Areas W07-12 to W07-15
Canning Basin
Basin Geology
The petroleum potential of the Offshore Canning and Roebuck basins is currently considered to be poor compared to other North West Shelf basins, primarily due to the perceived absence of a prolific source rock (Smith et al, 1999). Interpreted mild structural deformation in the Late Jurassic and Early Cretaceous suggests that restricted depocentres did not form and thus claystone source rocks typical of the adjacent Carnarvon and Bonaparte basins were not deposited in the Offshore Canning and Roebuck basins. However, seismic interpretation suggests that the proven onshore petroleum systems in Permian and pre-Permian sediments extend into the offshore area along the older Palaeozoic trends that underlie the Oobagooma Sub-basin and Broome Platform (Passmore, 1991). Reef plays similar to those on the onshore Broome Platform and northern flank of the Fitzroy Trough were probably deposited on the topographically higher offshore features, while more clastic sediments were deposited in the adjacent troughs. The organic-rich shales that are the source rocks for the onshore Ordovician and Carboniferous petroleum systems would have also been deposited in the now offshore part of the basin, and could be a source for shallower Palaeozoic and Mesozoic traps if they were preserved through pre-Permian erosion. Fluvial and deltaic clastics covered the present shelf by the mid-Jurassic, providing reservoir, source and seal (Passmore, 1991). Additionally, there is some potential for generation in the Mesozoic Rowley depocentre, with migration into the shallow Offshore Canning Basin a possibility (Smith, 1999).
This section includes a brief overview of the Palaeozoic petroleum systems documented for the Onshore Canning Basin by Kennard et al (1994), before providing details on potential Mesozoic offshore systems from Smith (1999), although the classification of Bradshaw et al (1994) has been retained in that the Early Triassic petroleum system has been grouped in the Gondwanan Petroleum Supersystem.
Palaeozoic Petroleum Systems
Several Palaeozoic petroleum systems are known in the Onshore Canning Basin: Larapintine 2, 3 and 4. The source rocks of the Gondwanan 1 and 2 petroleum systems range in age from latest Palaeozoic to earliest Mesozoic. The following section has been summarised from Kennard et al (1994) and Edwards et al (1997).
Larapintine 2
Source rocks in the Larapintine 2 Petroleum System comprise Ordovician organic-rich
marine shales, with the oil-prone alga Gloeocapsamorpha prisca, occurring
within the upper Goldwyer Formation (Llanvirn), with lesser abundances found
throughout the lower Goldwyer and Nambeet formations, as well as locally
in the lower Carribuddy Group (Foster et al, 1986). An additional potential
source rock is the calcareous shales and siltstones of the Willara Formation.
The Nambeet and lower Goldwyer source units matured prior to the Late Devonian,
and probably generated large amounts of oil during the Ordovician–Silurian.
The upper Goldwyer source unit is immature across much of the Broome Platform,
but where it has been down-faulted, maturation commenced after Devonian
deposition, and this source rock retains both some free expelled hydrocarbons
and some limited potential to generate more hydrocarbons on further maturation.
Most of the significant shows occur within Nita Formation carbonates where
the best porosity/permeability occurs within dolomitised, supra-and inter-tidal
parasequences. Other potential reservoirs are within Willara carbonates,
Goldwyer carbonates and submarine fans, aeolian sandstones of the Tandalgoo
and Worral formations. Where the Grant Group overlies and truncates the
Ordovician–Silurian succession, it is feasible that reservoirs could
be charged with hydrocarbons from the Larapintine 2 Petroleum System. The
critical factors for commercial accumulations appear to be the maturation/migration
history relative to timing of trap formation, and reservoir quality, particularly
the controls on porosity development and distribution within carbonates
of the Nita Formation.
Larapintine 3
The Larapintine 3 Petroleum System encompasses the classic Devonian reef
play. The primary sources are basinal and intra-shelf algal marine shales
of the Frasnian Pillara Reef complex. Where deeply buried, peak generation
of the Pillara succession probably occurred during the Carboniferous and
ceased prior to the Mesozoic. Along the Fitzroy Trough margins peak maturity
was attained from Carboniferous to Mesozoic time, and over the shallow portion
of the shelves peak generation was attained from Mesozoic to present. Production
from this petroleum system occurs at the Blina oil field, where oil flows
from transgressive peritidal carbonates of the Famennian Nullara reef complex
and highstand carbonates of the latest Famennian ramp succession. Lowstand
basin-floor and slope fans, outboard of the Frasnian–Famennian platform
margin, generally occur adjacent to transfer zones that formed focal points
for lowstand drainage channels. These plays could be more prospective than
the carbonate reef complexes, as they are positioned to receive hydrocarbons
from basinal shales. The critical factor for commercial accumulation appears
to be reservoir quality of both the reefal carbonates and the lowstand fans.
Larapintine 4
Source rocks in the Larapintine 4 Petroleum System comprise organic-rich
marine shales in the Early Carboniferous Laurel Formation. In the deeply
buried Fitzroy Trough, the Laurel Formation source attained peak oil maturity
from the Late Carboniferous to Mesozoic. In shallower portions, peak oil
maturity was attained from the Mesozoic to present, and on the flanking
shelves the Laurel Formation is immature to marginally mature and has less
source potential. Oil production occurs at Lloyd 1 and West Kora 1, and
gas was discovered at Point Torment 1 from sandstones of the Anderson Formation.
