in 2010 (Figure 2) in Regional Geology of the Northern Carnarvon Basin. The Pyrenees project achieved first production of heavy sweet crude oil with an estimated production life of 25 years; and the Apache operated Van Gogh development, part of the greater Vincent oilfield (Gascoyne Development Commission Offices, 2010). Domestic gas production is also expected from 2013 from the offshore Macedon gas field, using a new gas plant at Ashburton North, 17 km southwest of Onslow and via a connection to a pipeline between Dampier and Bunbury (Oil & Gas Journal, 2010). The pipeline linking the Griffin FPSO to the Tubridgi gas field onshore is located only 25 km east of the Release Area (Figure 1). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 2 of 24 RELEASE AREA GEOLOGY The geological evolution of the Northern Carnarvon Basin, and the Exmouth Sub-basin in particular, has been discussed in detail by many authors, and the summary presented below is derived from the work of Veevers (1988), Hocking (1990), Arditto (1993), Jablonski (1997), Tindale et al (1998), Bussell et al (2001), Norvick (2002), Longley et al (2002), Smith et al (2003) and Scibiorski et al (2005). Local Tectonic Setting Release Area W11-15 is located in the northern Exmouth Sub-basin. The north–south-trending Triassic high of the Alpha Arch marks the eastern boundary of the Exmouth Sub-basin and separates it from the Barrow Subbasin (Figure 3). The western margin of the Exmouth Sub-basin is bounded by northeast–southwest-trending Resolution Arch, which separates the subbasin from the broad, faulted Triassic platform of the Exmouth Plateau. Development of the Resolution Arch commenced during the Campanian and was enhanced during the Oligocene and possibly the Miocene. The rapid inversion along the Resolution Arch created a zone of instability, resulting in significant slumping and channelling over the western Exmouth Sub-basin and possibly over the Release Area. The Release Area is located in the northern end of the main depocentre, between the Alpha Arch and the Resolution Arch, to the northeast of the west-northwest to east-southeasttrending Novara Arch. Development of this latter structure commenced in the early Santonian and continued into the Oligocene, overprinting, reactivating and causing the erosion of earlier Valanginian structures. To the south of the Novara Arch, the east-northeast–west-southwest trending Ningaloo Arch formed during the Valanginian uplift, reactivating Triassic to Jurassic faults (Figure 3). Structural Evolution and Depositional History of the Sub-basin Along with the Barrow, Dampier and Beagle sub-basins, the Exmouth Subbasin formed as a series of northeast–southwest-trending, en echelon structural depressions during the Pliensbachian to Oxfordian (Tindale et al, 1998; Smith et al, 2003; Scibiorski et al, 2005). These sub-basins are Jurassic depocentres representing a failed rift system that developed during the early syn-rift phase of breakup of the northwestern Australian continental margin. The pre-rift section in the Exmouth Sub-basin consists of a sequence of Permian and Lower to Middle Triassic sediments. Pennsylvanian (late Carboniferous) to Early Permian rifting and subsequent Triassic thermal subsidence resulted in the formation of a wide basin. The Locker Shale was deposited in shallow shelf environments during a widespread Early Triassic marine transgression which is recognised all along the Western Australian margin from the Bonaparte Basin to the Perth Basin (Figure 4). The Locker Shale is overlain by a thick succession of mainly fluvio-deltaic to marginal marine sediments of the Mungaroo Formation (Figure 4; Tindale et al, 1998). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 3 of 24 A narrow basin, formed during Early-Middle Jurassic rifting between Australia and Greater India, resulted in the deposition in an open marine environment of the Dingo Claystone, the main source rock of the region (Figure 4). Extensional reactivation followed in the Late Jurassic and Early Cretaceous as Greater India separated from Australia. Rift-related uplift to the south of the Exmouth Sub-basin provided the sediment source for the main reservoir units in the area. These lie within the Barrow Delta which prograded northward over the Exmouth Sub-basin and the southern and central Exmouth Plateau. The delta had covered the Alpha Arch by the mid-Berriasian (Smith et al, 2003) and extended into the Barrow Sub-basin as far as the southern end of the Gorgon field (Figure 3). Northeast–southwest-trending syn-sedimentary faults affect the Barrow Group sediments and can provide local combined structural and stratigraphic traps. Continued separation of Greater India from Australia in the Valanginian (Veevers, 1988) is correlated with major structural inversion which resulted in the uplift of the Ningaloo Arch, with associated erosion of the Barrow Group and older Jurassic sediments across much of the sub-basin (Figure 4 and Figure 5a; Tindale et al, 1998). The delta sediments were reworked and redeposited in the parasitic deltaic wedges of the Zeepaard and Birdrong formations (Figure 4; Arditto, 1993; Tindale et al, 1998). This event is associated with the development of structural dip to the north by tilting of the east-west trending Ningaloo Arch to the south. This resulted in the formation of complex trapping architecture within the Late Berriasian arch that extends in a north-northeast direction across the western edge of the sub-basin (e.g., Eskdale structure). A regional marine transgression during the Hauterivian marked the beginning of thermal relaxation during the post-rift stage, and resulted in the deposition of the main regional seal for the Carnarvon Basin, the Muderong Shale (Figure 4). This formation thins to the south of the Release Area where it onlaps the Ningaloo Arch, which was a positive feature at the time of deposition (Tindale et al, 1998). The Muderong Shale is overlain by the Windalia Radiolarite, a porous but low permeability thief zone. Above the radiolarite, the Lower Gearle Formation, which consists of a thick sequence of Albian to