Summary

PLAY Purni Formation
SOUTH AUSTRALIA BASIN Pedirka
LOCATION Pedirka Basin, including Eringa Trough
AGE Early Permian
RESERVOIRPurni Fm. intraformational sandstones
SEAL Purni Fm. intraformational shales and coals; Walkandi Fm on easternmost side of basin
SOURCE ROCK/SPurni Fm. coals; Possibly Peera Peera Fm., Lower Poolowanna Fm.
HYDROCARBON PHASEGas/very light oil
PRODUCTIONNone; no hydrocarbon shows to date other than gas shows from coals
HYDROCARBON FLOW RATESCalculated water flow rate of 869BPD in Macumba 1
POTENTIAL VOLUMESThere is no estimate of undiscovered resources

Location of the Pedirka Basin in central Australia.

Location of the Pedirka Basin in central Australia

The Pedirka Basin has an area of around 60,000 km2, just under half of which is in South Australia and the remainder in the Northern Territory. It is an intracratonic basin unconformably overlying the SE Amadeus Basin and western Warburton Basin which were deformed during the Alice Springs Orogeny and possibly the Delamerian Orogeny.

A final NW–SE compressional phase of the Alice Springs Orogeny in the Mid to Late Carboniferous initiated deposition in the Pedirka Basin and created thrust faults such as occur at Mt Hammersley 1. Permo-Carboniferous sediments were subsequently deposited in a tectonically quiescent sag phase. In South Australia the Permian is entirely overlain by up to 2500m of Triassic to Late Cretaceous sediments of the Simpson and Eromanga basins.

The Purni Formation was deposited in an extensive coal swamp and floodplain environment crossed by high-sinuosity fluvial channels. The lower boundary of the Purni Formation is a disconformity with the Crown Point Formation over much of the Pedirka Basin, however, in Mt Hammersley 1 the Purni Formation disconformably overlies Mt Toondina Formation equivalent (Alexander et al, 2007), the only place in the Pedirka Basin where this occurs. The upper boundary is the unconformity with Triassic sediments of the Simpson Basin or Jurassic–Cretaceous sediments of the Eromanga Basin.

Youngs (1975) formally defined three facies suites in the type section in Purni 1 (drilled in 1963/4) on the basis of relative proportions of sandstone, shale and coal:

  • The upper member consists of fine-grained, silty cross-bedded sandstone and carbonaceous shale interbeds with abundant coal seams
  • The middle member is characterised by upward-fining cross-bedded sandstone with interbeds of grey carbonaceous siltstone and shale with minor coal
  • The basal member consists of thinly interbedded grey shale and fine to medium-grained sandstone, with minor conglomerate and coal stringers

The thickest Purni interval encountered to date is in Blamore 1, drilled between the Eringa and Madigan Troughs in the Northern Territory. A total of 564m was intersected, with a significant thickness of the upper member, consisting of thick coal seams with a total 160m of coal in seams greater than 0.2m, also assessed as a total of 132m of coal in seams greater than 2.0m thickness.

NNE-striking high-angle reverse faults separate NNW to NNE-trending anticlinal complexes. The most prominent is the Dalhousie–McDills Ridge separating the Pedirka Basin into a western and eastern portion. Permo-Carboniferous sediments are preserved across the ridge linking the two depocentres. The thickest sediments estimated from seismic interpretation (over 660m) are preserved in the Eringa Trough. In South Australia Mt Hammersley 1, drilled in 1987 on the eastern flank of the trough, intersected 281m of Purni Formation, with a maximum thickness on the eastern side of the Pedirka Basin of 350m at Mokari 1.

The sediments were subsequently uplifted and eroded during two major compressional episodes during the Early to Late Permian and during the Tertiary.

Geological summary of the Arckaringa, Pedirka and Simpson basins. Purni Formation highlighted in red. Potential source rocks highlighted in purple

Geological summary of the Arckaringa, Pedirka and Simpson basins. Purni Formation highlighted in red. Potential source rocks highlighted in purple.

