Basin Details and Geological Overview
The Mentelle Basin is a large offshore sedimentary basin located on the southwest Australian continental margin. It is a deep to extremely deep-water (500 to 4000m) frontier basin with no previous petroleum exploration. In the east the Mentelle Basin is separated from the southern Perth Basin (Vlaming Sub-basin) by a broad and shallow basement block, the Yallingup Shelf and in the west it is bordered by the Naturaliste Plateau (Figure 1). Based on differences in water depths and structure the basin is divided into the eastern Mentelle Basin and western Mentelle Basin (Figure 2).
Initial geological assessment of the Mentelle Basin (Bradshaw et al., 2003, A revised structural framework for frontier basins on the southern and southwestern Australian continental margin) suggested the presence of thick sedimentary successions that were interpreted to be of similar age to those in the adjacent southern Perth Basin. The basin therefore was identified as an area potentially prospective for hydrocarbons.
To enable petroleum prospectivity assessment of this frontier basin, Geoscience Australia acquired 1 060 kilometres of industry standard seismic data across the Mentelle Basin in 2004 (GA seismic survey 280: Figure 2) and 2 570 kilometres in 2008-09 (GA seismic survey 310: Figure 2; Foster et al 2009, Southwest margin surveys completed). Recent Geoscience Australia study based on these datasets included seismic interpretation combined with the development of a tectonostratigraphic framework and petroleum systems model for the basin (Borissova et al, 2010, New Exploration Opportunities on the Southwest Australian Margin: Deep-Water Frontier Mentelle Basin).
Results of the study have shown that initial rifting in the Mentelle Basin occurred in the Early Permian, followed by thermal subsidence during the Triassic to Early Jurassic. Permo-Triassic successions up to 7 kilometres thick are interpreted in the eastern Mentelle Basin. In the Middle Jurassic to Early Cretaceous during the breakup of eastern Gondwana, major extension and syn-rift deposition occurred in the western Mentelle Basin, which was located near a triple junction between Australia, Antarctica and India. During this time very thick (7 to 9 kilometres) sedimentary successions accumulated in half-graben depocentres. Continental breakup on the south-western margin in the Early Cretaceous was accompanied by extensive volcanism. Up to 1 kilometre thick syn-breakup volcanic succession comprising lava flows and volcaniclastic sediments formed in the western Mentelle Basin.
Assessment of the Mentelle Basin petroleum prospectivity is based on correlations to the south Perth Basin stratigraphy. It suggests that the basin is likely to contain multiple source rock intervals (coals and carbonaceous shales), regionally extensive reservoirs within fluvial strata, and regional seals comprising marine strata and volcanics. Petroleum systems modelling indicates that potential source rocks are thermally mature and generated hydrocarbons. The Mentelle Basin offers a wide range of play types, including faulted anticlines and highside fault blocks, sub-basalt anticlines and fault blocks, drape and forced fold plays, as well as a large range of stratigraphic and unconformity plays.
- Mentelle Basin Details and Geological Overview
- Eastern Mentelle Basin Details and Geological Overview
- Western Mentelle Basin Details and Geological Overview
Structural architecture of the Mentelle Basin reflects the multi-phase extensional history combined with the heterogeneity in the structural grain of the basement. Initial northeast-southwest extension between Greater India and Western Australia produced north-south trending faults and depocentres. Mid-Jurassic to Early Cretaceous northwest-southeast extension preceding the breakup between Greater India and Australia modified existing Permian structures and produced northeast-southwest trending structures (Figure 3).
The western and eastern Mentelle basins are separated by the Margaret Hinge Zone and have distinctly different fault and depocentre geometries (Figure 3). Major faults in the eastern Mentelle Basin strike predominantly north-northwest to north-northeast. Several mapped depocentres containing up to 7 to 9 kilometres of sediments have been interpreted as remnants of Permo-Trassic half-graben, which were mostly eroded at the onset of the Mid-Jurassic rifting.
In the western Mentelle Basin fault orientations and depocentre geometries are similar to those in the eastern Mentelle Basin in the north, but change significantly in the south (Figure 3). The majority of faults in the south-western part of the Mentelle Basin strike northeast-southwest, with the major rift bounding faults dipping to the southeast. The depocentres of the western Mentelle Basin are interpreted as extensional half-graben structures formed during Mid-Jurassic to Early Cretaceous rifting. The depocentres are interlinked and contain up to 11 kilometres of sediments. They have fairly simple geometries, but show signs of significant fault reactivation and inversion in the Cenozoic that are correlated to regional tectonic events. Fault-strata relationships suggest that the likely cause of fault reactivation and partial inversion in the western Mentelle Basin are the Mid-Eocene onset of fast spreading on the southern Australian margin and the Miocene collision between the Australian and Eurasian plates (I. Borissova et al, 2010).
