By Mohamed Khalifa Email: mohamed20au@yahoo.com
School of Biological, Earth and Environmental Sciences, University of New South Wales
Present address: 185B Richardson Road, Mt Albert-Auckland 1004, New Zealand

Abstract

High-resolution seismic reflection profiles and well data from the Blantyre Sub-basin are used to identify a fluvial to nearshore system within Mulga Downs Group sediments. Sedimentological analysis was applied, using lithologic samples (cores and cuttings), wireline logs and seismic line information, to aid in the development of a geologic model for the Mulga Downs Group in the Blantyre Sub-basin. Based on its geometric pattern and internal lithofacies distribution, the Mulga Downs Group displays, in sedimentological logs, three different lithofacies successions. The lower part of the Mulga Downs Group (Snake Cave Interval) is a succession mainly composed of braided fluvial and meandering fluvial lithofacies, and the upper part of the Mulga Downs Group (Ravendale Interval) contains braided fluvial and meandering fluvial lithofacies passing upwards into estuarine tidal channel deposits and a nearshore lithofacies complex. Additional seismic facies analysis was also applied to some of the seismic lines, comparing well log observation with reflection configuration and other seismic characteristics for the Snake Cave and Ravendale Interval. The seismic facies patterns show continuous, semicontinuous and discontinuous reflections with moderate amplitude and high to moderate frequency. The unit identified from the well data can also be recognised in the seismic profiles, with features such as low-angle and high-angle clinoforms, parallel and sub-parallel reflections and a few hummocky/wavy reflections indicating scour or secondary channel-fills, shallow channel-fills, and small-scale fluvial channels.

Introduction

The Blantyre Sub-basin is located in the central part of the Darling Basin and is currently included in Petroleum Exploration License (PEL) 8, held by the New South Wales Department of Mineral Resources (Figure 1) (Byrnes, 1985 and Pearson, 2003). This paper presents the results of a detailed subsurface lithologic and seismic sedimentological study of the Mulga Downs Group in the study area. The data on which the paper is based includes cuttings and core material, together with geophysical data from two wells in the Blantyre Sub-basin shown in Figure 1. The features identified in the well sections were compared with the features of equivalent sediments as described in the literature, especially with those of fluvial and shallow marine deposits. Much of the information outlined here has not been published before. One reason for the present study was to gather details of the lithology and lithofacies in the Blantyre Sub-basin, and to investigate the similarity to other areas already described in the literature on the Mulga Downs Group in the western Darling Basin (e.g. Glen, 1979, 1982a; Neef et al., 1996a, b; Bembrick, 1997a, b; Neef and Bottrill, 2001 and Neef et al., 2003).

The data presented here represent a detailed subsurface stratigraphical and sedimentological analysis, based on geophysical data (especially gamma-ray logs) and lithologic samples of cores and cuttings from Blantyre-1 and Mount Emu-1 wells. A number of two-dimensional (2D) seismic surveys (especially seismic lines SS134>HD-125, 128, 129, SS143>HD-205, 220, 223 and new line is DMR03-05) were also used for the seismic sedimentological observations. The few biostratigraphic interpretations used in this paper were extracted from published and unpublished reports, and mainly used to constrain the stratigraphic ages of the beds concerned. It was hoped that this comprehensive data set would allow many of the subtle, yet important aspects of the seismic sedimentological and lithofacies framework for the Mulga Downs Group to be better resolved.

Lack of detailed study of the genetic stratal architecture in the sub-basin reflects primarily the limits of currently available methodologies. The most commonly used approach is interpretation of well data (cores, wire-line logs, faunal data, etc.) for a high-frequency sequence framework, then tying of well data to seismic lines for regional sequence and seismic-facies mapping (Mitchum, 1977; Mitchum et al., 1977; Sangree and Widmier, 1977; Van Wagoner et al., 1990; Okamura and Blum, 1993 and Skelly et al., 2003). The interpreter’s ability to carry out such sequence and seismic-facies mapping depends on the abundance of seismic terminations (onlap, downlap, toplap, and truncation) and seismic-facies indicators (reflection configuration) that can be related to depositional processes (Okamura and Blum, 1993 and Skelly et al., 2003). The two-dimensional seismic data were used to correlate specific sequence boundaries between boreholes (e.g. Blantyre-1 and Mount Emu-1) and to define the unit geometry. The seismic data were also evaluated to identify any variations in seismic facies analysis, based on the reflection configurations, amplitude, and continuity within the different lithofacies successions. It is therefore necessary for these to be integrated to provide a coherent understanding of the seismic sedimentological and lithofacies across the area, as a framework for future petroleum and other exploration programs.

