another interesting read

Another interesting item on exploration for oil in Bass Strait.

This article was written by a Govt Department, so there should be less hype in it compared with comany prospectuses.

I like the parts that state that previous failures (dry holes) can be attributed to errors in applying siesmic depth conversion - BAS has proven its accuracy already in the current drill by only being 10 meters out in its prediction.

Another nice part to read is in the section on release area V97- G3 in which the Nannygai-1 well was drilled in 1972. This section makes statements such as "source rich units are mature for oil generation" ,and "a fault independent closure".

Reading this article has made me feel more at ease while waiting for the "big" announcement.

Best of luck.

TDF

Cover: VIMP Report 42 Hydrocarbon plays and play fairways of four vacant offshore Gippsland Basin areas, 1997 acreage releaseVictorian Initiative for Minerals and Petroleum Report 42Released: May 1997. Price: $165To order this item download the Petroleum Reports and Data order form here.

Executive summary

The offshore Gippsland Basin is Australia's premier petroleum province with reserves exceeding four billion barrels of crude/condensate, and 9 TCF of natural gas. Its production meets 70% of Australia's crude oil needs and nearly all of Victoria's gas requirements. The basin has an unknown volume of hydrocarbon source rocks since few wells reach mature units or economic basement. For the most prospective reservoir, the top of the Latrobe Group, hydrocarbon pays exist wherever valid closures have been drilled. The basin enjoys an anomalously high discovery strike rate, with inaccurate seismic depth conversion as the main cause of dry holes. Current approaches to pre-stack depth conversion alleviates this risk factor.

Four areas in the basin, V97-G1, G2, G3 and G4 are being released for exploration. These range in size from 754 to 2364 sq km. As indicated in Enclosure (Enc. 1), these are sparsely drilled, and proximal to significant discoveries. Previous explorers have mapped numerous prospects and exploration leads in the Release Areas from good quality seismic data - some at the historically low risk top Latrobe level. The technical merit of these, plus several stratigraphic plays are presented in this document. Some of the stratigraphic plays are potentially large, and yet to be explored in the basin. The Release Areas lack seismic time interval (isochron) mapping. These assist in delineating onlap depositional limits, and constrain the palaeogeographic mapping between wells by revealing the palaeotopography. As such, the Release Areas are considered to have a significant unrealised petroleum potential.

Release Area V97-G1 covers approximately 754 sq km and is in water depths of 20 to 80 m. It has a 0.5 to 2 km seismic grid, two dry holes (drilled in 1970 and 1982), and a 1981 oil discovery well (West Seahorse-1). The area is in the northwest part of the basin where several regional down-to-basin normal faults create a fault truncation play fairway which is sparsely explored (Enc. 1). Plays in this fairway involve Late Cretaceous Golden Beach Group reservoir sandstones and interbedded seals, juxtaposed against tight sections of Early Cretaceous Strzelecki Group and Palaeozoics. Seismic data indicates several closures, the largest of which, Triggerfish, has potential reserves of 160 MB (million barrels). However, the Orca Prospect in the Release Area is considered to have the lowest exploration risk with closure mapped at the top of the Latrobe Group. Its combined reserves could exceed 24.7 MB.

Seven other prospects and leads are mapped from seismic data in V97-G1. In addition there are stratigraphic plays involving laterally restricted intra-Latrobe sandstones such as encountered by West Seahorse-1 where a 7 m oil column flowed 1827 BOPD of 48 API crude. The find confirms that the V97-G1 area is charged with high quality oil. An extension of the West Seahorse discovery is considered to cross the Triggerfish Lead giving it dual objectives.

V97-G1 also straddles the depositional limit of several lower Latrobe Group interbedded sandstones and seals. This play fairway remains untested, and coincides with many of the mapped structural plays giving them a secondary objective. The V97-G1 area has had a prevailing dip towards the basin's depocentre where oil and gas are considered to have been generated for the discoveries to date in the region.

Release Area V97-G2 is the largest area on offer with approximately 2364 sq km. It has general water depths of 100 to 200 m, increasing rapidly to exceed 500 m in its southernmost part (refer Enc. 1). The area has a 0.5 to 4 km seismic grid and two dry holes, drilled in 1973 and 1982. Excluded from V97-G2 is a Retention Lease covering the Sole Field which contains dry gas reservoired at the top of the Latrobe Group.

Four structural prospects and leads are mapped from seismic data in V97-G2. Two of these, Scorpion and Barramundi, are top Latrobe Group closures considered to be fault independent by the previous permit holder. The others involve the Golden Beach Group truncated by a regional down-to-basin normal fault, and juxtaposed against tight Early Cretaceous Strzelecki Group. Basal and intra-Latrobe Group sandstones with interbedded seals provide secondary objectives for these plays.

A potentially large basin edge play, Northright, is indicated by regional seismic control in the northern part of V97-G2. The play involves the depositional and faulted limit of a 26 m intra-Latrobe sandstone which is sandwiched between thick shales. The occurrence of oil fluorescence throughout this interval at the Sole-1 well suggests that oil has been displaced updip from the Sole closure into the Northright play.