mid-Cenomanian claystones and siltstones, is deposited in an outershelf environment and is considered to be an effective top seal for some accumulations in the sub-basin (e.g., Pyrenees and Macedon fields; Bailey et al, 2006). Basin inversion and uplift in the Late Cretaceous formed the Novara Arch and the Resolution Arch (Figure 3) and shut down the Jurassic source ”kitchen”. Uplift began in the early Santonian and overprinted and reactivated previously formed structures (Tindale et al, 1998). The latest phase of tectonism is recorded in the Late Miocene by gross tilting of the margin to the west due to progradation of a thick Tertiary carbonate wedge and fault reactivation. During this interval, a renewed phase of compression enhanced the Pyrenees/Macedon structure and is interpreted to 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 4 of 24 have tilted many structures to the south and west and is likely to have modified existing hydrocarbon accumulations (Tindale et al, 1998). The Upper Cretaceous to Holocene passive margin sedimentary section in the Exmouth Sub-basin is dominated by deep-water fine-grained carbonate, including calcilutite and marl (Figure 4). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 5 of 24 EXPLORATION HISTORY The first flow of oil to the surface in Australia was recorded in 1953 at Rough Range 1 in the onshore Northern Carnarvon Basin, immediately east of the Exmouth Sub-basin. However, further drilling on the same structure, a surface anticline located to the south of Exmouth Gulf, could not replicate the initial success of Rough Range 1, which recorded an oil flow of 500 bopd (79.5 kL/d) from the Lower Cretaceous Birdrong Sandstone (Bradshaw et al, 1999; Ellis and Jonasson, 2002). Exploration in the Exmouth Sub-basin has been episodic over the last 35 years. The area did receive some attention during the first phase of island and shallow water drilling by Western Australian Petroleum Pty Ltd (WAPET) in the 1960s and early 1970s (Mitchelmore and Smith, 1994). The first gas shows were recorded in 1972 when West Muiron 1 was drilled on the feature later recognised as hosting the Pyrenees/Macedon gas and oil accumulation. This indicated that the Exmouth Sub-basin was petroliferous. However, the focus of exploration was elsewhere in the Carnarvon Basin, namely in the Barrow and Dampier sub-basins, where giant discoveries were made including a billion barrels (1.59 x 108 kL) of oil-in-place at Barrow Island in 1964 and multi-Tcf (>5 x 10 10 m 3) gas fields on the Rankin Platform in 1972. In the late 1970s and early 1980s, exploration in the region concentrated on deepwater drilling on the Exmouth Plateau. These initial exploration programs on the plateau were undertaken by Esso and Phillips (Barber, 1988) and resulted in the giant gas discovery at Scarborough (Walker, 2007). The general offshore exploration focus around Australia moved inboard in the early 1980s, as permit sizes and prospect volumes within the permits decreased. In the shallow water section of the Exmouth Sub-basin, Jurabi 1 was drilled by Esso Australia Ltd in 1982 as another test of the West Muiron structure. However, this failed to intersect a significant hydrocarbon column and it was not until the 1990s that there was eventual success on this structure (Mitchelmore and Smith, 1994). Eight deep-water wells were drilled in the southern Exmouth Sub-basin between 1999 and 2004, following the 1998 Vincent 1 oil discovery. The discovery in 1999 of the Enfield oil field followed by a string of oil discoveries, including Coniston, Laverda, Stybarrow, Ravensworth and Stickle increased interest in this sub-basin. A new oil province had been discovered. This drilling program was very successful, due to extensive quantitative interpretation of 3D seismic data, resulting in the numerous oil and gas discoveries (Walker, 2007). Combined initial production of major fields, including Enfield, Vincent, Pyrenees, Stybarrow and Laverda, indicates the province contains more than 48 GL (300 MMbbl) of heavy crude reserves. These are the most important oil development projects from Western Australia in the past five years. Production is estimated to approach 40,000 kL/d or 250,000 bbl/d (2008). Two new oil projects commenced in 2010; production 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 6 of 24 from the Van Gogh oilfield started in February and production from the Pyrenees project comprising the Crosby, Ravensworth and Stickle oilfields started in February-March 2010 (Department of Mines and Petroleum, 2010). Well Control Resolution 1 (1979) Resolution 1 was drilled by Esso Australia Ltd to test a Triassic Mungaroo Formation objective in a narrow, northeast-trending, faulted horst on the northwestern margin of the Exmouth Sub-basin. The well encountered reservoir quality sandstones in the Mardie Greensand, Barrow Group and Mungaroo Formation. The Dingo Claystone and Mungaroo Formation were identified as potential source rocks, with average TOC values of 2.36% and 2.53%, respectively. On well logs, a thin (2.5 m) and tight gas-bearing sandstone was identified at the top of the Mungaroo Formation. There was no closure at the Barrow Group level and no significant hydrocarbons were found at this level. No shows were recorded in sands of the Mardie Greensand Member. A seal was identified in the Dingo Claystone equivalent. The Muderong shale is not intersected in Resolution 1. A local heating event prior to the deposition of the Upper Jurassic sequence is interpreted from available data. A rapid early coalification in the Triassic was followed by significant erosion removing at least 500 m of section (Esso Australia Limited, 1980). The lack of significant hydrocarbons in the Resolution structure was attributed to overpressuring in the Mungaroo Formation transmitted from the Dingo Claystone, prior to thermal maturation of the source rock (Esso Australia Limited, 1980). Zeepaard 1 (1980) Zeepaard 1 was drilled by Esso Australia Ltd to test a narrow Triassic northeast-trending faulted horst in the northern edge of the Exmouth Subbasin. Closure is provided to the west and north