Lithofacies Interpretation

Analysis and interpretation of the geophysical logs, such as the gamma ray curves, has been used to supplement the major lithofacies descriptions presented above, to give a better understanding of lithofacies variations (Figures 2a, b & 3). Three major lithofacies have been identified: Braided fluvial lithofacies, Meandering fluvial lithofacies and estuarine tidal channel to nearshore complex lithofacies.

Braided fluvial lithofacies

The braided fluvial lithofacies is recognised at several horizons, and is represented by the following Unit BS-1, ES-1 and BR-2 (Figures 2a, b & 3). These units show abundant fine-grained sandstones, siltstones and few metres of intercalated shale. The sandstones are poorly to well sorted, with subangular to subrounded quartz grains, medium- to coarse-grained and sometimes very coarse to pebbly, and may contain small proportions of massive, gravel-supported conglomerate. The sandstone and siltstone horizons are red-brown to grey-brown, light grey and green-grey, and the siltstone is sometimes off-white in colour. They are commonly interbedded with very fine-grained sandstone and shale.

Identification of sediments deposited under braided fluvial lithofacies conditions is generally based on the associated sedimentary structures, using relationships documented by Allen (1970), Allen and Banks (1972), Ashley (1978), Miall (1977, 1985), Reineck and Singh (1980) and Selley (1985). The sedimentary features commonly displayed by such sediments in the sub-basin sequence include medium- to coarse-grained sandstones with medium to large-scale trough cross-bedding and small-scale planar cross bedding.

Meandering Fluvial Lithofacies

Sediments of the meandering fluvial lithofacies are represented by Unit BR-1, BR-3 and ES-1. Numerous descriptions of modern and ancient examples of meandering fluvial depositional systems are given in the literature, along with models of such sediments for lithofacies interpretation (e.g. Allen, 1964; Cant, 1978a; Ethridge et al., 1981; Miall, 1977; Rust, 1978a; Reineck and Singh, 1980 and Selley, 1985). The three principal depositional sub-environments recognized in the meandering fluvial lithofacies of the Mulga Downs Group are: (1) crevasse splay channel and floodplain complex sub-environment, (2) abandoned channel sub-environment.

Crevasse splay channels and floodplain complex sub-environment: A complete lithologic description of this sub-environment was given in Unit ES-1 (Figure 3). Unit BR-1 is probably most similar to the floodplain complex or channel-fill sandstone deposits found in modern sediments, and it is suggested that sedimentary Sub-unit BR-3A mostly represents floodplain deposits (Figure 2a). Lithologically, this unit show an abundance of sandstone and siltstone, with rare to very rare thin beds of shale. The sandstone is mainly a very fine- to- fine-grained quartz sandstone, with subrounded to rounded grains. The crevasse splay channel and floodplain complex sub-environment shows large nodules of dense brown limestone, and carbonaceous inclusions are present.

The sedimentary structures in the crevasse splay channel and floodplain complex lithofacies include small-scale cross-bedding and ripple marks, grading into tabular planar cross-stratified medium to coarse sandstone. Palaeontological and palynological analyses of samples from the wells show Arthrodire, flora and acritarch, remains that suggest the environments for this lithofacies may represent crevasse splay channel to floodplain complex settings.

