Abstract The Cambrian Donkey Bore Syncline exposes a salt-withdrawal minibasin filled with more than 500 m (1640 ft) of clastic sediments in the northern part of the Flinders Ranges, South Australia. The minibasin formed in the Early Cambrian as the Adelaide geosyncline passive margin was inverted. Analysis of salt geometries within the Flinders Ranges suggests that the Delamerian orogeny may have commenced during the latest Proterozoic (Mark G. Rowan and Bruno C. Vendeville, personal communication, 2006). If true, movement of the Willouran-aged Callanna Group salt would have been enhanced by shortening-induced folding and diapir squeezing. The doubly folded syncline exposes shallowly dipping sediments along its three limbs across an area of approximately 21 km 2 (8 mi 2 ) next to the Wirrealpa Diapir. The minibasin was situated on the upper slope, most likely below storm wave base. It was flanked to the south by a shallow-marine clastic depositional environment during deep-water sandstone (Bunkers Sandstone) deposition. Paleocurrent data within the basin as well as the axes of slumped sandstone beds indicate sediment input from the south. The main deep-water sediments of the Bunkers Sandstone are up to 350 m (1150 ft) thick. They have an overall net-to-gross of approximately 30% and form a transgressive package with each unit becoming successively less sand-rich. Three classes of architectural elements, ranging in thickness from 50-90 m (164-295 ft), are present in outcrop. These include 1) sandstone-rich sheets with a net-to-gross of up to 70% (Unit A), 2) sandstone-rich thin beds with net-to-gross of up to 60%

Abstract Despite increasing computer power, the need to upscale 3D geologic models for reservoir simulation is likely to continue in many commercial environments for some time. Therefore an understanding of the depositional environments in combination with modern and ancient analogs can provide the modeling team with geologically sensible scales for upscaled cell sizes in both geologic (static) and simulation (dynamic) models. A case study from the Flounder Field in the Gippsland Basin, southeastern Australia, is used to demonstrate the differences that understanding depositional environments can make in the approach to building 3D geologic models. The interpretation of depositional environments at an early stage can aid in the planning and building process of static and the resultant dynamic models. The interpretation of a certain depositional environment has implications for grid orientation, grid cell size, and the population of grids with facies and ultimately petrophysical properties. Placing the geologic interpretation into a sequence stratigraphic context can assist in the recognition of potential flow units and aid the prediction of flow-unit boundaries, facies distribution, and sandbody dimensions. Understanding the dimensions of the smallest sandbodies that are likely to influence fluid flow through the model is critical to the upscaling of facies models. Upscaling a model beyond half the length and/or width of the smallest influential depositional feature can result in changes to sand-body morphology. In addition, the relative influence of these bodies can be either magnified or decreased. In the Flounder Field a transgressive barrier-island-tidal-delta complex is interpreted. The smallest influential depositional features in this barrier-island system are the tidal channels, which are on average 600 m wide. Thus the side of the cells that cuts across a tidal channel should not be more than 300 m wide if the width and morphology of the tidal channels is to be modeled accurately. This places limitations on the maximum extent to which a model of this environment should be upscaled for reservoir simulation if the porosity and permeability trends of the reservoir are to be maintained. One of the keys to successful modeling is a clear understanding of the purpose of the models. A simple nomogram can be used to calculate the approximate number of cells in a 3D model before a model is built, enabling discussion between all interested parties about the potential dimensions of both static and upscaled dynamic models during the planning stage.

Abstract A seismic and multibeam sonar (swath) imaging study onboard the RV Southern Surveyor in Hervey Bay, east coast of Australia, revealed an incised valley (up to 50 m deep, 600 m wide, and at least 5 km long) cut into a Pleistocene shelf carbonate platform. The incision occurred in the Late Pleistocene, during the last eustatic sea-level lowstand. The subsequent transgression drowned the incision, but none of the fluvial or estuarine sediments that are typically deposited within incised-valley systems during passage through the coastal zone are evident on seismic. Instead, the seismic shows a low-amplitude fill with few internal impedance contrasts, suggesting a homogeneous, sandy composition. The incision is currently underfilled, and shows as a sinuous impression on the sea floor. Shelf sand dunes can be seen on the surrounding carbonate shelf migrating from the south towards the valley. At one location in the study area, the dunes migrate into the valley and appear to be filling it, because the dune crests can be traced across the valley margin and into the valley. Backscatter data from the multibeam survey also confirm the presence of extensive sand covering the valley floor. Scours can be seen adjacent to the steep valley walls, indicating the presence of strong tidal currents, which could be a mechanism for redistributing sand inside the valley. Implications for existing facies models of incised-valley systems are significant, because the filling of this incision did not occur in the coastal zone during transgression. Filling by shelf sediments could occur at any time during the sea-level cycle when the shelf and valley are not subaerially exposed. Depending on the dominant shelf process responsible for filling the valley, the valley-fill deposits can be anything from homogeneous sand, as in the case described here, to highly variable shelf facies, which would complicate any potential reservoirs in such a valley fill.