by a bounding fault which curves round to an east-west strike. Closure to the southeast is provided by dip of the beds towards the Exmouth depocentre. Possible erosion along the northern flank of the horst may have provided independent closure. The second objective was to evaluate the hydrocarbon potential of a Lower Cretaceous turbiditic sandstone stratigraphic trap. The well reached a TD of 4,214.8 m RKB. Good reservoirs were encountered in the Barrow Group and Mungaroo Formation equivalent. Two gas sands were interpreted in the Mungaroo Formation equivalent from electric logs, with average porosity of 14.5% and 10.6%, respectively. A possible third gas sand is interpreted in between. Residual hydrocarbons were found in the Barrow Group. The Lower Barrow Group and Dingo Claystone have good oil source potential, while the Mungaroo Formation equivalent could source both gas and oil. Reworked Triassic coals in the Dingo Claystone are also identified as a potential source. The Dingo Claystone and delta front siltstones of the Barrow Group and the Muderong Shale provide good seals. The lack of fluid movement into the horst trap resulting from over-pressuring in the Dingo Claystone may have preserved initial porosities (Esso Australia Ltd, 1981). 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 7 of 24 Vlaming Head 1 (1982) Vlaming Head 1 was drilled by CNW Oil (Australia) Pty Ltd to test a large stratigraphic pinch-out structure in the Barrow Group on a northeastsouthwest trending structural nose. The primary objective was encountered lower than predicted beneath the Muderong Shale and the Birdrong Sandstone and the predicted basal seal was instead the top of the Barrow Group. The well was deepened but no significant shales were intersected below the lower Barrow Group sands. The primary objective lacks a basal seal and the secondary objective an upper seal. The absence of seals resulted in the Barrow Group reservoirs being 100% water wet. The well was plugged and abandoned. West Muiron 3 (1992) The West Muiron structure (Pyrenees/Macedon fields) is a large antiform, dissected into a series of tilted fault blocks by several northeast–southwest trending faults. The prospect is in the Lower Cretaceous at the base of the Muderong Shale, overlying a deep Triassic high trend, the southerly extension of the Alpha Arch. West Muiron 1 and 2 were drilled by WAPET in 1972 and 1975 respectively, targeting the Lower Cretaceous sequence that was hydrocarbon-bearing at Barrow Island. West Muiron 1 was abandoned due to mechanical difficulties, while West Muiron 2 encountered no shows in the thin Barrow Group intersection. West Muiron 3 was drilled by BHP Petroleum in 1992, 3 km to the northwest of West Muiron 2, to test Birdrong Formation and Barrow Group sandstones within the West Muiron structure. West Muiron 3 well reached a TD of 1,200 mRT. The Birdrong Formation was absent, but the well intersected a 40 m dry gas column in highly porous and permeable unconsolidated sands of the Berriasian Barrow Group (Mitchelmore and Smith, 1994). An average porosity of 31% and gas saturation of 88% are calculated (BHP Petroleum Pty Ltd, 1994 a). The seal for these units is provided by the Muderong Shale and Lower Gearle Formation. A gas-bottom seal contact prevented an accurate assessment of the potential hydrocarbon column. The presence of gas is also reported in the Windalia Radiolarite but permeabilities are very low. The West Muiron 4 step-out well, drilled by BHP Petroleum in 1993, established a total gas column in the Barrow Group in excess of 91 m. The gas field discovered in West Muiron 3 was later named the Macedon gas field. Langdale 1 (2005) was drilled north of the Macedon gas field to evaluate the hydrocarbon prospectivity of the Berriasian Pyrenees Member (basal Barrow Group) in a combination structural-stratigraphic trap on the Langdale West fault block but did not achieve the principle objective as gas-bearing sands of variable quality were intersected instead of the prognosed oil-bearing sands. Helvellyn 1 (2007), drilled north of Langdale 1, failed to encounter commercial quantities of hydrocarbons (Department of Mines and Petroleum, 2008). BHP Billiton, operator with Apache Corporation will commence production of natural gas from the offshore Macedon gas field with four wells tied to a new gas plant onshore (BHP Billiton, 2010). The field is 100 km west of Onslow on production license WA-42-L. First production is expected during 2013. BHP, with a 71.43% interest, estimates reserves at 400-750 bcf of recoverable wet 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 8 of 24 gas and a total project cost of $1.5 billion. Apache holds the remaining interest. The gas plant will be built at Ashburton North, 17 km southwest of Onslow. From the plant, gas will flow via a sales pipeline connected to a pipeline between Dampier and Bunbury (Oil & Gas Journal, 2010). West Muiron 5 (1993) Pressure data acquired in previous West Muiron wells allowed for the possibility of an oil leg underlying the discovered gas, therefore the West Muiron structure was further appraised with the drilling of West Muiron 5 by BHP Petroleum (Mitchelmore and Smith, 1994). West Muiron 5 was drilled 5.7 km west-southwest of West Muiron 4 to test the downthrown fault block forming the western flank of the structure, which was closest to the interpreted oil migration direction and still within structural closure. The well encountered a 20 m thick column of dry gas overlying a 32 m thick column of 18 API oil within the Berriasian Barrow Group (BHP Petroleum Pty Ltd, 1994 b). Compositional variations of the gas, separate hydrocarbon contacts and minor pressure differences between West Muiron 5 and West Muiron 3 and 4 are indicative of two distinct fields. The oil and gas field discovered in West Muiron 5 was later named the Pyrenees oil and gas field. Both the gas and oil were biodegraded but producible, especially in high quality reservoirs (Smith et al, 2003). Residual hydrocarbon was found in the Windalia Radiolarite. York 1 (1993) York 1 was drilled by BHP Petroleum Pty ltd to test the Birdrong