Exploration of Contemporaneous Delta and Deep-Water Deposits: Berriasian Lower Barrow Group, Exmouth Plateau, North Carnarvon Basin, Australia
Published:December 01, 2012
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Minken Jon, C. Robertson Handford, Miller Josh, 2012. "Exploration of Contemporaneous Delta and Deep-Water Deposits: Berriasian Lower Barrow Group, Exmouth Plateau, North Carnarvon Basin, Australia", New Understanding of the Petroleum Systems of Continental Margins of the World, Norman C. Rosen, Paul Weimer, Sylvia Maria Coutes dos Anjos, Sverre Henrickson, Edmundo Marques, Mike Mayall, Richard Fillon, Tony D’Agostino, Art Saller, Kurt Campion, Tim Huang, Rick Sarg, Fred Schroeder
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An active exploration campaign in the Exmouth Plateau has yielded gas discoveries in a coeval wave-influenced delta and deep-water, sand-rich fan succession. Depositional elements were organized into clinoform seismic stratigraphic units that blanketed irregular topography created by extensional tectonics. Clinoform geometries revealed steady, rising and falling shelf-slope break trajectories. Slope successions associated with rising trajectories were devoid of deep-water feeder systems. In contrast, during steady to falling trajectories, the slope was characterized by numerous gullies. These gullies served as the main delivery system for sediment gravity flows into the basin. In some instances, an individual gully dominated and captured the flows of subordinate gullies and developed into a larger feeder system. The feeder systems were self-sourced and cannibalized the deltaic and slope successions through knickpoint retreat.
Arcuate strandplains organized into wave-influenced cuspate lobes characterized the deltaic succession. Littoral drift was locally to the east. Delta front well information indicated excellent reservoir quality. Sedimentological analysis of core data indicated different depositional processes as a function of the clinoform geometries. High quality delta front sands were fed into the slope and basin floor as sediment gravity flows and deposited as coalescing sand-rich fans. The fan cores were composed of high-density turbidites that graded into debrites and linked debrites along the margins. The deep-water fans were of favorable to excellent reservoir quality.
Recent exploration drilling in WA-390-P, Exmouth Plateau, Australia resulted in the discovery of gas-condensate at several stratigraphic levels (Fig. 1). The youngest accumulations were found in the late syn-rift Lower Barrow Group (Fig. 2). Trapping geometries and reservoir fairways were defined during the rifting of greater India from Australia, which formed a series of horsts and grabens (Barber, 1988; Longley et al., 2002). After rifting ceased, the Exmouth Plateau subsided and a fine-grained sealing succession was deposited over the porous Lower Barrow Group reservoir sandstones forming a reservoir-seal pair. Gas-condensate was generated and expelled from organic shales and coals in the Mungaroo Formation that migrated into the traps (Fig. 2) (Bussell et al., 2001). Ongoing subsurface analysis is currently delineating the resource range of the discoveries in WA-390-P; no fields are currently on production.
Seismic stratigraphic methods utilized a high-quality, 3D seismic data set to characterize the Lower Barrow Group reservoirs. Modern well log, core, and biostratigraphic data have calibrated the seismic data that allowed for the definition of architectural elements and integrated reservoir-scale studies from the sequence to pore throat. Architectural elements were populated within geocellular models that utilized ranges in rock properties as defined by discovery and offset wells. The purpose to constructing the geocellular models was to define ranges in gas initially in place (GIIP), estimated ultimate recoverable (EUR) and plan field development.
Late synrift extension caused uplift along the hinterlands that sourced the northerly prograding Lower Barrow Group deltaic complex (Jablonski, 1997). Local sedimentation patterns were controlled by fault subsidence. Barber (1988), described the complex as a delta-fed submarine ‘ramp’ consisting of deep-water shales that graded into an aggrading ‘ramp’ of submarine fan facies, shaley slope successions characterized by a network of small channels and sandy shoreface fluviatile units. In the middle Valanginian, the system was abandonment as the sediment source was removed during the final separation of greater India and Australia (Jablonski, 1997; Longley et al., 2002).