Key petroleum system events in the Pedirka Basin/Simpson Basin/Poolowanna Trough area

Key petroleum system events in the Pedirka Basin/Simpson Basin/Poolowanna Trough area

The key petroleum events chart for the Pedirka Basin/Simpson Basin/Poolowanna Trough region is similar to that for the Cooper-Eromanga Basins in that the critical moment was in the Late Cretaceous at the end of deposition of the Winton Formation, the last stage of the Eromanga Basin prior to uplift and erosion. Basin modelling suggests that major hydrocarbon generation and expulsion commenced around 100Ma (Ambrose et al, 2007) and lasted around 10-15Ma, so formation of structures prior to this was critical.

Structural reactivation occurred in the Miocene, and it is likely that this breached seals for some hydrocarbon accumulations where fault seal was critical, such as Poolowanna 1. Therefore, exploration is required of more subtle structures not affected by the reactivation in the Miocene.

Schematic structural history of the Pedirka Basin-Poolowanna Trough area (Giuliano, 1988)

Schematic structural history of the Pedirka Basin-Poolowanna Trough area (Giuliano, 1988)

Petroleum exploration commenced in the 1950s when licences covering the Cooper and Eromanga basins were first acquired by Santos, who went against conventional wisdom that commercial accumulations of oil would not be found in Mesozoic formations within the Great Artesian Basin.

Initial exploration involved surface mapping, stratigraphic drilling, aerial surveys, gravity and aeromagnetic surveys and seismic. The first petroleum well was drilled in 1959 and Cooper Basin gas was discovered in 1963.

The first commercial hydrocarbon to flow from the Eromanga Basin was gas produced from Namur 1 in 1976 (Cooper region). Oil was discovered in 1977 with uneconomic flows of oil from basal Jurassic and Triassic sediments in Poolowanna 1 in the Poolowanna Trough. The first economic oil flow was recorded from Strzelecki 3 (Cooper region) in the following year and this stimulated a major oil exploration program.

Exploration activity intensified in the Poolowanna Trough/Pedirka Basin area in the 1980s in response to the Poolowanna discovery. Six seismic surveys were carried out and five wells were drilled, three in 1985 and two in 1988.

Seismic reinterpretation by Santos indicated that five of the previously drilled wells were not valid structural tests being outside closure at all objectives. Of the 1985 wells, Oolarinna 1 had no independent fault closure and Glen Joyce 1 had little closure.

It should be noted that no wells have been drilled in the South Australian sector since the 1980s, although Central Petroleum drilled Blamore 1 and Simpson 1 in the Northern Territory in 2008. Exploration in the area would benefit from current ideas on the geology of the petroleum systems in the Cooper Basin region and modern technology and interpretation techniques.

Hydrocarbon shows have been recorded in most wells within the Pedirka Basin region. However, oil and gas has been recovered from only 2 wells, the Poolowanna and Peera Peera Formations in Poolowanna 1, and the Poolowanna Formation in Poolowanna 3.

The classification scheme used by Geoscience Australia has been used to determine the type of show present in a formation in the tables below.

Hydrocarbon shows and flow rates in South Australian wells in the Poolowanna Trough/Simpson Basin/Pedirka Basin region:

WellYear/ResultFormationShows/Recovery Flow rate
Pandieburra 1 1963
Oil shows
Peera Peera (?)

Minor fluorescence

 
Dullingari Group

Minor fluorescence. Recovered 190ft mud-cut water with a trace of hydrocarbons.

 
Mokari 1 1966
Dry hole
Poolowanna

Gas show; mudlog gas 14 times background. Minor fluorescence. Water to surface (WTS) in 12 mins.

387 cu ft/hr = rate of 1,654BWPD
Purni

Minor fluorescence. WTS in 17 mins.

Reported rate of 387 cu ft/hr
Poolowanna 1 1977
Oil shows
Poolowanna

Gas to surface (GTS) in 125 mins. Oil and water to surface in 3hrs 55 mins. Recovered 71¼ barrels water and 9½ barrels oil, 36.9o API gravity, with pour point of 41o C.

Gas at rate too small to measure (RTSTM). Calculated flow rate of 96BOPD

Peera Peera

Oil show. GTS in 120mins. Recovered 110ft mud and condensate.

