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The Phanerozoic Basin-Fill History Of The Roebuck Basin

Smith, Stuart A.

Doctor of Philosophy, 1999

University of Adelaide

Abstract

The Roebuck Basin is the least understood of four major sedimentary basins that makeup North West Shelf. During this study the key aim was to provide a structural and stratigraphic for the evolution of the basin and to evaluate its future petroleum potential.

Regional mapping has enabled accurate delineation of the structural features in the Roebuck Basin which were only crudely defined in older studies. Of note is the Oobagooma High, a newly identified Palaeozoic structure that separates the Oobagooma Sub-basin from the Rowley Sub-basin at the Palaeozoic level. Both the Oobagooma and Bedout Highs appear structurally related and are interpreted to have formed due to compressional forces over a major crustal detachment possibly related to an accommodation zone formed during earlier NE extension. Structuring along the margins of the basin are complex and changes from asymmetric half-graben extension onshore to symmetrical graben extension in the near offshore to thin-skinned detachment in the outer offshore area. Style of extension appears to be controlled by the location of the more rigid elements of the basin, with normal faults developing near the stable bounding Kimberley Craton and Leveque Platform to the NE and the Broome Platform to the SW. These structural provinces were used to divide the Oobagooma Sub-basin at the Palaeozoic level into three compartments separated by hardlinked transfer zones.

More detailed interpretation was conducted in conjunction with sequence stratigraphic analysis of available well data. This proved problematic in the upper part of the section which is carbonate-dominated. For carbonate-rich sediments wire-line methods have not been well documented. To resolve this, a new method of log interpretation was utilised for the upper part of the section. Using this and clastic wireline log interpretation methods, sixty-four boundaries were identified that could be partially correlated throughout the basin and used to date the section and divide the basin-fill.

Structurally, the Roebuck Basin is interpreted to have developed as a result of a multiphase rift history under four main stress regimes: NE-SW extension resulting in an intra-cratonic fracture sequence associated with the separation of the Chinese blocks from Gondwanaland during the Camabrian to Silurian; a transitional phase from NE-SW to NNW-SSE extension associated with the separation of the China-Buma-Malaya-Sumatra (SIBUMASU) blocks during the Late Carboniferous to Late Permian with development of both NW-SE and ENE-WSW structures; a NNE-SSW post Late Permian extensional phase resulting in formation of the Westralian Superbasin sequence (separation of Argoland and India); and a NE-SW compression phase during collision with Asia during the Middle Miocene to Recent. Each extensional basin phase is strongly related to the development of five break-up events that have effected the northwestern margin of Australia, four of which have been previously identified. A possible fifth break-up event which developed during the Early Jurassic has been identified during this study (previously identified as rift onset for the Middle Jurassic breakup episode).

Large scale differences in structuring between the Roebuck Basin (subtle) and its adjacent basins (pronounced) suggest that either the crustal composition or stress regime was different in this area. In contrast to the other basins, the Roebuck Basin has undergone NE extension prior to NNW extension which may have altered the crustal composition. Annealling of the upper crust by period heating and cooling during early rifting may strengthen it.

Major Triassic thermal sag and marine transgression has been punctuated by a series of NW-SE transpressional events that occurred along the margins of each sub-basin, known as the Fitzroy Movement. A theoretical model for progressive rift development around a pole or hinge point, that helps explain the formation of transpressional features during rifting, has been developed for the North West Shelf. The model demonstrates that as rifting progresses from NE to SE, transtensional/transpressional stresses will be developed due to differential rotational movement between adjacent blocks. During this study the Fitzroy Movement has been sub-divided into three tectonic episodes. Of these events Fitzroy Movement III appears associated with rift shoulder development on the margin during lower crustal/upper mantle thinning through pure shear processes during rifting in the unidentified rift event described above.

Due to limited availability of data, dating of events has been extremely difficult in the Roebuck Basin. Other authors identify similar surfaces in adjacent basins but with slightly different timing. This is interpreted to be a result of slight diachronous formation of sequence boundaries in response to different sediment budgets and tectonic regimes in each basin.

Forward sedimentary modelling using SEDPAK was employed directly as a tool for palaeogeographic and lithology prediction as well as hypothesis testing. Such predictions were used to directly develop hydrocarbon exploration play concepts and play element distribution and fairway maps in the area.

Six sea-level curves were used to derive sea-level during modelling. Simulations using locally derived Roebuck Basin sea-level curves, the Haq second and third order eustatic sea-level curves, produce similar overall geometries and internal characters which matched well with observed geometries in wells and seismic data. All offered reasonable and possible solutions. Simulations using the Roebuck Basin, Haq second and third order sea-level curve, although of differing frequencies and amplitudes, produced similar results. It appears that sediment supply rate is more important in controlling sediment geometries than higher order sea-level curve frequency and magnitude. Results from this study suggest that the curves derived by Haq et al. (1987) are reasonable approximations of the high-frequency relative sea-level oscillations that have occurred in the Roebuck Basin and could potentially be used as a first approximation to simulate basin-fill at other locations globally.

Failure to find commercial hydrocarbons in the basin is attributed to lack of source rock, seal potential and timing of maturation. During this study, source rocks were identified and their distribution predicted using a number of techniques, induding; sequence stratigraphic correlation, seismic mapping, forward sedimentary modelling and geochemical analysis. Using the above methods, ten source rock intervals were identified. These can be sub-divided into four groups of units depending on their tectono-depositional setting. Tr1 and Tr2 were deposited during rapid thermal sag, J1 to J5 were deposited during gentle thermal sag and infill, K1 and K2 were deposited during rift and post-rift sedimentation, and K3 was deposited during passive margin conditions. During maturity analysis vitrinite data suggests a thermal maturity lower than modelled maturities using present thermal conditions. The difference in modelled versus measured vitrinite reflectance is likely to be the result of vitrinite suppression which has been observed elsewhere on the North West Shelf. There is a general correlation between marine sequences and increased suppression in the Roebuck wells examined so far.

Sedimentary modelling of the Jurassic has been used to simulate the basin fill. Four marine incursions, each of which are followed by the development of a progradational wedge, are clearly identifiable. Thickest delta front development occurred in the inner Rowley Sub-basin to the north-west of the Bedout High and may offer many attractive stratigraphic exploration targets such as isolated channel systems, coastal sand deposits and barrier-bar systems. These reservoir-trap systems are potentially sourced/sealed by high-quality source rich, pro-delta, shale systems.


Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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