Sequence Stratigraphic Framework
Seismic stratigraphic units were characterized clinoforms that prograded towards the north (Fig. 3). Calibration from well information demonstrated the clinoforms were sand-prone toesets, shale-prone foresets, and sand-prone topsets. Seismic sections indicated clinoform morphology varied between sigmoidal, complex sigmoid-oblique, and oblique (Mitchum et al., 1977). Several well penetrations through the clinoforms enabled detailed biostratigraphic analysis and the identification of chronostratigraphic markers (Fig. 4). The marker tie lines illustrated contemporaneous depositional systems between the delta top and the deep water. Accommodation and sediment supply controlled the clinoform geometry. To determine the variability of sediment supply and accommodation through the succession, the clinoform trajectory was mapped through a series of clinoform sets. The clinoform trajectory was the successive positions of the shelf-slope break (Steel and Olsen, 2002).
Rising shelf-slope break trajectories
Complex sigmoid-oblique and sigmoidal clinoform sets were associated with a rising trajectory (Fig. 5). These geometries have well-developed topsets that indicated sediment supply kept pace, or was exceeded by shelf accommodation during deposition. The topset comprised a shallow marine platform developed by regressive and transgressive cycles of the shoreline (Steel and Olsen, 2002). Regressive components of the shallow marine platform were dominated by deltaic and shelf deposits that show an increase in wave energy towards the shelf-slope break. The increased wave energy near the shelf-slope break was attributed to obstruction by the sea floor and the brunt of the energy focused on the strandplain. In contrast, wave energy was progressively impeded by the sea floor on the shelf and diminished up the seaward-deepening profile towards the paleo-coastline (Swift and Thorne, 1991). In the proximal shallow marine succession, an increase in tidal and fluvial energy was observed. Foreset reflectors had a smooth profile that converged down-dip into a smaller number of toesets. The convergence of the reflectors indicated a decreased sediment volume reaching the lower slope and basin floor.
In planform, the clinoform topsets and foresets were organized into a series of laterally continuous, amalgamated, arcuate to cuspate, shore parallel edifices comprised of individual strand plains (Fig. 6). The cuspate nature of the lobes indicated wave influence. Within the study area, no distributary channel networks were identified within the seismic or well data. Therefore, it has been postulated that most of the sediment was transported by longshore drift into the study area from the west. The geomorphology was consistent with a westerly longshore drift and the delta showed a decrease in thickness and sand content towards the east (Fig. 7). Other studies have illustrated that a large amount of sediment composing a deltaic edifice can be derived from longshore drift (Bhattacharya and Giosan, 2002).
Steady to falling shelf-slope break trajectories
Flat to falling clinoform set trajectories have oblique geometries that were highly progradational (Fig. 5). The foreset reflectors were discontinuous, had varying degrees of reflectivity, and were commonly associated with thick toesets. Steel and Olsen (2002) have used similar clinoform trajectories to predict time equivalent deep-water fans. The oblique geometries were associated with periods when sediment supply exceeded shelf accommodation, promoting deposition on the foreset and toeset (Fig. 5). The absence of topsets in these geometries implied that deltaic environments existed at or near the shelf-slope break, and fed sediment directly into the slope and deep-water.
Deposition in the deep-water was influenced by late synrift tectonics that developed irregular antecedent topography in the basin. Toeset reflectors were observed to onlap and blanket the preexisting topography, smoothing out irregularities with basinward progradation (Fig. 8). Deposition began in the grabens and half-grabens, but as accommodation was filled, the depocenter moved to the dip-slope of the tilted fault blocks and the graben margins. Detailed seismic extractions on individual toeset reflectors revealed a hierarchy of coalescing fans (Fig. 9).
Deep-water feeder systems
A line-sourced feeder system of gullies demarcated the foresets of oblique clinoforms that transported sediment gravity flows into the deep-water (Fig. 10). Individual gullies were low sinuosity, ranged in width from 50 meters to 600 meters, and expanded rapidly down depositional dip. These gullies passed seaward into a down-dip sediment apron. Gully heads commonly reached the shelf-slope break, but in some instances have been observed to retreat into the topset.