Anderson Formation sandstones are the main exploration targets, but they
are difficult to locate without detailed facies information. The relative
timing of generation and structuring during the Fitzroy Movement is crucial
in determining whether the large anticlines produced by this tectonism can
be charged from the Laurel Formation.
Gondwanan 1
The major potential sources of the Gondwanan 1 Petroleum System are Early
Permian transgressive marine shales of the Poole Sandstone and Noonkanbah
Formation; however, they are thermally immature except along the southern
margin of the Fitzroy Trough. Marine shales of the upper Grant Group are
locally organic-rich, but have poor generative potential. Reservoir facies
are widespread through the lower and upper Grant Group but there are problems
with inadequate seals and water flushing. Other critical factors are suitable
maturation histories and migration pathways from source units in underlying
petroleum systems.
Gondwanan 2
The potential source rocks of the Gondwanan 2 Petroleum System are Early
Triassic marine and deltaic shales of the Locker Shale equivalents and Keraudren
Formation. These shales were first deposited during a phase of rapid thermal
sag shortly after the Bedout Movement. The source rocks have average total
organic carbon (TOC) values of 4 %, with values up to 6 %. The algal content
of many Triassic samples in Keraudren 1 exceeds 40 %. Triassic samples are
mature for oil generation in Phoenix 1 in the Bedout Sub-basin. While the
Triassic sediments are in the early oil window and the system extends across
the Oobagooma Sub-basin, the Triassic is poorly developed. Onlap plays have
been identified in the Oobagooma Sub-basin through the Early and mid-Triassic.
Mesozoic Petroleum Systems
The following section has been modified from Smith (1999) and Smith et al (1999; Figure 6). The Westralian 1 Petroleum System as described in Smith’s studies has been included as the Gondwanan 2 Petroleum System in this report. The Westralian 2 Petroleum System as described by Smith (1999) and Smith et al (1999) is documented as Westralian 1, and Smith’s Westralian 3 has been divided into the Westralian 2 and 3 petroleum systems, herein.
Source
Source rocks were identified from the Mesozoic section of the Offshore Canning
and Roebuck basins, and their distribution predicted using geochemical analysis,
sequence stratigraphic correlation, seismic mapping and sedimentary modelling.
The source rocks were sub-divided into four groups depending on their tectono-depositional
setting.
Westralian 1
The potential source rocks of the Westralian 1 Petroleum System were deposited
under fluvio-deltaic conditions during declining thermal sag in the Early–Middle
Jurassic. Sapropelic and coal-rich material was deposited in lakes and restricted
embayments that developed during period of rifting and subsequent rapid
transgression. The source units have average TOC values from 4 % to 25 %.
Algal-rich units have good to excellent oil generative potential, but these
facies are thinly developed and highly variable. The Jurassic and earliest
Cretaceous are predicted to lie in the oil window in the Rowley Sub-basin.
Westralian 2 and 3
The potential source rocks for the Late Jurassic Westralian 2 and Early
Cretaceous Westralian 3 petroleum systems are sediments that prograded into
gently sagging post-rift basins without major faulting. Average TOC contents
of the source rocks are between 2 and 10 %, but maturity for oil generation
only occurs beneath the Tertiary carbonate wedge in the Rowley Sub-basin,
and reservoirs are scarce due to declining sediment input post-breakup.
Reservoirs
Early Permian clastic sediments form both reservoir and seal in several
onshore fields and, where intersected in the offshore Fitzroy Trough, these
sandstones are known to have good porosity.
Middle to Late Triassic fluvial sandstones of the Keraudren Formation are reasonable reservoirs, which have an interpreted 110 m of net gas pay in tight sands below 4322 m in Phoenix 1. These have not been intersected by wells in the Oobagooma Sub-basin, but seismic shows the Triassic pinching out over the sub-basin forming a potential stratigraphic trap similar to the postulated Hammerhead prospect in the Willara Sub-basin (Lipski, 1993).
Early to Middle Jurassic fluvio-deltaic sandstones of the Depuch Formation are an example of a shallower Mesozoic objective, but structuring associated with these reservoirs are mild and migration pathways from mature Triassic sources are probably limited away from basin margin faults.
Seals
A thick Early Cretaceous marine claystone (the Muderong Shale and equivalents)
forms a regional seal over the North West Shelf; however, in this part of
the basin the Early Cretaceous succession is relatively sandy as it grades
from the Broome Sandstone to the predominately silty Mermaid Formation.
The thick Late Triassic to earliest Jurassic red-bed claystone of the Bedout Formation is expected to form a regional seal over the Late Triassic sandstones.
A Middle Triassic regional limestone marker (the Cossigny Member of the Keraudren Formation), which grades into claystone near the basin margins, acts as an intraformational seal between Middle Triassic sandstones, as with Phoenix 1 in the Bedout Sub-basin.
Traps
Structural movement associated with rifting and Miocene reactivation has
resulted in limited trap formation in the Mesozoic section.
Stratigraphic pinchout traps are formed against the Oobagooma High and the basin margin.
Thermal Maturity
1D modelling of thermal maturity at Wamac (Smith, 1999) suggests the Early
to Middle Jurassic section is early mature. The higher measured values in
the well are likely related to dolerite intrusions. Based on the modelling,
Carboniferous source rocks are presently within the oil window in much of
the Oobagooma Sub-basin. Older (Ordovician and Devonian) rocks have not
been sampled, but generated hydrocarbons are likely to have already migrated
into Palaeozoic traps.