Abandoned channel sub-environment: Sediments deposited in an abandoned channel sub-environment in two distinctive units at different levels in the Blantyre-1 well. One is Sub-unit BR-3B (Core sample 19/ 1424.64 to 1429.51 m and core sample 18/ 1338.07 to 1342.64 m; in Figures 2b & 4) and the other is Unit BR-1 (Figure 2a). These facies are interpreted to have been formed as infilling channels. They are dominantly composed of very fine- to fine-grained sandstone interbedded with shale and very minor thin siltstone laminae.

The sediments of this association show mainly horizontal lamination, with local ripple cross-lamination and traces of plants and burrowing organisms. The cross-sections show the contact relationships and distribution of the meandering fluvial lithofacies based on a combination geophysical log data and the lithologic description of cuttings and core samples. Sediments of this lithofacies have a wide distribution in the study area.

Estuarine Tidal Channel to Nearshore Complex Lithofacies

The lithofacies and well-log analysis suggests that the estuarine tidal channel to nearshore complex lithofacies is restricted to the upper Ravendale Interval, and has only a relatively limited geographic distribution. Such sediment, however, reaches a maximum thickness of 850 m in the Blantyre-1 well (Figure 4). The estuarine tidal channel to nearshore complex lithofacies is represented by Unit BR-4. This can be further subdivided into main sub-facies, Sub-unit BR-4A and BR-4B (Figure 2a). Identification is based on relationships documented by Allen (1971, 1984); Allen et al. (1979, 1980); Cant and Walker (1978); Harms (1975a); Rahmani, 1988; Reineck and Singh (1980); Reading (1986) and Selley (1985).

The lithofacies consists of sandstones that are generally composed of fine- to medium-grained quartz and feldspar, with occasional skeletal remains and a few traces of glauconite and oolites. The interbedded siltstone has a wide variety of textures and is reddish in colour. Usually there is good grain sorting, with the frequent development of fining or coarsening upward sequences. The associations of sedimentary structures and general features point to an estuarine-fluvial environment, with small-scale sand-filled fluvial and tidal channels associated with tidal deltas. The following supports this interpretation:

Sediments deposited in this sub-environment exhibit a typical serrated gamma-ray log response (Figure 2a). They can be distinguished in each of the individual unit of the Ravendale Interval in the Blantyre-1 well, where they have an overall thickness of around 350 m. Sub-unit BR-4A consists of very pebbly or medium- to fine-grained sandstones and thin siltstones with sandstone laminae.

The paleodepositional environment of Sub-unit BR-4B (Figure 5), which is about 500 m thick, varies from tidal channel associated with tidal deltas to estuarine sand-filled channels. Its dominant sedimentary structures are small-scale ripple cross-bedding, cross-lamination and horizontal to low-angle planar or slightly wavy lamination, although sets of cross-stratification and cross-lamination in the opposite direction can frequently be observed. Bioturbated muddy sandstone occasionally overlies the silty shales, associated with a few trace fossils, and bone fragments have been recognized in the sediments (Campe and Cundill, 1965).

Interpretation of Sequence Boundaries and Seismic Facies Analysis

The continuity of the two strong seismic reflectors (SBMDG-1 and SBMDG-2) that have been traced on the key profiles (SS143>HD-205, SS134>HD-125, 128, 129 and DMR03-05, see Figure 1 map showing the distribution of the seismic profiles) and other sections throughout the Blantyre Sub-basin (Figures 6, 7, 8 & 9) suggests that they represent major stratigraphic boundaries. They are also interpreted as regional unconformity surfaces (cf. Evans, 1977; Bembrick, 1997a, b; Alder et al., 1988), and therefore are taken to represent significant sequence boundaries in the present study. There is, however, more than one possibility for the position of these sequence boundaries, within the framework of the large-scale sequences.

Two seismic markers interpreted as sequence boundaries within the Mulga Downs Group (SBMDG-1 and SBMDG-2) are recognized on the seismic profiles. The lower sequence is equivalent to the Snake Cave Interval, which is the lower part of the Mulga Downs Group sequence (SBMDG-1). The Ravendale Interval is equivalent to the upper part of the Mulga Downs Group sequence, the base of which corresponds to sequence boundary (SBMDG-2). These have been identified in the seismic lines shown in Figures 6, 7 and 8. These markers have also been tied to well data (e.g. Blantyre-1 and Mount Emu-1). In some cases (e.g. Figure 9), the markers cannot be tied to well data because no wells have been drilled, but the character of the seismic reflections suggests that they can still be traced as sequence boundaries within the stratigraphic succession.