Gas at RTSTM
Walkandi 1 1981
Oil shows
Poolowanna

Oil shows. Recovered 650ft muddy water and 1450ft gas-cut slightly muddy water.

 
Peera Peera

Oil shows. Recovered 46ft rat hole mud and 31ft slightly gas-cut mud.

 
Walkandi

Oil shows. Recovered 480ft water cushion and 4900ft slightly gas-cut mud.

 
Erabena 1 1981
Oil shows
Poolowanna Minor fluorescence  
Peera Peera Minor fluorescence  
Kuncherinna 1 1981/2
Oil shows
Poolowanna Recovered 90ft rathole mud with trace of oil  
Peera Peera Minor fluorescence  
Poolowanna 2 1985
Oil shows
Wallumbilla Minor fluorescence  
Cadna-owie Minor fluorescence  
Algebuckina Minor fluorescence  
Poolowanna Minor fluorescence. Recovered 1026ft VSGCMW, 306ft SGCM.  
Peera Peera

Minor fluorescence. Recovered 240ft very slightly gas-cut muddy water.

 
Oolarinna 1 1985
Dry hole
Poolowanna Minor fluorescence  
Peera Peera Recovered 5310ft gas-cut water (slightly muddy at top)  
Killumi 1 1985
Dry hole
Poolowanna Minor fluorescence  
Miandana 1 1985
Oil shows
Poolowanna Minor fluorescence  
Poolowanna 3 1989
Oil shows
Algebuckina Minor fluorescence  
Poolowanna

Minor fluorescence. GTS in 141 mins. Recovered 5550ft slightly oil and gas-cut water and 2290ft water in 171 mins.

Gas at RTSTM. 80BWPD.

Peera Peera Minor fluorescence  

Hydrocarbon shows in Northern Territory wells in the Poolowanna Trough/Simpson Basin/Pedirka Basin region:

WellYear/ResultFormationShows/Recovery
McDills 11965
Trace hydrocarbon indication
PurniMinor fluorescence
Colson 11978
Trace hydrocarbon indication
PoolowannaMinor fluorescence
Peera PeeraMinor fluorescence
WarburtonLog interpretation “oil show”
Thomas 11981
Oil indication
Poolowanna

Residual oil. Recovered 4951ft water with minor mud in 68 mins.

Poeppels Corner 11984
Trace hydrocarbon indication
AlgebuckinaMinor fluorescence
PoolowannaMinor fluorescence
Peera PeeraMinor fluorescence
Beachcomber 11988
Trace hydrocarbon indication
Cadna-owie

Minor fluorescence in coals

Algebuckina

Minor fluorescence in coals

Peera Peera

Minor fluorescence in coals

Blamore 12008
Oil and gas show
AlgebuckinaResidual oil
WalkandiMinor fluorescence
PurniGas shows in coals. Trace fluorescence.
CBM 93 12008
Trace hydrocarbon indication
PurniMinor fluorescence
Simpson 12008
Trace hydrocarbon indication
PoolowannaMinor fluorescence
WalkandiMinor fluorescence

The South Australian sector of the Pedirka Basin is currently covered by PEL 160, PEL 288 and PELAs 159, 332, 333 and 663.

Petroleum exploration permits, regional reserves and conservation parks in the Poolowanna Trough area as at August 2021

Petroleum exploration permits, regional reserves and conservation parks in the Pedirka Basin area as at September 2021

The Purni Formation play occurs within the Pedirka Basin, although the play has not been proven as there have been no hydrocarbon shows to date other than gas shows from coals.

Extent of the Purni Formation play in the Pedirka Basin

Approximate extent of the Purni Formation play in the Pedirka Basin

Schematic section across the Eromanga, Pedirka and Simpson basins. Pedirka Basin highlighted in red.

Schematic section across the Eromanga, Pedirka and Simpson basins. Pedirka Basin highlighted in red.

Purni 1 (1963/4)

The Purni Formation in Purni 1 is the type section for the formation in the Pedirka Basin.

