Click here to see the deliverables from LEBARG Phase II
Overview of the Lake Eyre Basin
The Lake Eyre Basin (1.3 Million km2) is a vast, intracratonic, internally-draining, low accommodation dryland fluvial-lacustrine basin. The modern basin is a wide shallow Cainozoic structure superimposed upon the even more extensive magnesium-rich pre-existing megalake of the Mesozoic Eromanga Basin and early Tertiary Birdsville Basin which occupied much of central Australia. Rainfall ranges from 500-400 mm/y in the headwaters to 120 mm/y around Lake Eyre with rivers showing extreme flow variability. Major depositional elements in the Lake Eyre Basin are ephemeral streams, floodouts, dune fields and playas (Fig. 1). Between major rivers, Cretaceous sandstones and with local Tertiary strata form uplands capped by sheets of wind-abraded gravel (gibber plains) and thick duricrusts.
FIGURE 1. Overview of the Lake Eyre Basin with its major depositional elements
Within the south western corner of the basin lies Lake Eyre (Fig. 1), which is the fifth largest terminal lake in the world and the largest salina-playa in Australia. It is located some 1000 km north of the city of Adelaide, South Australia (Fig. 1). Lake Eyre intermittently becomes a true playa lake only when flooded but is usually a dry salina with an evaporite crust covering the southern low-lying areas. Flood waters enter Lake Eyre from the several river systems that form the catchment. During the most common minor floodings, and even during the early stages of rare great fill floodings whilst lake level is rising, the terminus of each of these terminal splay complexes lies above the level of lake fill and is neither inundated by nor abutting the lake waters. The cessation of sedimentary deposition at the terminus of each of these terminal splays therefore implies that mechanisms other than the simple interaction of fluvial waters with standing lake waters are involved.
A variety of terminal splay complexes can be found along the shoreline of Lake Eyre North. Three of these, the Neales, Umbum and Douglas Terminal Splay Complexes (TSC), were studied for detailed facies distribution, element architecture and reservoir geometry during Phase I and II (Fig. 2).
FIGURE 2. Satellite image showing Lake Eyre North and adjacent areas.
The modern Neales terminus covers an area of approximately 25 km2 and consists of a fluvial-dominated, high constructive triangular lobate terminal splay complex (Fig. 2). Three active rectilinear avulsion distributary channels (ADCs) dominate the system. The distributary complexes are generally highly constructive lobate terminal splays, with a broad middle-ground bar complex incised by deeper distributary channels (Fig. 3).
The Umbum TSC is a radial distributive system covering about 15 km2 (Fig. 2). It comprises five major erosive ADCs that deposit locally reworked and flood-borne sediment as splays at the end of each channel (Fig. 4). Beyond the terminal splay and across the playa floor, suspended fine silts and parallel laminated muds are deposited by a series of braided channels. Following flood events, suspended sediment within ponded water settles out of suspension, forming mud lenses at the base of channels, which are later desiccated.
The approximately 4 km2 Douglas TSC is characterized by two ADCs which terminate basinwards, causing propagation through sheetfloods (Fig. 2). With increasing radial distance from the source, the splay shows a decrease in grain size, lithofacies thickness, syn-depositional incisional surfaces and primary sedimentary structures. The exposed surface of the splay is etched by a network of minor distributary channels that are often draped by a thin layer of clay.
FIGURE 3. Schematic block diagram showing the distribution of architectural elements of the distal part of the Neales TSC (modified after: Hicks, 1998).
FIGURE 4. Surficial facies map of the lower Umbum Creek and the Umbum Terminal Splay Complex (Mark Reilly, 2005).
Objectives of Phase III
This proposal builds on the completed Phase I and II projects and extends the research program to focus on five specific research objectives.
Specific research objectives
1) Expanding Terminal Splay Complex Facies Model
The established facies model under Phase II is based on detailed studies of three coarse-grained, high-energy TSCs (Neales, Umbum, Douglas) with comparable catchment geology. The two proposed new TSCs investigated in Phase III are the Neales Overflow and the Kalaweerina (Fig. 2).
The Neales Overflow TSC is the product of three individual creeks converging in Belt Bay in the northwest corner of Lake Eyre North. The catchments of these creeks are short and underlain by Cretaceous bedrock comprising shales and sandstones causing more finer-grained splay deposits and thicker channel mud-plugs. Furthermore, it is planned to investigate the avulsion history of the lower Neales River and its overflow as this will be crucial for understanding how avulsion operates spatially and temporarily in dryland rivers and how it influences the development of terminal splay complexes.
The Kalaweerina TSC is situated on the eastern Lake Eyre and is characterised by an extremely long river course, large catchment and a very low river gradient. Because of the low gradient the terminal splay itself is only fed by a local sandy catchment composed of Pleistocene aeolian dunes. These dunes are believed to have a major effect on sediment composition and facies geometry of the TSC and constitute a perfect site to study fluvio-aeolian interactions.