Abstract Mutiara field produces hydrocarbons from middle Miocene fluvio-deltaic successions within the Kutei Basin, East Kalimantan, Indonesia. In strata exposed over the doubly plunging Sanga-Sanga and Samboja anticlines, a large number of cores and downhole logs provide an excellent opportunity to integrate surface and subsurface data to improve reservoir characterization for exploration and development. Sedimentary facies and paleocurrent analysis were used to gain insight into the distribution of the dominant channel deposits within the succession. Channelized sandstone bodies identified in outcrops and cores constitute the main hydrocarbon reservoirs. They commonly comprise single-story distributary-channel sandstones and occasionally multistory alluvial-channel sandstones. Paleocurrent analysis revealed the distributary channels flowing in an overall southward direction, roughly parallel to the strike of the anticlines, while two of three multistory channels trend northeast. The third multistory channel has a strong westward flow direction, which might indicate a valley incision of a previously more sinuous channel. The orientation of the single-story distributary channels can be explained as a result of active tectonism during the middle Miocene. Incision of the Mahakam River into the uplifting hinterland means that a point source of sediment supply has existed to the north of Mutiara Field since middle Miocene times. Growth of anticlines through regional inversion of older, extensional basement faults has restricted the eastward progradation of the paleo–Mahakam Delta. As a result, the delta distributary channels and delta progradation was merely towards the south and north, parallel to depositional strike, and not perpendicular, as commonly thought. In addition, low directional variance among the single-story and multistory channels suggests that the paleo–Mahakam Delta comprised low-sinuosity channels, which has strong implications for the exploration of stratigraphic traps. Periods of dominant alluvial sedimentation produced roughly west–east striking multistory channels, which can sometimes be linked to incised-valley fills. These sandbodies might have had sediment sources to the southwest of the Mutiara field, and thus belong to a totally different fluvial system.

Abstract The Late Devonian Rockfields Member of the Bulgeri Formation in northeast Queensland, Australia, was deposited in a tectonically active alluvial basin. The source area was an uplifted igneous and metamorphic terrain to the south of a major oblique-slip fault zone, forming the basin margin. The member is characterized by relatively uniform, very fine- to medium-grained, soft-sediment-deformed sandstones, interbedded predominantly with slightly reddened siltstones. Detailed mapping of sediment body geometries and internal structures has resulted in the recognition of eight architectural elements. Channel-fill elements include laminated sand sheets, dune complexes, scour fills, laterally-and downstream-accreting macroforms, and linguoid barforms. Floodplain elements include levees, crevasse splays, and overbank fines. The abundant soft-sediment deformation structures (mainly convolute lamination) were produced by liquefaction of sandy bedforms, during or immediately after rapid deposition. Larger scale structures, however, deform multiple erosion surfaces. These may be due to liquefaction of saturated unconsolidated sands during earthquakes of magnitude >5, with inferred epicenters along the fault zone 20 km to the south. The proposed depositional model comprises a broad, sandy braidplain with marked discharge variation, resulting in widespread seasonal flooding. Channel-avulsion events, combined with a relatively high subsidence rate associated with tectonic activity, resulted in the accumulation of laterally extensive, sheet-like, channel sandbodies and fine-grained floodplain deposits. A modem analog is the seasonally inundated low-sinuosity channel system of the alluvial plains and fans surrounding the Gulf of Carpentaria, north Queensland. This model bears some resemblance to published models for low-sinuosity streams, but sufficient differences warrant recognition as a distinct fluvial style. The unit represents an example of a strongly layered sequence with laterally extensive (>1,500 m) channel sandstone bodies 3-10 m thick, separated by floodplain deposits 0.3-2 m thick of comparable lateral extent (1,500 to >4,000 m). Hydrocarbon reservoirs with similar architecture, uniform grain size, and a general lack of internal fine partings could be considered to behave isotropically with respect to fluid flow within individual bodies.