Purni Formation in type well Purni 1 with subdivisions from Youngs (1975) (excerpt from Composite Log)

Purni Formation in type well Purni 1 with subdivisions from Youngs (1975) (excerpt from Composite Log)

Analogue:        Patchawarra Formation in the Cooper Basin

Patchawarra Formation type log (Strong et al, 2002)

Patchawarra Formation type log (Strong et al, 2002)

The Purni Formation was deposited in an extensive coal swamp and floodplain environment crossed by high-sinuosity fluvial channels. The lower boundary of the Purni Formation is a disconformity with the Crown Point Formation over much of the Pedirka Basin, however, in Mt Hammersley 1 Purni Formation disconformably overlies Mt Toondina Formation equivalent (Alexander et al, 2007) the only place in the Pedirka Basin where this occurs. The upper boundary is the unconformity with Triassic sediments of the Simpson Basin or Jurassic–Cretaceous sediments of the Eromanga Basin.

Youngs (1975) formally defined three facies suites in the type section in Purni 1 on the basis of relative proportions of sandstone, shale and coal:

  • The upper member consists of fine-grained, silty cross-bedded sandstone and carbonaceous shale interbeds with abundant coal seams
  • The middle member is characterised by upward-fining cross-bedded sandstone with interbeds of grey carbonaceous siltstone and shale with minor coal
  • The basal member consists of thinly interbedded grey shale and fine to medium-grained sandstone, with minor conglomerate and coal stringers

The middle member is likely to exhibit the best reservoir quality, but potentially good reservoirs occur throughout the formation.

Table 1: Purni Formation porosity and permeability values extracted from well completion reports (Wohling et al, 2013)

Well NameReference Porosity (%) Permeability
(mD)
Derived Hydraulic
Conductivity (m/d)*
Etingimbra Osborne and Edwards (1990) 31–32   
Colson Beach Petroleum (1979) 13–16   
Hale River No 1 Amerada Petroleum (1966) 15–22
(25)**
632 (Kv)#
2529 (Kh)##
0.53
2.44
McDills No 1 Amerada Petroleum (1966) 15–25
(19–22)**
135–187 0.11–0.16
Mokari French Petroleum (1966) 3.7–10.4   
Macumba Delhi International (1978) 13 2 0.002
Purni French Petroleum (1966) 19
(16.2–22)**
  
Dalmatia New (1988) >20   
CBM93-1 Central Petroleum (2008) 16.7 0.2–96^ 1.7x10-4-0.004
CBM107-001 Central Petroleum (2008)   36.7^ 0.03

* Hydraulic conductivity values converted from permeability measurements assuming water temperature of 20°C
** Bracketed porosity values indicate laboratory measurements of core samples, other values derived from geophysical logging
^ Permeability measurements taken over coal measures
# Vertical hydraulic conductivity
## Horizontal hydraulic conductivity


Carne and Alexander (1997) reported that the high energy channel sequence has a maximum measured sand thickness in South Australia of 45m (147 ft) in Mokari 1, with porosity increasing towards the west and around the edges of the Pedirka Basin, where it reaches 11% in Glen Joyce 1 and Mokari 1 and 12 % in Macumba 1. However, in Oolarinna 1 porosity is only 6% in the Purni Formation, although the reservoir quality is better in the underlying Crown Point Formation.

Blamore 1, drilled in 2008 between the Eringa and Madigan Troughs in the Northern Territory, encountered a significantly thicker Purni Formation (564m) than anywhere else in the Pedirka Basin (350m at Mokari 1). Most of the additional section is due to a significant amount of coal in the upper part of the Purni Formation with a total 160m of coal in seams greater than 0.2m, also assessed as a total of 132m of coal in seams greater than 2.0m thickness. Sandstones within the Upper Purni were light grey, loose, fine to very coarse, predominantly medium, subangular to angular, slightly micaceous. This suggests they are immature, with their provenance being relatively local, rapidly deposited in an actively subsiding basin, which is clearly shown in the isopach of the formation.

Thickness of the Purni Formation in the Pedirka Basin

Thickness of the Purni Formation in the Pedirka Basin


Cores were cut in the Purni Formation in Purni 1 and Mokari 1 in South Australia and McDills 1 in Northern Territory.

