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U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY


TOTAL PETROLEUM SYSTEMS OF THE NORTHWEST SHELF, AUSTRALIA:

THE DINGO-MUNGAROO/BARROW AND
THE LOCKER-MUNGAROO/BARROW

by

Michele G. Bishop1

Open-File Report 99-50-E










Exploration History
Oil was discovered onshore at Rough Range field in 1955 on the North West Cape in the deltaic Birdrong Sandstone (Fig. 4)(Table 1). Subsequent surveys and mapping by the Geological Survey of Western Australia were extended offshore to include Barrow Island and the Dampier Archipelago. Exploration activity increased in the early 1960’s and Barrow-1, drilled in 1963, was the first oil discovery in the Northwest Shelf Province. Barrow-1 tested oil, gas, and condensate from Upper Albian to Callovian rocks of a faulted anticline. Detailed water-depth maps were used to target ridges offshore. In 1967 the Ashmore-1 was drilled 350 km from the coast and was dry. Legendre-1, 180 km NE of Barrow Island, found noncommercial oil in Lower Cretaceous deltaic deposits of the Neocomian Barrow Group. North Rankin-1, drilled in 1971, was a giant gas and condensate discovery in Late Triassic, marginal-marine Mungaroo Formation. Subsequently, Angel field was discovered, finding gas and condensate with oil shows in a Late Jurassic marine interval. In 1972, Goodwyn-1 discovered gas and condensate in a Lower Jurassic channel fill and oil in an Upper Triassic fluvial interval.

This encouraging start in a remote location with no infrastructure, technical challenges, and high finding costs was dampened in 1972 when the Labor Party won the Federal election and the new Minister for Energy declared an embargo on hydrocarbon exports. The effect was to slow exploration in Australia for several years (Purcell and Purcell, 1988).

Excitement again was high in 1979 for the Northwest Shelf Province when Scarborough-1 found an estimated 15 trillion cubic feet of gas (TCFG) accumulation in Cretaceous deep-water sands on the Exmouth Plateau in 912 meters of water. From 1979 to 1984, five gas fields, four oil fields, and three oil and gas fields were discovered. These include discoveries in Cretaceous marginal marine and deltaic strata, Triassic fluvial strata, and Cretaceous deep-water strata. Gas production commenced in 1984, Harriet field came online in 1986, and North Herald and South Pepper fields in 1987

PETROLEUM OCCURRENCE
Mature source rocks of the Dingo-Mungaroo/Barrow 394801 TPS are located in the Exmouth, Barrow, and Dampier sub-basins (Fig. 2). Accumulations of this primary system occur 1) within the sub-basin trend, 2) directly adjacent to the sub-basins in high, tilted fault blocks (such as the Rankin Platform trend on the eastern edge of the Exmouth Plateau, and at the Leatherback discovery, offshore North West Cape), and 3) in strata that regionally overlie the sub-basin trend.

Mature and over-mature source rocks of the Locker-Mungaroo/Barrow TPS occur across the entire area (Fig.1). Accumulations are found in 1) tilted fault blocks and sediments overlying tilted fault blocks, of the Exmouth Plateau, 2) structures along the southern and eastern flanks of the sub-basin trend, and 3) on the inner structural terraces adjacent to the stable continent.

In Province 3948 there are approximately 14 oil and gas fields, 7 gas fields and 10 oil fields with reservoirs in the Neocomian Barrow Group. Jurassic reservoirs account for another 11 fields and other Cretaceous reservoirs an additional 9. The Rankin Platform trend and Exmouth Plateau account for fields containing estimated reserves of 7 billion barrels of oil equivalent (BBOE) (13% liquids), compared to 1.1 BBOE (45% liquids) for the sub-basins and 0.9 BBOE (69% liquids) for the eastern inner structural terraces (Kopsen, 1994).

Fields and discoveries of the Northwest Shelf are classified in the literature as "pre main-unconformity" or "post main-unconformity" with reference to the hydrocarbon reservoir. The main unconformity, also known as the breakup unconformity (Fig. 3 and Fig. 4), was a regional Callovian (Jurassic) event when widespread erosion occured on tilted fault blocks of the high Exmouth Plateau, the rifted and subsiding sub-basins, and the inner structural terraces. This unconformity was the result of a prolonged lowstand of sea level associated with a westward shift of continental rifting and the initiation of sea floor spreading in the Argo Abyssal Plain (Stagg and Colwell, 1994; Barber, 1994).