Sandstone on an unfaulted depositional drape anticline with four-way dip closure and 15 m vertical relief, overlying an older Triassic–Jurassic horst block. The time closure is lacking due to lateral velocity variations in the Tertiary carbonates, “pulling-up” the eastern flank of the structure, however, depth conversion indicated a closure at the York location. York 1 well reached a TD of 3,372 mRT in a water depth of 365 m. Good reservoir sandstones with high net-to-gross ratios were intersected in the Birdrong and underlying Zeepaard Formation of the Barrow Group. The Birdrong Sandstone has core plug porosities of 15 to 20% and permeabilities of up to 3,000 mD. The well intersected the entire Zeepaard Formation and bottomed in the upper Barrow Group, without encountering significant hydrocarbon shows (BHP Petroleum Pty Ltd, 1994 c). This well did not attempt to drill into a deeper target of possible slope fan sandstones that may form stratigraphic traps in the intraBarrow Group on the York structure. Altair 1 (1995) Altair 1 was drilled by West Australian Petroleum Pty Ltd to test a turbidite sandstone pinch-out within the deltaic bottom-set unit (Malouet Formation) of the Barrow Group. This was a pure stratigraphic trap test on a monoclinal structure, based solely on a strong seismic amplitude anomaly. The well intersected sandstone with excellent reservoir properties, but the sandstone is water-bearing. The top seal is provided by progradational prodeltaic shales over a turbiditic reservoir. The bottom seal is provided by the underlying transgressive and highstand condensed pelagic shales. A wireline formation test recovered water and a tiny quantity of solution gas from the sandstone. A sharp lithological contrast is likely the cause of the amplitude anomaly. It was 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 9 of 24 concluded that gas was once present in the Altair Sand and probably in the Paleogene, during inversion folding, the hydrocarbons were lost. Subsequently, the sands were not recharged nor the trap re-established. This suggests that potential for recharge by hydrocarbons matured during the Late Cenozoic must be carefully evaluated in the area (West Australian Petroleum Pty Ltd, 1995). Vincent 1 (1998) Vincent 1 was drilled by Woodside in 379 m water depth on the flank of the Novara Arch trend, to test a three-way dip/fault closure at the base of the Muderong Shale. Vincent 1 encountered hydrocarbon-bearing sandstones in the Lower Barrow Group objective, with an 8 m gas leg and a 19 m oil leg. The well was tested, achieving maximum flow rates of 4,301 bopd (683.8 kL/d), with 1.9 MMscf/d (53,808 m3/d) gas through a 2” choke. The API gravity of the oil was 17°. H 2 S gas was detected during the test, reaching a maximum of 80 ppm. The excellent quality of the reservoir encountered in Vincent 1 is the primary reason for the good test result and it is clear that reservoir quality is a key factor for prospects with a similar hydrocarbon charge. The Vincent oil discovery at the top of the Barrow Group is significant in that it proved producible oil in an area where oil was previously considered to be too biodegraded and heavy to produce (Polomka et al, 1999). The Vincent 1 well became the harbinger of successful exploration drilling campaigns in this area. Van Gogh is the name given to the northern part of Vincent field where production commenced in February 2010. Van Gogh is Apache’s first oil development using a floating production, storage and offloading (FPSO) system, the Ningaloo Vision. The project is expected to produce 6,360 kL/d (40, 000 bbl/d) of oil (Department of Mines and Petroleum, 2010). Enfield 1 (1999) Enfield 1 was drilled by Woodside to test a terraced fault block on the western flank of the Novara Arch. Enfield 1 intersected a gross 21.9 m oil column in the primary objective, Macedon Formation sandstones, at 2,022.3 mRT, and a gross gas and oil column of 14.8 m at 1,478.3 mRT in the secondary objective, the Lower Cretaceous Barrow Group–Mardie Greensand Member. The well reached a total depth of 2,192 mRT in the Dingo Claystone. A production test, carried out within the Macedon Formation, achieved a maximum stabilised flow rate of 4,800 bopd (763.1 kL/d) through a 44/64” choke (Woodside, 1999). Enfield 1 was drilled in 544 m of water and had 128 MMbbl (2.035 x 10 7 kL/d) in initial reserves. The well came into production in 2006 and was the first field to do so in the Exmouth Sub-basin (Walker, 2007). Coniston 1 (2000) Coniston 1 was drilled in 415.5 m of water by BHP Petroleum Pty Ltd to evaluate the hydrocarbon prospectivity of the upper Berriasian shoreface sands of the Barrow Group within the large, fault-dissected, domed anticline that was also tested by Novara 1 in 1982. Petrophysical interpretation of the well showed that Coniston 1 intersected an 11.5 m gas column over a 13 m oil column within the Barrow Group sands. However, in contrast to Novara 1, 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 10 of 24 Coniston 1 achieved a flow of 2,119 bopd (336.9 kL/d) from a 13 m oil column below 11.5 m of gas (Smith et al, 2003). Early Cretaceous migration of oil into the Novara structure appears to have been modified during the Late Cretaceous compressional tectonism that created the Novara Arch, and on which the broad Coniston anticline is developed. The 18 m residual oil column intersected in Novara 1 is interpreted to be the result of spillage into the newly developed Coniston structure. The base of the present-day oil column in Novara 1 coincides with the spill point into the Coniston structure. The latter, then, is substantially under-filled (and with an oil–water contact approximately 10 m above the Novara oil–water contact) due to inadequate spilled hydrocarbons available to fill the trap. In addition, little fresh oil was expelled and migrated from the Exmouth Sub-basin following Late Cretaceous uplift of the source rocks into a much cooler thermal regime. Reburial of Jurassic source rocks into the generative zone, due to deposition of Pliocene to Holocene carbonates, has contributed a minor thermal