Given the poor well control, any interpretation of depositional environments based on seismic facies is necessarily speculative. However, the environmental interpretations are supported by the unit analysis discussed above, which indicates that the lower part of the Mulga Downs Group consists of multiple fluvial channel-fill sequences. The upper part of the Mulga Downs Group (as seen in Blantyre-1) is mainly characterized by lithofacies that indicate an initial environment with large-scale fluvial channels that changed up-sequence to an estuarine or near-shore complex. As part of the study, an investigation was made to determine if further information on the different depositional environments might be obtained from recognition of distinctive seismic reflection patterns, thus providing an improved basis for constructing a series of depositional models for the Blantyre Sub-basin.

Seismic facies investigations are based on interpretation of the internal reflection patterns within individual sedimentary sequences (Mitchum, 1977; Mitchum et al., 1977 and Sangree and Widmier, 1977), Only limited work has been carried out to date aimed at interpreting continental sediments in this way, but key references include Okamura and Blum (1993) and Skelly et al. (2003). The application of seismic facies analysis to the lower and upper part of the Mulga Downs Group sequence within the Blantyre Sub-basin is discussed below, in relation to similar successions described elsewhere in the literature:

The architecture of the channel-belt deposits of the Niobrara River, northeast Nebraska, USA, recording the response of a sandy braided river to a rapid base level rise, has been studied by Skelly et al. (2003). Okamura and Blum (1993), studied the seismic stratigraphy of Quaternary stacked progradational sequences beneath three forearc basins off southwest Japan and in this study suggesting that the stack patterns in the three basins show similar successions of seven progradational sequences. A number of the more recently acquired seismic lines in the Blantyre Sub-basin, particularly seismic lines SS143>HD-205, SS134>HD-128, 129, 125 and DMR03-05, have been interpreted in a similar way, by more closely evaluating high-resolution and medium-resolution seismic data from a sedimentological point of view.

Seismic Facies Analysis for sequence MDG-1

Sequence boundary SBMG-1 is identified as the base of the Snake Cave Interval in the Mulga Downs Group sequence. The sequence boundary is equivalent to seismic horizon B identified by Evans (1977), Bembrick (1997a, b) and Alder et al. (1998). The internal seismic facies in the lower part of the Mulga Downs Group sequence (Snake Cave Interval) is characterised by reflections that become more continuous to semicontinuous and parallel, with locally sub-parallel reflections. In the seismic lines SS143>HD-205, SS134>HD-125 and DMR03-05 (see Figures 6, 7 & 8), the lower part of the Mulga Downs Group sequence features generally curved, concave-up reflections that are similar in form to erosion surfaces identified in other areas by Skelly et al. (2003) as marking a shallow channel complex or channel-fill facies and a scour or secondary channel-fill facies and low-angle clinoforms (cf. Skelly et al., 2003 and Okamura and Blum, 1993).

The Snake Cave sediments in the Mount Emu-1 well, in the centre of the seismic lines evaluated in this way, are interpreted as representing an upward transition from meandering to braided fluvial deposits (Figure 3), which is consistent with such a seismic facies interpretation. However, the two-way travel time associated with the features in the Blantyre Sub-basin (Figure 8) is significantly larger that those described by Skelly et al. (2003). The Blantyre Sub-basin features may represent similar sedimentlogical structures on a much larger scale, or they may represent artifacts developed in some way during data collection or data processing.

SBMDG-1 is the sequence boundary for the lower part of the Mulga Downs Group sequence (Snake Cave Interval), SBMDG-2 is the sequence boundary for the upper Mulga Downs Group sequence (Ravendale Interval), and SBWK is the sequence boundary at the base of the Winduck Interval (i.e. the seismic horizons originally identified by Evans, 1977). The northwestern portion of the line has been migrated. In the seismic line illustrated in Figure 10, the resulting reflections may be reduced high-angle clinoform reflections are discontinuous, as documented by Skelly et al. (2003).