Three cores were cut in the Purni Formation in Purni 1, the type well for the formation:

Core 2 – 4,731’-4,778’, 100% recovery; Upper Purni

  • Grey white, fine grained, carbonaceous sandstone, very rich in coal fragments, micaceous; overlying
  • Grey white carbonaceous sandstone, with kaolinitic cement
  • 4,740ft – 19.6% porosity
  • 4,742ft – 22.0% porosity
  • 4,743ft – 16.2% porosity

Upper Purni, Excerpt from Purni 1 Composite Log

Upper Purni, excerpt from Purni 1 Composite Log


Core 3 – 5,066’-5,096’, 90% recovery; Middle Purni

  • Grey medium sorted sandstone; overlying
  • Carbonaceous silt, very compact; overlying grey brown fine to medium, well sorted and subangular sandstone

Middle Purni, Excerpt from Purni 1 Composite Log

Middle Purni, excerpt from Purni 1 Composite Log


Core 4 – 5,225’-5,315’, 100% recovery; Lower Purni

  • Fine grained silty sandstone within carbonaceous veins; overlying
  • Grey shales with grey sandstone lenses and thin coal levels

Lower Purni, Excerpt from Purni 1 Composite Log

Lower Purni, excerpt from Purni 1 Composite Log


There are no photographs of any of these cores. However, a photograph was taken of part of core 3 in Mokari 1.

Photograph of the Purni Formation at 1998m from Core 3 in Mokari 1, showing finely laminated fine-grained, silty cross-bedded sandstone and carbonaceous shale (Alexander et al, 2007)

Photograph of the Purni Formation at 1998m from Core 3 in Mokari 1, showing finely laminated fine-grained, silty cross-bedded sandstone and carbonaceous shale (Alexander et al, 2007)


Drill stem tests (DSTs) were conducted over the Purni Formation in Mokari 1 (1966) and Macumba 1 (1977):

Mokari 1:

  • DST 2   - 6,468ft to 6,557ft (Upper Purni)      – misrun
  • DST 3   - 6,593ft to 6,693ft (Middle Purni)
    • The DST data sheet in the WCR Appendix 9 Interpretation of Test Results records “Good blow when opened increasing to very strong. Mud to surface in 13 mins. after opening. … Water flowing at rate approx. 2400 gall/hr (387 c.ft/hr.)”
    • However, there must be some doubt about the recovery and hence the rate:
      • WCR Appendix 8 Record of Tests suggests “Fluid to surface in 17 min.”, with water recovered at 387 cu ft/hr.
      • Coincidentally, this is precisely the same rate reported for the water flow from DST 1 over the Poolowanna Formation, which is reported elsewhere in the WCR as having much better porosity than the Purni (14.9% vs 10.4%) and hence better flow rates
    • NB 387 cu ft/hr equates to a flow rate of 1,654BWPD

Macumba 1:

  • DST 1   - 8,211ft to 8,283ft (Lower Purni)
    • Recovered 4,350ft of slightly gas-cut water in 90 minutes. Top 200ft muddy.
    • Calculated water flow rate of 869BPD

Adequate seals for Purni Formation reservoirs are likely to be provided primarily by intraformational flood plain and lacustrine sediments. The upper member of the formation consists primarily of carbonaceous shale interbeds with abundant coal seams, so is likely to act as a good seal to the mainly sandy middle member, as well as any sandstone reservoirs within the upper member.

Over the eastern and central parts of the basin the Triassic Walkandi and Peera Peera Formations overlie the Purni Formation, providing additional seal.

Intense east-west compression and local wrenching during the Miocene resulted in structural rejuvenation and breach of fault seal in the Poolowanna Trough region, resulting in residual hydrocarbons in the Poolowanna and Peera Peera Formations in Poolowanna 1. This may have occurred with Purni structures as well so must be considered when drilling for Purni targets.