gas charge that has displaced the oil pool downwards to its present level (Smith et al, 2003). Crosby 1 (2003) Crosby 1 was drilled by BHP Billiton to test the validity of an elongate, northnortheast-trending structural–stratigraphic trap located on a northeast trending fault terrace between the Ravensworth and the West Muiron 5 oil and gas discoveries. The primary objective was the shallow marine uppermost Tithonian to lower Berriasian Pyrenees Member of the Barrow Group. The well reached a total depth of 1,226 mRT. Wireline log analysis indicated that the primary reservoir was oil-bearing, with 34.0 m of net oil pay. This was confirmed by RCI pressure testing and fluid recovery, with good quality samples of 18.6°API oil obtained from the Pyrenees Member. Geochemical analysis of the Crosby 1 oil and gas indicates that the accumulation represents a mixture of an early charge of oil and associated gas, a later charge of mature wet-gas/condensate (including gasoline-range hydrocarbons), and a late charge of very mature dry gas, all of which were subsequently biodegraded (BHP Billiton, 2004). In February and March 2010, the Pyrenees project, comprising the Crosby, Ravensworth and Stickle oilfields, started production ahead of schedule in the BHP Billiton Petroleum operated permit WA-42-L. The full project includes a subsea gathering system and the Pyrenees Venture FPSO vessel capable of producing up to 15,263 kL (96,000 bbl) of oil and reinjecting 1.7 Mm 3 (60 MMscf) of gas per day. Gas produced by the development will be reinjected into the reservoir of the nearby Macedon gasfield for future recovery (Department of Mines and Petroleum, 2010). Eskdale 2 (2004) Eskdale 1 and 2 were drilled by BHP Billiton in 2003 and 2004, respectively, to test the hydrocarbon potential of a complex structural/stratigraphic channel trap beneath the collapsed core of the north-northeast-oriented late Berriasian Resolution Arch. The trap is defined by sand distribution within a midTithonian submarine canyon-fill complex that is dissected by the cross-cutting Berriasian–Valanginian structural dip and fault components. Eskdale 1 encountered a residual oil column within the target Dupuy Formation. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 11 of 24 Eskdale 2, located approximately 2 km south of the Eskdale 1, intersected a 36.9 m thick gas- and oil-bearing sandstone succession in the Dupuy Formation, with a 24 m gas leg and a 12.9 m oil leg (BHP Billiton, 2005). An oil–water contact was not penetrated but extrapolation of MDT pressure data suggests a total oil column of 49.2 m. Modelling suggests that the Oxfordian basal Dingo Claystone is the likely source rock, and although the Dingo Claystone is capable of generating both oil and gas, the majority of the gas is likely the result of biodegradation of trapped oil (biogenic methane). The oil discovered in Eskdale started production in 2007 as part of the BHP Billiton (operator) and Woodside Stybarrow project. Stybarrow Project produced 11.40 million barrels of crude oil during 2009 and has a current capacity of 11.97 million barrels per year. The field life of Stybarrow Project is expected to be around 10 years with complete abandonment by September 2016. The field is expected to generate $3.73 billion in revenue (undiscounted) during its remaining life, starting 1/1/2010 (Articles Hub, 2010). Beg 1 (2007) Beg 1 was drilled by Apache Northwest Pty Ltd less than 5 km southwest of the Acreage Release area and discovered hydrocarbons (Department of Mines and Petroleum, 2008). The well was drilled in 345 m of water (below AHD) and reached a total depth of 3,936 mMDRT (Apache Energy, 2008). No further data is publicly available at the time of writing. Bleaberry West 1 (2007) Bleaberry West 1 well was drilled by Apache Energy about 25 km southwest of the Release Area and discovered hydrocarbons (Department of Mines and Petroleum, 2008). No further data is publicly available at the time of writing. For further details regarding wells and available data follow this link: http://www.ret.gov.au/Documents/par/data/documents/Data%20list/data%20li st_exmouthsb_AR11.xls 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 12 of 24 Data Coverage Release Area W11-15 has a good coverage of 2D seismic data with between 1 km to 5 km line spacing, comprising surveys of different vintages from the 1970s to the early 1990s: Barrow 3 (DW) 324 (1971), Barrow 4 (DW) 352 (1972), Hilda (1973), X78A (1978), Ningaloo Round 1 (1979), Cape (1980), Vlaming Head (1980), Vlaming 2D (1992) and GPCT93 (NEPS 2D) (1993). The area is also intersected by some regional seismic lines acquired by Geoscience Australia: BMR 17 (1972) and AGSO 110–Barrow/Dampier (1992) surveys, examples of which are shown in (Figure 5a.) Most of the Release Area is covered by high quality 3D seismic of the Carnarvon HCA04A survey acquired in 2005 by BHP Billiton Petroleum Ptd Ltd and with the PGS Ramford Vanguard vessel. In 2007, PGS acquired the New Dawn Survey, a multi-client 2D seismic survey (PGS DATA LIBRARY, 2007) that provides long offset 2D data in deep water along the North West Shelf of Australia, close to the Release Area. Gravity and magnetic data were acquired in conjunction with the 2D seismic (PGS Data library, 2007). To view image of seismic coverage follow this link: http://www.ga.gov.au/energy/projects/acreage-release-andpromotion/2011.html#data-packages The North West Shelf Digital Atlas (NWSDA) also covers the Release Area and provides a regional understanding on a continental scale and delivers a detailed insight into NWS petroleum provinces (PGS Data library, 2010). This package also contains regional grids for bathymetry, gravity, magnetic, TOC, HI and VR data. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 13 of 24 PETROLEUM SYSTEMS AND HYDROCARBON POTENTIAL Table 1: Petroleum Systems Elements Summary Sources Jurassic Dingo Claystone – source of oil in fields in the sub-basin Triassic Mungaroo Formation – deltaic sediments are a source of gas Mardie Greensand Member Cretaceous Barrow Group, Zeepaard and Birdrong Sandstones Jurassic Dupuy Formation Triassic sandstones Cretaceous Muderong Shale (regional seal) Intraformational seals within the Upper Triassic and Lower Cretaceous deltaic sequences Cretaceous inversion anticlines and structural/stratigraphic traps Jurassic complex structural/stratigraphic channel trap beneath Late Berriasian anticline Triassic fault blocks and associated drape Reservoirs Seals Play Types Petroleum Systems Two petroleum systems are prospective in the Release Area. The extensive Locker/Mungaroo–Mungaroo/Barrow petroleum system, which has sourced some of the giant gas fields in the Northern Carnarvon Basin, was proven south the Release Area with the discovery of gas in the Mungaroo Formation at Falcone 1A. The Triassic sedimentary succession in the Release Area has proven potential for mature source facies, including possible organic-rich units in the Lower Triassic (marine Locker Shale equivalents) and Upper Triassic (deltaic Mungaroo Formation facies and marine equivalents). The Upper Jurassic Dingo Claystone is the principal source for oil in the Exmouth Sub-basin (Tindale et al, 1998). This unit is relatively thick within the Release Area W1115. Accumulations of the productive Dingo–Barrow petroleum system of the Exmouth Sub-basin lie only 25 km southwest to the Release Area and even 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 14 of 24 closer in the Griffin field, in the adjacent Barrow Sub-basin. Migration of hydrocarbons from the Upper Jurassic source kitchens in the central Exmouth Sub-basin was focused towards the Release Area during the Early Cretaceous (Tindale et al, 1998; Smith et al, 2003). Source Rocks Potential source intervals in the Exmouth Sub-basin occur in the fine grained, deepwater and pro-delta facies deposited in the Late Jurassic and Early Cretaceous. Some hydrocarbon generation from the gas-prone Mungaroo Formation system and older Triassic source rocks presumably occurred in the Exmouth Sub-basin during the Jurassic. The Lower Barrow Group and Dingo Claystone have good oil source potential. The synrift anoxic marine shales of the Late Jurassic Dingo Claystone are the principal effective source for oil in the Dampier, Barrow and Exmouth Sub-basin (Tindale et al, 1998; Longley et al, 2002). Modelling suggests that although the Dingo Claystone is capable of generating both oil and gas, the majority of the gas is likely the result of biodegradation of trapped oil (biogenic methane). The Mungaroo Formation equivalent could source both gas and oil. Reservoirs Proven reservoirs in the Exmouth Sub-basin are listed in Link to Table 1 and potentially extend into Release Area W11-15. Good reservoirs were encountered in the Late Triassic Mungaroo Formation equivalent at Zeepaard 1 (Esso Australia Ltd, 1981; Figure 4). Two gas sands were interpreted in the Mungaroo Formation equivalent from electric logs, with average porosity of 14.5% and 10.6%, respectively. The Mungaroo Formation is interpreted to consist of low-sinuosity river, levee bank and overbank deposits in Zeepaard 1 (Esso Australia Ltd, 1981). Several point-bar deposits and a 57 m thick stream sequence with some good quality reservoirs were intersected in Resolution 1(Esso Australia Ltd, 1980). However, porosity reduction due to siderite, calcite and pyrite cements and abundant kaolinitic clay matrix is frequently observed in these sandstones. The Triassic reservoir is probably too deep across most of the Release Area W11-15 to be a good reservoir (Figure 5b) The Upper Jurassic Dupuy Formation was recognised as a reservoir within the Eskdale structure (Figure 4). It has a reservoir quality and sediment accumulation strongly influenced by fluvial discharge. The Eskdale Member (Dupuy Formation) is a 36.9 m thick gas- and oil-bearing sandstone succession at Eskdale 2 with an average porosity of 27.8% and an average clay content of 7.2%. It is composed of a fine-grained subarkose with mainly quartz, K-feldspar, unaltered/chloritised faecal pellets and argillaceous intraclasts. Similar reservoirs may exist in the northwestern part of the Release Area, close to the Resolution Arch structure. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 15 of 24 The Early Cretaceous Barrow Group equivalent (Macedon Member, Pyrenees Member, Barrow Group sandstones) host significant hydrocarbon accumulations throughout the Exmouth Sub-basin and extend into the Release Area (Figure 4). The Macedon Member sandstone is interpreted to have been deposited as turbidites or mass flow deposits and has very good reservoir properties, with a net:gross ratio of 81% and the net sands having an average porosity of 26.7%. It hosts a gross 21.9 m oil column at Enfield 1 (Woodside, 1999). However, the 13.1 m gross (13 m net) Macedon Member sandstone of excellent reservoir quality is water saturated at Eskdale 2 (BHP Billiton, 2005). The shallow marine, uppermost Tithonian to lower Berriasian Pyrenees Member of the Barrow Group was intersected in Crosby 1 (BHP Billiton, 2004) and the pay interval has an average porosity of 29.3% with an average water saturation of 21.2%. The Pyrenees Member sandstones are good reservoirs throughout the interval at West Muiron 5 with a gas-bearing layer, with a net:gross ratio of 91%, a high average porosity of 29% and an average water saturation of 15%, overlying an oil bearing layer with net:gross of 70%, a high average porosity of 25% and an average water saturation of 45% (BHP Petroleum, 1994b). The Pyrenees Member sandstones at Crosby 1 are clean and variably argillaceous/sideritic quartzarenites and subarkoses grading to argillaceous, subarkosic sandstones with increasing depth. Most sands have been little affected by diagenesis and are consequently unconsolidated with well preserved intergranular porosity and good permeability (BHP Billiton, 2004). The Altair Sand is unique among the Barrow Group sands encountered in Altair 1 well and extends into the northern portion of the Release Area. These sands have good reservoir properties with significantly higher porosity than the Barrow Group sands above and below (25% compared to 13% and 18%), a property due to the near absence of quartz