Extending the study to the Neales Overflow and the Kalaweerina TSC will allow the documentation of commonalities and differences between depositional elements. Despite the overall dryland depositional setting the five TSCs are morphologically and compositionally very different and therefore provide important insights into potential variability encountered in subsurface analogues.
The overall aim of this research objective is in determining the overriding influences that effect sand distribution patterns in dryland fluvial terminal splay complexes. Furthermore, a better understanding on the influence of fluvio-aeolian interaction in these setting will help to establish more diagnostic criteria for dryland environments.
2) Application of dataset to reservoir modelling and 3-D reservoir models
Detailed statistical analysis and modelling will be performed on the data generated during Phase II and the new data generated during Phase III. This will include Monte Carlo simulation of grain size distributions and reservoir geometries, the creation of semi-variograms from the grain size and compositional data, and the building of 3-D models in PETREL or RMS.
In addition, the scalability of geometry data from all rivers and TSCs will be tested against those from existing fields. This requires companies to supply feedback either indirectly or through interactions during the free consulting week offered by us as part of consortium membership (see below).
3) Preservation and Correlation
A key question regarding the application of any modern analogue is the preservation potential of the observed depositional products and their impact on reservoir connectivity. The Lake Eyre Basin is an area of slow tectonic subsidence in the continental interior, and in Lake Eyre itself this leads to a relatively low rate of increase in accommodation to permanently preserve sediments. This tends to promote sand connectivity because of the reduced accommodation space to store fine-grained sediments of the floodplain and the distal terminal splay complex. The low accommodation rate is also impacted by the strong aeolian deflation excavating sediments accumulating temporarily in and around the playa lake. This may, however, be offset by climatic wet phases when increased rainfall and discharge in the fluvial systems re-deposits these aeolian sediments back into Lake Eyre. These processes all contribute to the development of an extensive, relatively coarse-grained sandy and gravelly sand sheet with high potential reservoir connectivity. The quantitative measurement of these influences is the subject of Phase III through correlations of dryland cycles between the Neales and Umbum systems and through OSL/TL dating of these deposits.
4) Lake Eyre Basin
During Phase I and II the question emerged how mega-scale components relate temporarily and spatially throughout the basin. The north eastern rivers are characterised by extensive catchments and receive the highest precipitation from tropical cyclones in far-northern Australia, whereas rivers in the western part have smaller catchments and comparatively little precipitation. These shorter western rivers terminate within Lake Eyre forming sandy terminal splay complexes. However, the larger north eastern rivers deposit most of the their coarse-grained sediment load in large floodouts 400 km inland from the playa margin, and mainly fine-grained sediment is transported to Lake Eyre only. The floodouts are up to 300 km long and 100 km wide and comprise anabranching rivers separated by extensive floodplains. The understanding of these floodplains and how to distinguish them from playa deposits is of crucial importance for the development of basin-scale palaeo-geographic and palaeo-environmental reconstructions for existing fields from producing basins elsewhere (e.g. North Sea; Chad, Algeria). The floodplains are dominated by broad Quaternary longitudinal and transverse, aeolian dune complexes (Simpson, Strzelecki, Tirari deserts) that represent a significant sand storage area, ready to be reworked into fluvial systems in a future wet phase.
The high variability of depositional elements at various scales within the basin emphasizes the care that needs to be taken when using the Lake Eyre Basin as an analogue for both regional and field scale palaeogeographic reconstructions. The aim of this research objective is to establish a basin-scale facies model for dryland sedimentary basins in an intracratonic setting through selected studies on the various elements occurring within the basin.
5) Subsurface Applications & Technology Transfer
The comprehensive data set collected from the Lake Eyre Basin offers the opportunity to transfer and apply the outcomes to the subsurface. This will provide the sponsors and researchers with a verification of the usability of this analogue data set. It is proposed that as part of consortium membership each sponsor will get the opportunity to invite LEBARG team members to review and discuss their (confidential) analogue data sets (i.e. core, outcrop, well and seismic data). This hands-on technology transfer aims at disseminating all information and experiences gained by both sides and will aim to improve the characterisation of fields in dryland settings.
In return for participation in this project, consortia members can expect immediate access to accumulated knowledge on the Lake Eyre depositional system and reservoir architecture in digital form via our password protected web page.
New consortium members will receive a copy of the Phase II final report on DVD as part of the sign-on fee. Throughout the duration of this consortium regular web-based updates on the project with preliminarily results delivered by Mid 2007 as part of the proposed field workshop (field workshop guide) can be expected. The first field workshop will play a key role for participating consortia members and invited experts to familiarize themselves with the data first hand and direct the research project within the scope set out in this proposal.
A field workshop is planned for Mid 2009. Note that the costs for participation in the field workshops are not budgeted for in this proposal; participants will need to fund their attendance but consortia funding will cover the preparation for the workshops. Individual sponsors may wish to fund additional field training courses for their staff.
A comprehensive final report in digital format (DVD based web document) with accompanying databases and PETREL/RMS models will be delivered at the end of the project.
Interested parties should contact Kathryn Amos ().