Gordon Wakelin-King
Pacific Oil & Gas Pty Ltd. Alice
Springs, N.T.
Recent seismic surveys and drilling have opened up a new and regionally extensive play in the Upper Proterozoic of the Amadeus basin, central Australia.
The 1 Magee well drilled by Pacific Oil & Gas Pty. Ltd. in late 1992 intersected a thin wet gas zone in the previously undrilled Heavitree quartzite (870 million years old). While the thin pay zone and low production rate prevented the discovery well from being commercial, the vast untested regional extent of the reservoir and sealing evaporite, together, represent a new opportunity, in the basin.
REGIONAL GEOLOGY
The Amadeus basin (Table 1) occupies the southern part of the Northern Territory of Australia (Fig. 1).
The basin was initiated and developed during the late Proterozoic in an intracratonic setting. The late Proterozoic units are regionally extensive and have correlatives in adjacent basins, leading some geologists to postulate the existence of a superbasin across central Australia at this time.
Deposition became more localized during the Paleozoic as the basin became divided into several depocenters. The current form of the Amadeus basin is strongly asymmetric with a deep trough, abruptly truncated in the north and a gently shallowing platform in the south.
Proterozoic rocks are generally very deeply buried in the northern trough but come within drilling depth south of an arcuate line marking the southern limit of the Paleozoic depocenters (Fig. 1). The Proterozoic sequence consists of a basal shallow marine transgressive sandstone, the Heavitree quartzite, which appears to thin southward and which onlaps basement paleotopographic highs. This is overlain by a thick evaporite/carbonate/shale unit called the Bitter Springs formation.
Unconformably overlying this are clastic units composed of glacial and marine sequences locally differentiated into various formations, but here referred to as the Upper Proterozoic clastics.
An unconformity at the top of the Upper Proterozoic heralds the development of more localized deposition in which the Ordovician sequence hosting the current Amadeus basin hydrocarbon production was deposited. Deformation during the Cambrian and the Carboniferous caused intense structuring driven by movement within the Bitter Spring salt.
EXPLORATION HISTORY
The Amadeus basin has had a checkered exploration history.
Spectacular surface anticlines have attracted explorers since the late 1950s and continue to grace the pages of remote sensing textbooks worldwide.
Disappointingly however, most of them proved to be scalped to below the Ordovician source/reservoir sequence that has been the traditional target in the basin (Fig. 1). The few that remain intact host prolific Palm Valley gas field and Mereenie oil and gas field, the largest single reserve in onshore Australia.
Since exploration commenced only 33 wildcat wells have been drilled. In all cases the presence of thick salt in the Upper Proterozoic Bitter Springs formation deterred explorers from venturing deeper. The thick and internally deformed halite proved impenetrable to seismic, and it was recognized early that the Paleozoic anticlines were salt influenced and therefore surface structure could not predict structure below the salt.
After a diverse beginning, exploration then became focused on a narrow area in the north of the basin where the Ordovician is preserved.
PACIFIC'S EFFORT
Proterozoic oil has been noted many times worldwide with the best known examples being the Siberian platform in eastern Siberia and the South Oman basin. Pacific Oil & Gas, as part of an Australia wide review of Proterozoic sedimentary basins, noted the significance of regional sealing evaporites in "ancient" petroleum provinces and identified the Bitter Springs formation salt sequence in the Amadeus basin as possessing similar qualities.
The large area available for tenement acquisition and the regional extent of the Proterozoic stratigraphy also made the Amadeus basin an attractive proposition for exploration. Work commenced in the Amadeus basin in 1989, and the Heavitree quartzite was targeted as the reservoir.
No preserved source material could be identified in the deeply weathered outcrops of the overlying, stromatolitic Bitter Springs formation; however, based on geological reasoning it was felt that the transition from shallow open marine to a restricted hypersaline environment should have included a phase of accumulation of organic material.
Exploration commenced using regional gravity data and surface geology to target possible structures at Heavitree quartzite level. Some 103 km of seismic were acquired and one well, 1 Murphy, was drilled in 19901991 on what was believed to be a structure at reservoir level.
The interpreted "basement" turned out to be an intrasalt reflector. The strong internal deformation which resulted in energy attenuation within the salt had prevented seismic penetration to the reservoir level beneath the salt.
The well was terminated short of its target, leaving the play concept untested and a question mark on its explorability in the basin. Various geophysical techniques including magnetotellurics and regional aero-magnetic surveys were carried out but identification of basement structure remained elusive.
During this period, Pacific was part of an information exchange program with a Russian state enterprise NPO Sibgeo, which was involved in the Russian success in Proterozoic rocks of the East Siberian basin. NPO Sibgeo specialists were contracted to interpret all existing data including satellite imagery and aerial photographs, and this work resulted in the definition of an area in the east of the basin as a potential uplift or "buried hill."
The eastern part of the Amadeus basin was attractive to Pacific already, as thermal maturity studies from Upper Proterozoic rocks in the unsuccessful 1 Murphy suggested that the south central part of the basin was substantially over-mature. The airphoto anomaly defined by Sibgeo provided the impetus for seismic acquisition in this new area.