overgrowth cement (West Australian Petroleum Pty Ltd, 1995). This sand contains formation water less saline than the water occurring in the Barrow Group sands above and below and is significantly isolated, preventing equilibration of pore fluids with those in surrounding sands (West Australian Petroleum Pty Ltd, 1995). The Barrow Group sandstones are good reservoirs, composed predominantly of quartz grains, weakly cemented by siderite and pyrite, with a small amount of clay matrix and log-derived porosities in the 20-26% range (Esso Australia Ltd, 1980). In Coniston 1 (BHP Petroleum Pty Ltd, 2001) the Barrow Group is a massive quartz sandstone 88 m thick with excellent reservoir quality, with average porosities of 27% and permeabilities calculated at 4,565 mD and measured in core at 3,547 mD. Overlying the Zeepaard and Birdrong formations are good quality reservoir sands with a high net:gross ratio (Figure 4). These reservoirs were encountered in York 1 well, located northeast of the Release Area, however 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 16 of 24 they are 100% water saturated at this location (BHP Petroleum Pty Ltd, 1994c). The overlying Mardie Greensand Member, commonly a hydrocarbon thief zone, hosts a gas and oil column at Enfield 1 (Figure 4). This unit is also an excellent reservoir, with a net:gross ratio of 80.4% and the net sands having an average porosity of 29.5% (Woodside, 1999). It is composed of thin, moderately hard, glauconitic sandstone, dark green to medium grey, with poorly sorted, very fine to coarse grains (up to 85% glauconite with 30% clay, 10% silt and traces of pyrite). Only 1.6 m of Mardie Greensand Member is recognised in Coniston 1 where it is composed of moderately silica-cemented silty claystones and glauconitic sandstones (BHP Petroleum Pty Ltd, 2001). This reservoir unit at Resolution 1 is composed of friable glauconitic sandstone. Seals Both regional and intraformational seals are present in the Release Area. The Lower Cretaceous Muderong Shale is the regional seal across the Exmouth Sub-basin (Figure 4). Interbedded claystones within the deltaic sequences of the Triassic Mungaroo Formation, the Dingo Claystone and delta front siltstones of the Lower Cretaceous Barrow Group (Macedon Mudstone) have the potential to form intra-formational seals. The Eskdale discovery proves the viability of lateral pinch-out seals for the Tithonian canyon plays in the subbasin. The Lower Gearle Formation can also provide a seal (West Muiron 3). Generation and Expulsion The complex history of hydrocarbon charge in the Exmouth Sub-basin has been discussed by Tindale et al (1998) and Smith et al (2003). Some hydrocarbon generation from the gas-prone Mungaroo Formation system and older Triassic source rocks presumably occurred in the Exmouth Sub-basin during the Jurassic with the deposition of kilometres of Dingo Claystone and other sediments in the main depocentre. In the Early Cretaceous, generation from the Triassic source rocks may have extended across the sub-basin and onto parts of the Exmouth Plateau. Burial by the Barrow delta sediments, possibly coupled with elevated heatflow related to continental breakup, may have been sufficient to push suitable source rocks into the gas and oil generation window. Generation and expulsion from the Upper Jurassic Dingo Claystone oil source rocks commenced in the Early Cretaceous in response to loading by the Barrow delta system. Modelling suggests that the initial hydrocarbon charge from the southern and central Exmouth Sub-basin occurred as early as Late Tithonian and Berriasian, prior to the deposition of the regional Muderong Shale seal. The modelled peak expulsion occurs prior to the interpreted Late Berriasian faulting episode. Stratigraphic trap formation is interpreted to have occurred prior to peak expulsion, with the Late Berriasian faulting providing post-charge re-configuration and possible conduits for additional hydrocarbon charge (or leakage). The presence of hydrocarbons in the primary objective in 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 17 of 24 Eskdale 2 indicated that the prospect had access to a potentially significant volume of mature hydrocarbons. MDT fluids from the Eskdale 1 residual zone appeared not to be biodegraded. Traps reliant on Muderong seals may have been charged from the northern part of the sub-basin where generation occurred from the Hauterivian to mid-Cretaceous. Campanian inversion (growth of the Novara Arch) and uplift terminated hydrocarbon generation from the Jurassic source “kitchen” (Smith et al, 2003), re-ordered migration pathways, formed new traps and kept oil reservoirs shallow and cool, maximising the risk of biodegradation. The progradation of the Miocene to Holocene carbonate wedge over the eastern part of the Exmouth Sub-basin has produced a late gas charge which has mixed with biogenic gas in accumulations located to the east of the Release Area. Play Types The Seismic line 110/12 on Figure 5b illustrates proven and potential petroleum plays within the Release Area. The proven traditional Triassic fault block play (Figure 5b, play type 1), which hosts most of the hydrocarbon reserves in the Northern Carnarvon Basin, is adjacent to the Release Area, to the west and east. Mungaroo Formation sandstones in fault block traps are sealed by either the Dingo Claystone or intraformational seals. Gas sands interpreted in the Mungaroo Formation equivalent from electric logs at Zeepaard 1, north of the Release Area W1115, is an example of this play type. Confined (Eskdale) and partially confined channels have been proven to contain hydrocarbons within the Jurassic sandstones plays (Figure 5b, play type 3). The Eskdale 2 trap is a complex structural/stratigraphic channel trap beneath the collapsed core of the north-northeast-oriented late Berriasian Resolution Arch. Similar traps with Jurassic sandstones may exist within the Release Area. Complex combination structural-stratigraphic trap within the Lower Barrow Group represent the major producing traps for the oil province within the subbasin (Figure 5b, play type 4). A similar