Extent of the seals for Purni Formation reservoirs in the Pedirka Basin

Extent of the seals for Purni Formation reservoirs in the Pedirka Basin

No hydrocarbon shows have been recorded over the Purni Formation in the Pedirka Basin to date, other than gas shows associated with coals. However, there are three intervals in the Pedirka Basin region that contain suitable material to have acted as potential source rocks for charge of hydrocarbons into Purni reservoirs:

  1. Purni Formation – intraformational coal-rich organic shales lying adjacent and above the reservoirs are the most likely,
  2. Middle to Late Triassic Peera Peera Formation of the Simpson Basin
  3. The Early Jurassic Lower Poolowanna of the Eromanga Basin

The Peera Peera and Lower Poolowanna are both stratigraphically younger than the Purni and lie above the target, so are likely to have sourced hydrocarbons only in potential stratigraphic closures on the flanks of the basins, where they may act as seals as well.

Purni Formation, Pedirka Basin

The Early Permian Purni Formation in the Pedirka Basin is the equivalent of the Patchawarra Formation, which is the source for much of the gas and oil discovered in the Cooper and Eromanga Basins. It contains the richest source rocks in the Pedirka Basin-Poolowanna Trough region, with extensive coal-rich organic shale which appears to be both oil and gas-prone. It contains up to 4% dispersed organic matter, with vitrinite and exinite macerals present in moderate abundance.

Radke (2009) reported both the upper and lower members of the Purni Formation as having good to excellent source rock potential. They consist of alternating shale, coal and siltstones deposited in lacustrine, swamp and floodplain settings, with the coals being the richest source rocks, comprising predominantly vitrinite and inertinite, with exinite generally less than 5% (but up to 20%). DOM in carbonaceous shales is generally poor. In the eastern Pedirka, Purni Formation sediments contain both H–rich exinite and vitrinite, and H-poor vitrinite rich source rocks

2008 drilling in the Northern Territory by Central Petroleum established extremely thick coal accumulations away from structural highs. Blamore 1 encountered thick coal seams in the upper part of the Purni Formation with a total 160m of coal in seams greater than 0.2m, also assessed as a total of 132m of coal in seams greater than 2.0m thickness. The isopach of the Purni Formation suggests there may be over 660m of section in the Eringa Trough in the Northern Territory, and the large amount of coal present had the potential to generate reasonable quantities of hydrocarbons.

Maturity of the Purni Formation source rocks in the Pedirka Basin:

Maturity of the Purni Formation source rocks in the Pedirka Basin

Peera Peera Formation, Simpson Basin

The Triassic Peera Peera Formation offers fair to good source potential. The Poolowanna 1 Well Completion Report indicated “The uppermost 60 feet of this section consists of black, silty, very organic rich shale, and appears to be an excellent source rock”, and that the formation “appears to be less prospective for liquid hydrocarbons than the Lower Jurassic section, although higher yields of kerogen have been attained in the upper part of the Peera Peera Formation.” The WCR also noted “A large sonic travel time shift (18 microseconds per foot, maximum) indicates the high organic content of this silty shale, which seems to be the main source of hydrocarbons in the area.”

The Peera Peera is rich in Type III kerogen organic matter (TOC around 4-5% although up to 18% in places). Ambrose et al (2007) reported Hydrocarbon Index of 100-200 with yield of 5-11kg/tonne in Macumba 1 (1977), suggesting a source for mixed gas and liquids, which is consistent with the gas flowed to surface from the Peera Peera in Poolowanna 1, albeit at a rate too small to measure. Radke (2009) noted that generally the DOM is higher towards the top of the unit and inertinite is dominant, and that much of the source potential of this unit lies in its abundant coals which have comparable macerals to those in the overlying Poolowanna Formation. He stated that the formation appears to be predominantly gas-prone but with potential for a modest oil yield.

The Peera Peera Formation of the Simpson Basin is stratigraphically younger than the Purni, and so is only likely to have been a source in the easternmost Pedirka Basin, where there may have been direct migration of hydrocarbons into the Purni Formation reservoirs if they were in communication at time of migration.

Maturity of the Peera Peera Formation source rocks in the Simpson Basin

Maturity of the Peera Peera Formation source rocks in the Simpson Basin

Lower Poolowanna Formation, Eromanga Basin

The middle section of the Poolowanna Formation is considered to have good potential as a source rock in the Poolowanna Trough: the Poolowanna 1 Well Completion Report indicated “A 40 foot interval within the Poolowanna Formation offers the best source rock potential of the well. The kerogen from this interval included about 25 % diffuse sapropelic material that is rich in exinite, but also in inertinite.”