All prospects and leads mapped in V97-G2 are open to downdip mature source areas to the south. The Basker-1 well, 14 km southwest of the Release Area, flowed 5720 BOPD of 42 API crude plus 5 MMCF/D from an intra-Latrobe sandstone (refer Enc. 1 for well location).

Release Area V97-G3 is approximately 1876 sq km and has water depths of 50 to 80 m. It has a 0.5 to 4 km (2D) seismic grid, plus partial 3D coverage. The Release Area has 10 abandoned wells, drilled from 1969 to 1990. Of these, Omeo-1 recovered gas and traces of oil in an RFT programme. The others are considered to lack valid closure at the tops of their reservoir objectives (original mapping was done at the top of an overlying seal in some cases).

V97-G3 has challenging depth conversion problems, and a varying thickness of top Latrobe Group seal (Gurnard Formation). The area lacks evidence of pre-stack depth conversion being applied. The mapped closures are regarded here as exploration leads, and six of these are discussed in this report. The Stargazer Lead is an intra-Latrobe fault dependent closure mapped updip of the Nannygai-1 well which encountered intra-Latrobe oil shows over a 16 m interval. The lead is 7 km from the giant Kingfish top Latrobe oilfield (Enc. 1).

Below the TD of the Gurnard-1 well, a faulted dome is mapped at the base Latrobe/top Golden Beach Group level. The hydrocarbon recoveries at the Omeo-1 well occur across this stratigraphic interval.

Lobster-A is a top Latrobe lead in the southern part of V97-G3 (Enc. 1). It involves rollover into the down side of a regional normal fault along the basin's southern margin. In contrast with two earlier wells targeting a similar play in this sector of the basin, this feature has some fault independent closure. Adjacent to this lead is the Lobster-B Lead - a potentially large stratigraphic trap. It occurs at the onlap depositional limit of a 50 m intra-Latrobe sandstone, against the basin margin palaeotopography. The play has similarities to the 63 MB (in-place) Wapella oilfield, located along the eastern margin of the Western Canadian Sedimentary Basin.
Previous explorers discounted the prospectivity of this part of the basin since it lacks suitable migration paths from the basin's depocentre to the north. However, detailed recent work (presented here) indicates that source-rich units are mature for oil generation within the Release Area, in a syncline along its northern part. Plays such as Lobster are directly updip of this local kitchen.

Release Area V97-G4 is approximately 870 sq km and has water depths of 80 to 200 m, increasing beyond 300 m along its eastern edge (Enc. 1). It has a 0.5 to 1 km (2D) seismic grid, plus a reprocessed 3D survey and parts of other 3D surveys. The Release Area has seven abandoned wells, drilled from 1972 to 1990. Three of these discovered oil and gas/condensate accumulations in basal Latrobe and Golden Beach Group sandstones. These have been interpreted as subeconomic by the previous permit holder. The Anemone-1A discovery is estimated to have proved and probable GIIP of 11 BCF plus 0.6 MB of condensate, while the nearby Archer-1 well has 7.6 MB of oil-in-place plus 66.9 BCF of GIIP. These fields have numerous pay zones, with hydrocarbon-water contacts projected from RFT data. No follow-up drilling was conducted on these, and all dry wells in the area are considered to be beyond valid closures.

Nine structural closures are mapped as prospects and leads in V97-G4. Most of these are down-side fault rollover features as tested by Anemone-1A and Archer-1. The trapping of hydrocarbons at these two wells confirms that oil and gas has migrated into V97-G4, and that fault plane leakage is not an exploration risk factor.

The Southwest Kingfish Prospect is the largest structural play in V97-G4. It is a four-way dip (fault independent) closure mapped at several intra-Latrobe horizons and the top of the Golden Beach Group. This play is estimated to contain up to 117 MB of recoverable oil.

The Whitepointer Lead is a potentially large stratigraphic trap across the southeastern part of V97-G4. The basal Latrobe sequence has a facies change from aggrading marshy seal units on the west side of the Release Area, to shoreface reservoir sandstones on the east side. Trapping in the updip direction (to the south) is facilitated by the onlap depositional limit of the sequence, while top sealing is provided by a marine highstand shale. This overlying seal unit exceeds 100 m in thickness in nearby wells, and onlaps beyond the reservoir's depositional limit to the south. The play could be explored by testing one of several structural plays mapped within the Whitepointer Lead area itself. The oil accumulations discovered to date in V97-G4 (Hermes-1 & Archer-1) occur within the objective reservoir area of the Whitepointer play. These pay zones in the Archer-1 well could actually define the play's lowest known hydrocarbons.

Bibliographic reference: Chiupka, J.W., Megallaa, M., Jonasson, K.E. & Frankel, E., 1997. Hydrocarbon plays and play fairways of four vacant offshore Gippsland Basin areas, 1997 acreage release. Victorian Initiative for Minerals and Petroleum Report 42. Department of Natural Resources and Environment.


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