In complete contrast to previous results, the seismic in the east of the basin provided a reliable image of the stratigraphy, and most importantly, of the basement. The structure identified by the Russian team proved to be formed above the Bitter Springs salt and of no immediate interest for the Heavitree quartzite play; however, a tie line to the only nearby well identified a fault related anticline with both basement and the Heavitree quartzite clearly involved.
At this point, with four years devoted to the project and without any clear indications that the play was valid, it was decided to drill the structure as a concept test on one line of seismic. Some degree of comfort of closure was provided by a 1964 single fold seismic survey on which the nearby well, 1 Mt. Charlotte, had been drilled. This well, like all the others, had terminated in the Bitter Springs salt. Pacific's exploration effort is summarized in Table 2 and Fig. 2.
DRILLING
Drilling in the Amadeus basin is often made arduous by the extreme induration of the rocks, particularly the sandstones. Penetration rates of around 1 m/hr and 30 m bit runs are not uncommon. The rocks do respond well to air drilling.
Development drilling at Mereenie and Palm Valley fields is routinely carried out with air. A drilling plan for 1 Magee was developed based around air and air hammer drilling to a TD of 2,500 m in 6 in. hole. Formation damage was also a factor in this choice of technique as the Heavitree quartzite was considered likely to have low permeability, and to be possibly fractured.
Air drilling proved to be highly successful, with an average penetration rate of over 10 m/hr being achieved. Drilling economy was also assisted by favorable geology in the eastern section of the basin where most of the indurated sandstones are not present.
RESULTS
The sequence below the salt in 1 Magee proved to be composed mainly of dolomite and anhydrite, becoming shaly toward the base.
The Heavitree quartzite was intersected at 2,342 m KB. Gas flowed to surface at a stabilized rate of 63.1 Mscfd. (Fig. 3).
The extraordinarily high helium content (Table 3) seems to confirm the integrity of the seal since the Proterozoic.
Considering the high C2 + content it is surprising that no condensate was produced during two days of open hole flow. This may be explained by the extremely low flowing pressure of 38 psi at surface. It is thought that some condensate has precipitated out at the sandface during testing, thus exacerbating the already low permeability of the reservoir. The gas composition is similar to most oil-and-associated-gas reservoirs, particularly when the probable algal (Type 1) source, and the likelihood of heavier ends being left in the formation due to the draw-down across the sandface, are considered.
SOURCE ROCK
An interval of 20 m of black shale above the reservoir was found to be bitumen stained.
From this bitumen an MPI of 1.02 was derived demonstrating that the sediments are still within the oil window.
No primary source material was identified at the 1 Magee location indicating that the structure was charged by migration from an area deeper in the basin, probably to the west.
PROBLEMS, OPPORTUNITIES
The main obstacle to the commerciality of the discovery is the very thin reservoir intersection. The 3.6 m of net "pay" recorded is insufficient to justify or technically support stimulation.
The Heavitree quartzite is not usually this thin. Outcrop thicknesses in excess of 100 m are usual, thus raising the possibility of the sandstone thinning over the structure. It is also probable that the reservoir is pinching out toward the basin margin location of 1 Magee.
This fact alone may indicate the potential for a major stratigraphic accumulation beneath the regional salt seal. Perhaps also the high nitrogen and helium contents are indicative of very long gas migration pathways. Unfortunately the existing seismic data is insufficient to resolve these issues. The absence of a plentiful source rock is also a concern, but the evidence of the gas and extracted oil is that a source rock, still within the oil window, does exist downdip from 1 Magee.
The presence of the salt seal assures that whatever has been generated will still be there waiting for the intrepid explorer.
FUTURE WORK
The 1 Magee was the first well to test the sequence below the Proterozoic Bitter Springs formation salt in the Amadeus basin.
The well discovered helium-rich wet gas, indicating the possibility for a new hydrocarbon province in central Australia, sourced, sealed, and reservoired in late Proterozoic rocks 870 million years old (Fig. 4).
To follow up this opportunity a seismic program of approximately 100 km has been devised. Emphasis will be placed on imaging the top and bottom surfaces of the reservoir to detect thickening and/or a pinchout. This requirement for high resolution at some 2,500 m depth below a salt sequence, and also the relatively - isolated location, has increased acquisition costs.
Careful planning of line location is necessary to optimize the data recovered to help in the selection of a robust future drilling location.
The seismic survey must answer two questions:
- Where does the reservoir thicken to economic proportions? and
- How extensive is the field?
Pacific hopes to carry out this work in 1994.
MARKETS
Gas production in the Amadeus basin is sold for electricity generation via pipeline to Darwin. Oil is shipped south to refineries via rail.
Current domestic gas requirements are satisfied by existing production; however, various resource projects in northern Australia would be potential markets for new petroleum resources.
Expanding helium markets in Southeast Asia could also be serviced from this play.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