The Poolowanna Formation source rocks are primarily at the top of the Lower Poolowanna. Coals and coal-related lithologies are volumetrically important in the sequence, with maceral composition ranging from vitrinite and vitrinite-rich clarite through a range of trimacerites to durite (Radke, 2009). Inertinite is dominant in dispersed organic matter (DOM) but vitrinite is also abundant. At Poeppels Corner 1, woody herbaceous DOM is rich in exinite. The coals have good to excellent source richness with total organic carbon (TOC) up to 15%, possess good oil generative potential, and have reached peak maturity of 0.9 % Ro in the Poolowanna Trough.

The Lower Poolowanna is stratigraphically above the Purni so is likely to have sourced hydrocarbons only in potential stratigraphic closures where it is in direct communication with the immediately underlying Purni on the flanks of the Poolowanna and Eringa Troughs, where it may act as a seal as well.

Maturity of the Lower Poolowanna Formation in the Poolowanna Trough and the Eringa Trough

Maturity of the Lower Poolowanna Formation in the Poolowanna Trough and the Eringa Trough

Source rock maturity

Onset of oil generation (Ro=0.5%) would generally be expected with burial at 1500m, and peak oil generation (Ro=0.7%) at 2200m Radke (2009), although of course this depends upon the types of organic matter present and the local geothermal gradient. Gas generation is expected from 0.6% Ro at 1900m.

In much of the Pedirka Basin geothermal gradients are low and so sediments may not have generated significant quantities of hydrocarbons, but the Early Jurassic (Poolowanna) and Permo-Triassic sequences have reached the main oil generative window (Ro 0.7-0.9%) over large portions of the Pedirka Basin and Poolowanna Trough.

Basin modelling in the Poolowanna Trough by Ambrose et al (2007) suggested that early onset of oil generation commenced from the Peera Peera Formation in the Early Cretaceous around 135Ma; this suggests that this may have occurred potentially earlier from the Purni Formation in the eastern Pedirka Basin. Peak generation and expulsion from all source rocks occurred primarily in the early part of the Late Cretaceous(100-90Ma) following significant subsidence during the deposition of the Winton Formation.

Cross section of the Poolowanna Trough with isoreflectance contours, showing maturity of the sequence up to the basal Jurassic, and a progressively shallower mature sequence to the east (from Radke, 2009, modified from Ambrose et al, 2007)

Cross section of the Poolowanna Trough with isoreflectance contours, showing maturity of the sequence up to the basal Jurassic, and a progressively shallower mature sequence to the east (from Radke, 2009, modified from Ambrose et al, 2007)

Three intervals contain suitable material in the Pedirka Basin area to have acted as potential source rocks for charge of hydrocarbons into the Purni reservoirs:

  1. Direct charge from Purni source rocks into adjacent and underlying Purni reservoirs in the Pedirka Basin
    • The Purni sandstone reservoirs are interbedded and directly overlain by potential source rocks of the same age, and therefore charge into reservoirs is likely from areas where the source is mature, including updip of the mature source due to the porous and permeable carrier bed system
    • Purni source rocks are mature over most of the Simpson Basin
  2. Lateral charge from Peera Peera source rocks in the Simpson Basin into Purni reservoirs in the Pedirka Basin
    • The Peera Peera is stratigraphically above the Purni on the western flank of the Simpson Basin so may have sourced hydrocarbons in potential closures
    • This could occur on the eastern flank of the Pedirka Basin
  3. Lateral charge from Lower Poolowanna source rocks in the Eromanga Basin into Purni reservoirs in the Pedirka Basin
    • The Lower Poolowanna is stratigraphically above the Purni and directly overlies it in the western Pedirka Basin so may have sourced hydrocarbons in potential closures
    • However, there is little overlap between the distribution of the mature Lower Poolowanna source rocks and the Purni Formation reservoirs, so charge should be deemed more unlikely than probable

Pedirka, Simpson and Eromanga Basins play types and potential migration pathways (Ambrose et al, 2002)

Pedirka, Simpson and Eromanga Basins play types and potential migration pathways (Ambrose et al, 2002)

Up to 20km migration distance from mature source rocks is generally regarded as a reasonable migration distance within porous reservoirs, although Borazjani et al (2019) stated that in the Cooper-Eromanga basins “the migration of expelled oils to Jurassic reservoirs was relatively fast given the short pathway distances of <1 km (fault migration) and ~40 km (carrier bed pathway)”. In this play analysis study 20km migration distances have been used outboard of mature source rock areas.