Pre-main unconformity
Pre-main unconformity proven reserves are approximately 505 million barrels of oil (MMBO) and 7.5 TCFG and are found primarily in Late Triassic Mungaroo Formation on the Exmouth Plateau/Rankin Platform trend, inner structural terraces and high fault blocks (Delfos and Dedman, 1988). Drilling indicates only the first few fault blocks on the eastern edge of the Rankin Platform trend portion of the Exmouth Plateau have hydrocarbon accumulations. Fault blocks further back into the Plateau have proven to be sub-commercial or dry supporting the theory of hydrocarbon migration out of the Jurassic section of the adjacent sub-basins, updip along and under the main unconformity and into Triassic reservoir rocks of the Rankin Platform trend.

There is, however, a gas-condensate discovery that may be important in determining source rock maturity and hydrocarbon migration. North Rankin West field with two wells is located west of the North Rankin field, which is in the first fault block adjacent to the Dampier sub-basin. It is fault separated and downdip of North Rankin field, which produces from Triassic Mungaroo in a tilted fault block under the main unconformity and is sealed by Muderong Shale. The North Rankin West reservoir rocks were deposited in a local graben on the Exmouth Plateau as Jurassic Bathonian age, high energy, nearshore sands. The reservoir is beneath the main unconformity and is sealed by Muderong Shale. A gas column of at least 130 m has produced at flow rates up to 180 million cubic feet per day (MMCFD) since 1991 and the structure is thought to be filled to spill point (Tilbury, 1994 and Tilbury and Barter, 1992). Unlike other reservoirs of the Rankin Platform trend, this accumulation is not of a configuration or in a position favorable to be filled by hydrocarbons from mature Jurassic Dingo Claystone in the sub-basin trend.

Gas-condensate shows and oil shows in pre-main unconformity reservoirs are present on the Exmouth Plateau in Vinck-1, Sirius-1, Investigator-1, Jupiter-1, Zeewulf-1, and Resolution-1. Jurassic shales, deposited in individual fault blocks or fault-block trends, are thin and immature or absent and, consequently, these hydrocarbons are sourced by Triassic rocks.

A Rankin Platform trend type fault-block trap was drilled in 1991 offshore of the northern tip of the North West Cape. In this trap, the Leatherback-1 flowed 2332 barrels of oil per day (BOPD) from an active oil column of 6 meters overlying a residual oil column of 156 meters in the Mungaroo Formation. This discovery is on the east margin of the Exmouth sub-basin and may be sourced also by Kimmeridgian Upper Dingo Claystone (Bauer and others, 1994).

Post-main unconformity
Proven reserves from post-main unconformity fields are estimated at 1618 MMBO and 41 TCFG, occur across the entire area, and are sourced by both Triassic and Jurassic source rocks. The giant 15 TCFG Scarborough discovery and the >1 billion barrels of oil (BBO), 475 billion cubic feet of gas (BCFG) Barrow Island field are examples. Interpreted depositional environments of Jurassic age reservoir rocks include deep-water fan, transgressive marine, and deltaic. The most important post-main unconformity reservoirs are deltaic and deep-water deposits of the Cretaceous Barrow Group. Younger reservoirs have been sourced by migration along faults that breached the regional seal, as in Barrow Island field where the major producing horizon is the Cretaceous Windalia Sandstone Member of the Muderong Shale (Fig. 3).

In the Dingo-Mungaroo/Barrow TPS 394801, oil gravity of post-main unconformity fields ranges from 19° -50° API. In pre-main unconformity fields the gravity range is 36° -49° API. These hydrocarbons are attributed to Jurassic source rocks. Barrow Island field crude is termed "sweet and light" (Miyazaki, 1989). Highly degraded oil and oil above gas relationships are also present (Kopsen and McGann, 1985), indicating multiple episodes of migration.

 


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U. S. Geological Survey Open-File Report 99-50E