prospect is interpreted within the Release Area, in the lower Barrow Group sandstone oil play, sourced from the Dingo Claystones, sealed by Muderong Shale or inter-bedded claystones. The Tomcat prospect, located in the central and southern part of the Release Area, was formerly known as Ponsonby has been reassessed by Octanex and presented in the RPS report (Octanex, 2009). The reassessment suggests that, due to its depth of burial, the Tomcat Prospect is likely to be a gas and possibly condensate play. It was identified from a group of seismic amplitude anomalies, which are interpreted to represent hydrocarbon-bearing, Cretaceous, deep-water channel and fan sandstones. The prospect has been mapped using data from a high quality 3D seismic survey acquired during the HCA04 3D seismic survey. This mapping identified and mapped two seismic reflectors at the level of the anomalies. Depth structure mapping at these levels shows these horizons generally plunge towards the north-east with no independent structural closures. The extent of the prospect is defined by 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 18 of 24 seismic amplitude anomalies at and below these mapped horizons. The two target levels are separated by a thin stratigraphic interval interpreted from seismic to be shale (Octanex, 2009). Trapping potential is also confirmed by the presence of a hydrocarbon column within the Mardie Greensand Member (Enfield 1) which may also exist within the Release Area (Figure 5b, play type 5). Play types based on Lower Cenozoic sands have proven to be viable elsewhere in the adjacent Barrow Sub-basin (Maitland field; Figure 5b, play type 6). Critical Risks For the Mungaroo Formation play, charge and reservoir quality are the main risks. The gas charge is considered to be locally derived from the underlying Triassic sequence. Diminished reservoir qualities due to diagenetic overprinting are another risk. However, the application of amplitude analysis with 3D seismic coverage can image gas within reservoirs and improve success rates. The depth to the Triassic across much of the Release Area limits this play type. Risk for the Jurassic play (where sand is found within the shale-prone sequence) is suggested to be related to capillary breach or burial and both could have led to the residual column in Eskdale 1 (BHP Billiton, 2005). The greatest risks for the Barrow Group plays are believed to be associated with the effectiveness of the seal and to a lesser extent the reservoir effectiveness. In the Tomcat prospect, the degree of connection between the sand bodies within unconfined fan deposits at the end of a channel feeder system and the sealing capacity of potential seal sediments contribute to a high seal risk. Assuming the seals are present, the ability of the seal to retain large columns of gas is still a risk. For these sandstones, their depth of burial is such that reservoir quality may have been reduced due to diagenetic processes. The GPoS (Geological Probability of Success) of the Tomcat prospect falls within the high risk category (Octanex, 2009). Risks for this play that have been identified in the wider Exmouth Sub-basin also apply, including hydrocarbon charge, with migration required from the north of the Release Area, and biodegradation of an early oil charge. Smith et al (2003) noted that these risks can be mitigated by high quality reservoirs that allow viscous oil to flow, and that less biodegraded oil will be hosted in the deeper and hotter reservoirs, beneath seals other than those of the Muderong Shale. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 19 of 24 FIGURES Figure 1: Location map of Release Area W11-15 in the Exmouth Subbasin, Northern Carnarvon Basin. Figure 2: Graticular block map and graticular block listings for Release Area W11-15, in the Exmouth Sub-basin, Northern Carnarvon Basin. Figure 3: Structural elements of the Exmouth Sub-basin over a residual Bouguer gravity map (from Morse, 2010), and showing Release Area W11-15, key discoveries and location of seismic section 110/12 shown on Figure 5a. Figure 4: Generalised stratigraphy of the Exmouth Sub-basin based on the Northern Carnarvon Basin Biozonation and Stratigraphy Chart (Nicoll et al, 2010). Geologic Time Scale after Gradstein et al (2004) and Ogg et al (2008). Major accumulations are shown. Figure 5a: AGSO seismic line 110/12 across the Exmouth Plateau and Exmouth Sub-basin. Wells, Release Area and inset location for Northern Carnarvon Basin EXMOUTH Sub-basin RELEASE AREA Figure 5b are indicated. Location of the line is shown on (Figure 3). Figure 5b: Enlargement of AGSO seismic line 110/12 showing conceptual petroleum plays across the northern Exmouth Sub-basin and in the Release Area. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 20 of 24 REFERENCES APACHE ENERGY, 2008—Beg-1, WA-357-P, Well Completion Report, Basic Data, issued November 2008. ARTICLES HUB, 2010—[Web page] Aarkstore Enterprise - Stybarrow Project, Australia, Commercial Asset Valuation and Forecast to 2016 http://www.articleshub.org/article/18565/Aarkstore-Enterprise---StybarrowProject,-Australia,-Commercial-Asset-Valuation-and-Forecast-to-2016.html (last accessed 25 October 2010) ARDITTO, P.A., 1993—Depositional sequence model for the post-Barrow Group Neocomian succession, Barrow and Exmouth Sub-basins, Western Australia. The APEA Journal, 33 (1), 152–160. BAILEY, W.R., UNDERSCHULTZ, J., DEWHURST, D.N., KOVACK, G., MILDREN, S. 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WALKER, T.R., 2007—Deepwater and frontier exploration in Australia – historical perspectives, present environment and likely future trends. The APPEA Journal 47(1), 15–38. WEST AUSTRALIAN PETROLEUM PTY LTD, 1995—Altair 1, WA-213-P, Carnarvon Basin, Well Completion Report, Interpretive volume, September 1995. WOODSIDE, 1999—Enfield-1, WA-271-P, Well Completion Report, Interpretive data, November 1999. Front page image courtesy of Petroleum Geo-Services. 2011 Release of Australian Offshore Petroleum Exploration Areas Release Area QQ11-15, Exmouth Sub-basin, Carnarvon Basin, State Release Area Geology Page 24 of 24