Seismic Facies Analysis for sequence MDG-2

As shown in Figures 8 and 9, sequence boundary SBMDG-2 is identified at the base of the Ravendale Interval, within the upper part of the Mulga Downs Group sequence. It marks the boundary between the top of the Snake Cave and base of the Ravendale Intervals, equivalent to seismic horizon C according to Evans (1977), and Bembrick (1997a, b) and Alder et al. (1998). Sequence MDG-2 has similar interpreted reflections to sequence MDG-1, but the reflections are more continuous, smooth and parallel on seismic line SS143>HD-205 (Figure 9). They are similar to the relatively continuous, sub-parallel, and partly concave-up reflections that typically represent fluvial channel facies with shallow channel-fill complex facies (cf. Skelly et al., 2003).

The top part of sequence MDG-2 in seismic line SS134>HD-125 (Figure 7) shows fairly continuous reflections, with forms that are similar to those produced by channel-fills or shallow channel complexes with scours or secondary channel-fills. Smaller and less common high-angle clinoforms are present in the lower part of the sequence, along with parallel reflections, similar to those interpreted as braided fluvial deposits by Skelly et al. (2003).

Interpretation of the geophysical logs and cored intervals in the Blantyre well (Figure 2a) suggests that the lower part of the Ravendale Interval was deposited under meandering to braided fluvial conditions, but that the upper part represents mainly shallow marine deposits. The seismic facies appear to be consistent with the interpretation from the well data as fluvial deposits. The greater abundance of parallel reflections in the Ravendale Interval (cf. Okamura and Blum, 1993), compared to the underlying Snake Cave Interval, may possibly be related to the presence of shallow marine deposits, but this is not very clear from the available seismic data.

Concluding Remarks

Sedimentological analysis based on the well logs (e.g. Blantyre-1 and Mount Emu-1) and the lithological features of relevant core and cutting samples, have suggested a threefold lithofacies classification of the Mulga Downs Group sequence:

· Braided fluvial lithofacies association, with units representing a multi-story fluvial channel sub-environment and probably a sandy braided channel-fill sub-environment.

· Meandering fluvial lithofacies association, containing three main units: a crevasse play channel/floodplain complex sub-environment and an abandoned channel sub-environment).

· Estuarine tidal channel to near-shore lithofacies association, with units including deposits of an estuarine fluvial/small-scale fluvial channel sub-environment, a tidal channel and tidal delta sub-environment, and an estuarine sand-filled channel sub-environment.

Recent advances in sequence stratigraphic concepts provide a framework within which seismic lines can be utilized, in conjunction with other stratigraphic tools, in an integrated approach to seismic sedimentological analysis within the Blantyre Sub-basin. Details of the seismic lines have also been used to interpret the distribution of seismic facies in the subsurface sections of the Snake Cave Interval and Ravendale Interval. This is based on the form of the seismic reflection patterns and the variations in continuity and amplitude within the two intervals as seen in the seismic lines. Reflection continuity is generally fair to good, although it is poor in some places through all two stratigraphic intervals.

The principal results of the interpretation are presented in the seismic reflection patterns of the Snake Cave Interval and the Ravendale Interval. The seismic reflection patterns recognised include low-angle and high-angle clinoforms, and scour or secondary channel fills. These have been correlated to features encountered in the exploration wells (Blantyre-1 and Mount Emu-1) within the Blantyre Sub-basin.

Acknowledgements

Thanks to PESA News for permission to publish this material. The data for this paper represent part of a Ph.D. thesis completed in 2005, under the supervision of Professor Colin Ward and co-supervisor, Dr Derecke Palmer, at University of New South Wales. This thesis was supported by scholarship from the Libyan Government. I would like to record grateful thanks to Mr Phillip Cooney, Senior Geophysicist, for valuable discussions and help. Many thanks are expressed to the New South Wales Department of Mineral Resources for provision of the seismic lines, geophysical log data and samples (cuttings and core) used in this paper.

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