The Mentelle Basin has only one stratigraphic well, DSDP 258 (Davies et al, 1974, Initial Reports of the Deep Sea Drilling Project, Volume 26, Washington), which was drilled on its boundary with the Naturaliste Plateau. DSDP 258 intersected the upper part of the post-rift succession down to mid-Albian strata. The stratigraphy of pre-Albian section in the Mentelle Basin (Figure 4) is based on seismic interpretation integrated with geological knowledge of the adjacent basin areas that formed during the same regional tectonic events. Correlations between the Vlaming Sub-basin and the Mentelle Basin are based on seismic character and stratal relationships of the major sequences. The Vlaming Sub-basin was also used as a proxy to interpret lithologies, depositional environments and petroleum systems elements of the syn-rift and early post-rift successions of the Mentelle Basin.
The main supersequences of the Mentelle Basin are shown on the tectonostratigraphic chart (Figure 4) and can be summarised as follows:
- Supersequence Mentelle 1A, mapped in the eastern Mentelle Basin, may contain Permian coal measures similar to those in the Perth Basin and correspond to the initial syn-rift deposition in the Early Permian. Supersequence Mentelle 1B is correlated to the phase of thermal subsidence and may contain sediments similar to the Upper Permian Willespie Formation, Lower Triassic Sabina Sandstone and Triassic Lesueur Sandstone, while supersequence Mentelle 1C is correlated to the Lower Jurassic Cattamarra Coal Measures (Crostella and Backhouse, 2000, Geology and petroleum exploration of the central and southern Perth Basin, Western Australia). These units were extensively deformed, faulted and eroded at the onset of second phase of rifting in the Middle Jurassic.
- Mentelle 2, 3, and 4 supersequences were deposited during the Middle Jurassic to Early Cretaceous major extensional phase in the basin.
- Mentelle 2 Supersequence is correlated to the mid-Bajocian - Kimmeridgian part of the Yarragadee Formation in the Vlaming Sub-basin;
- Mentelle 3 Supersequence (Tithonian - Berriasian) is correlated to the upper Yarragadee Formation and lower part of the Parmelia Group;
- Mentelle 4 Supersequence (Berriasian) is correlated to the upper part of the Parmelia Group in the Vlaming Sub-basin.
- Mentelle 5 is a syn-breakup supersequence, up to 1 km thick, that shows evidence of massive syn-depositional volcanism. The supersequence comprises a thick basal igneous unit (Mentelle 5a) that extends throughout the western Mentelle Basin, and often contains prominent volcanic edifices. A sedimentary package (Mentelle 5b) overlies and, in places, interfingers with the basal igneous unit, onlaps the volcanic edifices, and is often intruded by younger igneous bodies. The igneous succession is confined to the western Mentelle Basin, while the sedimentary succession pinches out over the eastern Mentelle Basin. The sedimentary part of the Mentelle 5 succession is correlated to the marine to deltaic Gage Sandstone and South Perth Shale in the Vlaming Sub-basin. The volcanic succession is correlated to breakup-related volcanics in the Perth Basin and on the Naturaliste Plateau (Crostella and Backhouse, 2000; Crawford et al, 2006).
- Mentelle 6 and Mentelle 7 (Hauterivian to Albian) supersequences were deposited during the initial post-breakup thermal subsidence. Mentelle 6 and Mentelle 7a are correlated to interbedded marine sandstones and mudstones of the Leederville Formation in the Vlaming Sub-basin, whereas Mentelle 7b is correlated to the Albian black marine mudstones at DSDP 258, and glauconitic sandstones and siltstones from the Osborne Formation (Henley Sandstone Member) in the southern Perth Basin.
- Mentelle 8 (Albian to Cenomanian) supersequence corresponds to the establishment of open marine environment and development of a deep intra-slope basin system in the western Mentelle Basin. The age and geology of this sequence are well constrained by DSDP 258.
- Mentelle 9 (Cenomanian to Campanian) supersequence corresponds to the onset of deep marine, carbonate-dominated sedimentation. This sequence is equivalent in age to the shallow marine Molecap Greensand, Gingin Chalk and Poison Hill Greensand in the southern Perth Basin.
