Perth, May 6, 2008 AEST (ABN Newswire) - Central Petroleum Limited (ASX:CTP)(PINK:CPTLF) Recently completed seismic over the Johnstone (320 UOIIP), Gypsum, Stuart and Surprise oil prospects in EP 115 has, inter alia, delivered some interesting results over the Gypsum prospect. A previous Technical Note, (240408), likened some elements of the widespread variously oil and gas mature Horn Valley Siltstone to the Bakken fractured shale oil play in the USA.
The Gypsum prospect fractured shale oil play in the northwestern Amadeus Basin- the Horn Valley Siltstone.
(CTP Technical Note 050508)
The Gypsum structure is a robust, westerly plunging anticlinal nose located to the southeast of the Johnstone oil prospect in EP 115. Application of the previously described Horn Valley Siltstone (HVS) fractured shale oil play concept (see Technical Note 240408) over this structure indicates that a fracture dependent Undiscovered Oil Initially in Place (UOIIP) resource at "high" estimate may be up to 30 MMbbls in volume. This type of fractured shale oil play ("Bakken" style play analogies in the USA) has never been tested in the Amadeus Basin to date although up to 7,000 km2 of the HVS has been mapped in the oil window, much of which has unproven potential for oil accumulations in zones of high intensity natural fractures.
There is also significant potential in the same structure for conventionally reservoired oil in 4 way dip closure under the Parke Siltstone seal (Mereenie Sandstone) and under the Lower Stokes Siltstone seal (Pacoota Sandstone) subject to additional mapping indicating an updip seal to the east possibly of salt intruded fault planes.
The unconventionally reservoired oil potential of the Larapinta Group source rock (Horn Valley Siltstone-HVS) over the Gypsum Anticline is discussed here. The anticline is a large, symmetrical tight fold intruded at depth by Gillen Salt. The structure has dip line rollover (wavelength) of about 14 km and a vertical height of c. 640 m at the top Pacoota level. In addition to conventionally reservoired oil potential in the afore - mentioned Ordovician reservoirs, Central believes fractured shales of the Horn Valley Silstone (HVS) at or near the axis of this tight fold may have potential for oil production. The Horn Valley Siltstone is an excellent mature source rock consisting predominantly of marine shale (Gorter, 1983) and nearby data from Mt Winter-1 suggests the HVS is in the oil window at that location.
It is thought that the HVS fractured shale oil play may not require a top seal to be a valid trap with oil in this play being effectively non-dependent on top or updip seal.
The structure may have a 4-way dip closure component, the definition of which is dependent on the outcome of new seismic mapping which is currently underway. A large portion of the eastern extension of the Gypsum structure appears to be faulted and may be also intruded by salt which, in combination, may have resulted in a functional updip seal to prevent updip migration from the west. To the north, the Gypsum structure is bounded by a very well defined and significant oil kitchen trough zone with another to the west and south. The Parke Siltstone and the Lower Stokes Siltstone seals necessary for 4 way dip closure in the Mereenie Sandstone and the Pacoota Sandstone are probably not breached based on the anticipated low tenor of associated faulting. There appears to be some possible Direct Hydrocarbon Indicators near the base of the Mereenie Sandstone and above the Horn Valley Siltstone evidenced by a series of tentatively identified flat spots in the seismic which may deserve additional investigation. There is potential for a significant conventional oil play at the level of the Ordovician Larapinta Group (Pacoota Sandstone and Stairway Sandstone) if a structural trap can be established by the new round of mapping.
At least 1.0 second of two way time equivalent of Palaeozoic sediments (2,500m plus) have been eroded from the crest of the Gypsum structure and this suggests the HVS at Gypsum is either at or through the oil window although conadont alteration index derived maturities derived from nearby outcrops are in the early oil window (Gorter, 1983). Regional studies of organic content type and richness indicate HVS source richness and oil generation capacity increases to the northwest in the Amadeus basin.
Regional isopach studies indicate that the HVS is probably 50-60 m thick over the structure and could have provided sufficient source rock volumes to fully charge the fracture systems in the Gypsum structure as well as any four way dip closure structural component. The Gypsum structure is unusual in that :
1) The structure is steep and areally extensive, with a very large vertical height varying from 0.45 seconds of two way time (1,000m vertical) at the Arumbera Sandstone level to 0.2 seconds (350m vertical ) at the base of the Hermannsberg Sandstone.
2) Target petroleum systems and conventional reservoir-seal couplets appear to be unbreached by salt intrusion, faulting or erosion. However, confirmation of a conventional structural trap at the top Pacoota / Stairway levels awaits remapping of newly acquired seismic data.
3) The fold, which formed during the latter stages of the Alice Springs Orogeny, is relatively "tight" compared with Mereenie and Palm Valley structures; production from the latter is totally dependent on fractures in Ordovician sandstones which bodes well for fracture development in the HVS over this structure, most noteably along its axis
The potential area of intense fracturing (note this is not areal closure which could be much greater) is about 4,000 acres. This is the area where the axis of the plunging nose is clearly defined. An estimate of undiscovered-oil-in place uses the following assumptions:
1) area assumed to be fractured is 4,000 acres
2) Fractures make up ~ 1.5% of the gross rock volume,
3) The relevant gross fractured shale interval is 45 m thick,
4) Bo = 1.5
5) Oil saturation in fractures Sh ~0.75
6) Geometric Factor = 0.9
Undiscovered Oil Initially in Place "high" estimate = 30 MMbbls *
* No account has been taken in this assessment of the potential for artificial fracturing (fraccing) or horizontal drilling, techniques used so successfully in the Bakken fractured shale oil play in the USA and the figure quoted is not an estimate of recoverable oil.
Conclusions
A apparently largely unbreached anticlinal structure has been highgraded in the northwest Amadeus Basin (Gypsum Prospect EP 115) and new seismic in the area may help define a structural trap in 4 way dip closure. In addition, an unconventional fractured shale oil play centred on the Horn valley Siltstone has potential over this relatively tight structure. Undiscovered oil-in place resource potential at "high" estimate has been assessed at 30 MMbbl UOIIP for the fractured shale oil play concept ignoring any potential contribution from 4 way dip closure which may be considerable. The play type is probably tenable over a wide area of the basin where the Horn Valley Siltstone lies in the oil window and was tightly structured during the Alice Springs Orogeny.
About Central Petroleum Limited
Central Petroleum Limited (
ASX:CTP) is an established ASX-listed Australian oil and gas producer (ASX:CTP) with exploration and appraisal permits in the Northern Territory (NT). Central has grown to become the largest onshore gas operator in the NT, supplying residential and industrial customers in the NT and wider Australian east coast market.
Central is seeking to become a major domestic energy supplier, in addition to helium and naturally occurring hydrogen, with exploration, appraisal and development plans across 169,112 km2 of tenements the NT, including some of Australia's largest known onshore conventional gas prospects in the Amadeus Basin.
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