Potential limit of migration extent from marginally mature Purni Formation source rocks in the Pedirka Basin

Potential limit of migration extent from marginally mature Purni Formation source rocks in the Pedirka Basin (JPG 465 KB)

Potential limit of migration window within Purni reservoirs of hydrocarbons generated from mature Peera Peera source rocks

Potential limit of migration window within Purni reservoirs of hydrocarbons generated from mature Peera Peera source rocks (JPG 499 KB)

Potential limit of migration window within Purni reservoirs of hydrocarbons generated from mature Lower Poolowanna source rocks

Potential limit of migration window within Purni reservoirs of hydrocarbons generated from mature Lower Poolowanna source rocks (JPG 462 KB)

Trapping mechanisms in the Pedirka Basin are likely to be dominated by anticlines with 4-way dip closure over pre-existing, “basement” highs, although onlap and pinch-out traps against Warburton Basin highs are expected along the eastern flank of the Dalhousie-McDills Ridge.

Structures to be investigated must have been in place prior to major generation and expulsion of hydrocarbons, so those that formed during the Cainozoic are less favourable prospects than those formed prior to end of deposition of the Late Cretaceous Winton Formation.

Intense east-west compression and local wrenching during the Miocene resulted in structural rejuvenation and breach of fault seal in the Poolowanna Trough area. This would explain why only residual hydrocarbons were encountered in the Poolowanna Formation in Poolowanna 1. Therefore, exploration is required of more subtle structures not affected by the reactivation in the Miocene.

Stratigraphic traps are also potential targets, in particular pinch-outs on the flanks of the basin.

In the Cooper-Eromanga Basins a common technique for estimating whether a present day structure is likely to have been charged with hydrocarbons or not is to look at the isopach between the top of the Winton Formation and the top of the target formation; this map approximates the target surface structure at time of migration, with thin isopach areas indicating potential structures at time of migration. If there is no top Winton surface mapped in the area of interest the isopach between the top of the Cadna-owie Formation and the target formation is used as a proxy. This technique is applicable to the Purni Formation in the Pedirka Basin as well.

It must be borne in mind that the coverage of 2D seismic data within the Pedirka Basin is limited compared to that in the Cooper Basin to the south-east. Consequently, structures in reality may be quite different to the way they have been mapped.

Possible structure at time of migration of hydrocarbons into Purni reservoirs in the Pedirka Basin

Possible structure at time of migration of hydrocarbons into Purni reservoirs in the Pedirka Basin (JPG 520 KB)

Structural closures within the Purni Formation at potential time of migration

Structural closures within the Purni Formation at potential time of migration (JPG 486 KB)

Structures within the Purni Formation at time of hydrocarbon migration (pink) and Present Day (green)

Structures within the Purni Formation at time of hydrocarbon migration (pink) and Present Day (green) (JPG 350 KB)

Seismic coverage used to define structures within the Purni Formation play area, at time of hydrocarbon migration and Present Day

Seismic coverage used to define structures within the Purni Formation play area, at time of hydrocarbon migration and Present Day (JPG 588 KB)

Play elements for the Purni Formation in the Pedirka Basin

Play elements for the Purni Formation in the Pedirka Basin


Approximate extent of the Purni Formation play in the Pedirka Basin

Approximate extent of the Purni Formation play in the Pedirka Basin

Proven production from the Purni Fprmation:None
Number of producing wells at 28 February 2021:None
Total production at 28 February 2021:None
Hydrocarbon flow rates:Calculated water flow rate of 869BPD in Macumba 1

There is no estimate of undiscovered resources.

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