- Within the Cenozoic strata (Mentelle 10-12) individual sequences are difficult to differentiate and correlate due to the condensed, and often discontinuous and extensively eroded nature of this section. Some stratigraphic data is provided by DSDP 264, which shows three Cenozoic units separated by unconformities: a middle Paleocene-lower Eocene chalk; an upper Eocene ooze; and an upper Miocene-Holocene ooze.
Figure 5 - Cross-section through the eastern and western Mentelle Basin, showing supersequences (Geoscience Australia seismic line 280/04)
© Geoscience Australia
Figure 6 - Cross-section through the central part of the western Mentelle Basin, showing supersequences (Geoscience Australia seismic line 280/07).
© Geoscience Australia
Figure 7 - Conceptual play diagram for the Mentelle Basin
© Geoscience Australia
The eastern Mentelle Basin is interpreted to contain predominantly Permian, Triassic and Lower Jurassic strata with only thin Middle Jurassic to Holocene successions (Figure 5). Permian to Lower Jurassic strata were extensively deformed, faulted and eroded at the onset of Middle Jurassic rifting in the eastern Mentelle Basin, which makes interpretation and correlation of sequences very difficult. In contrast, the western Mentelle Basin contains mostly Jurassic successions (up to about 9 kilometres thick, Figures 5 and 6), similar to those in the central part of the Vlaming Sub-basin. Permo-Triassic strata appear to step down along a series of southwest-dipping fault blocks beneath the hinge zone of the western Mentelle Basin (Figure 5), and may form a pre-rift section beneath the Jurassic depocentre.
Petroleum Systems and Hydrocarbon Potential
Petroleum prospectivity assessment of the Mentelle Basin (Borissova et al, 2010) confirmed significant potential to become a new petroleum province. The basin is likely to contain multiple source rock intervals associated with coals and carbonaceous shales, as well as regionally extensive reservoirs and seals within fluvial, lacustrine and marine strata. Petroleum systems modelling indicates that potential source rocks are thermally mature and started to generate during the Early to Middle Jurassic in the eastern Mentelle Basin and during the Early Cretaceous in the western Mentelle Basin. Some source rocks probably continued to generate and expel hydrocarbons after breakup, charging existing and newly-created traps. A wide range of play types have been identified in the Mentelle Basin (Figure 7), including faulted anticlines and highside fault blocks, sub-basalt anticlines and fault blocks, drape and forced fold plays, and a large range of stratigraphic and unconformity plays.
In the western Mentelle Basin the main potential plays include highside fault blocks and sub-basalt anticlines. Highside fault blocks (Play Type 1, Figure 7) which developed during the syn-rift phase form potential structural traps, where these blocks have favourable location and timing to be charged from the Middle Jurassic to Lower Cretaceous coals and lacustrine mudstones. Fluvial sandstones of the same age are likely reservoirs. Sub-basalt anticlines (Play Type 2, Figure 7) are structural traps that formed during the breakup incorporating Lower Cretaceous fluvial reservoirs sealed by thick syn-breakup volcanics. A number of potential plays formed after the breakup (Play Type 3, Figure 7). Lower Cretaceous marine sandstone, including turbidite units, are likely to form good reservoir intervals, while overlying marine mudstone are potential regional seals. Trap types include:
- dome structures and forced folds which originated during Cretaceous to Cenozoic igneous events
- inversion anticlines that formed during the Cenozoic margin tilting and subsidence
- drape structures.
Combined structural-stratigraphic traps (Play Type 4, Figure 7) are likely to occur along the hinge zone separating the eastern and western Mentelle basins. This is where Lower Cretaceous fluvial sandstones onlap on basement or pre-rift strata. This trap type has favourable location and timing for charge from syn-rift source rocks in the western Mentelle Basin, and may be a significant exploration play.
In the eastern Mentelle Basin potential trap types include faulted anticlines, highside fault blocks, and sub-unconformity traps (Play Type 5, Figure 7). Reservoirs are likely to be present throughout the pre-breakup succession and Lower Permian coals should provide mature source rocks. Potential stratigraphic traps (Play Type 6, Figure 7) occur within post-rift strata, both in the eastern and western Mentelle basins. These traps include turbidite sandstones sealed by slope mudstones within prograding shelf-slope wedges and ponded turbidite sandstones encased in slope mudstones.
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