Occasionally, some gullies developed into larger features, which could be classified as canyons that reached up to 1-2 kilometer in width (Fig. 11). Formation of a canyon was facilitated by progressive capture of subordinate gullies and being favorably positioned to access shelf-generated sediment gravity flows. Gully capture was facilitated by knickpoint propagation and erosion that caused retreat and flow divergence from a subordinate gully. A canyon was formed by erosion and widening owing to the higher sediment discharge. Progressive erosion at the gully head was not limited to the slope, but was also observed to cannibalize the shelf succession (Fig. 11). No fluvial systems were observed in the shelf and therefore the gullies were considered to be self-sourced by erosion of the slope and shelf. It was expected from the well information that erosion of the shelf contributed sand-prone constituents to the gravity flows, where degradation of the slope added finer-grained material (Fig. 4 and Fig. 7). Additionally, the wave-influenced delta morphology indicated favorable conditions for the development of longshore drift cells and the transportation of sediment by longshore drift into the gullies and canyons (Fig. 6).
Morphologically, the gullies resembled Type II canyons described by Jobe et al., 2011 in that only in some instances do they indent the shelf-slope break. The gullies contrasted from the Type II canyons in minimal sediment supply and low depositional energy origins (Jobe et al., 2011). The described gullies were formed and modified by high energy processes as indicated by the v-shaped geometry and terraces within the canyon indicating mass-wasting and erosion (Cf. Jobe et al., 2011, Type I canyons). No levees were observed on the gully margins that indicated the sediment gravity flows were largely confined.
Figure 12 illustrates a potential candidate for a gully fill that has eroded into a topset. The fill was defined at the base by a fining-upward, mud clast breccia. The clasts were intra-formational and had angular geometries indicating a local source. The clasts were randomly oriented within a well-sorted, medium-grained sandstone matrix. Their origins were attributed to the degradation of the gully margins. Overlying the mud clasts were homogenous, medium-grained, well-sorted sandstone having equivocal low-angle cross-bedding. This bed graded into a medium- to very finegrained sandstone having lower flow regime parallel laminations that indicated a decrease in depositional energy. The succession was finally capped by climbing ripples.
Architectural elements are genetically related, hierarchically organized sedimentary bodies possessing similar facies, geometries, and bounding surfaces (Pickering et al., 1989). Originally defined by Miall (1985) based on fluvial systems, the concept can be applied to most depositional systems by the integration of seismic stratigraphic methods and well data. In this study, the data sets included a modern 3D seismic data set, 20 wells with recent well log suites and biostratigraphic information, six full-bore cores, and associated data sets. Integration of this data allowed for definition and a robust understanding of the Lower Barrow Group architectural elements. The architectural elements were used as building blocks for geocellular models.
Delta architectural elements
The deltaic shallow marine platform architectural elements were arcuate to cuspate strandplains that were laterally continuous along depositional strike and narrow but amalgamated along depositional dip as illustrated in seismic horizon slices (Fig. 6). Integration of the seismic data with the well data indicated the shallow marine platforms were organized into an overall coarsening-upward succession of higher order parase-quences (Fig. 13). A complete parasequence in this example represented the shoaling-upwards of environments from distal, lower, middle, and into upper shoreface with the succession capped by a flooding surface. The parasequence succession in Figure 13 is a regressive, wave-influenced, deltaic shoreface that was deposited near the shelf-slope break. In more proximal positions, the equivalent parasequences were expected to be more tidally influenced.
Lower shoreface successions were medium- to fine-grained argillaceous, low-angle cross-stratified sandstones containing Diplocraterion, Skolithos, Rossella, and Ophiomorpha trace fossils. Deposition of the lower shoreface was below fair-weather wave base and above storm-weather wave base. Middle shoreface sandstones were medium-grained, well sorted, and cross-stratified, possibly indicating hummocky cross-stratification. Moderate bioturbation was observed from Skolithos and Arenicolites ichnofauna. Deposition occurred near the fair-weather wave base. Upper shore-face sands were coarse-grained, well sorted, posessed large-scale cross-stratification, and patchy calcite cement. Partial bioturbation was observed from Skolithos and Ophiomorpha trace fossils. The upper shoreface sandstones have been deposited above the fair weather wave base by shoaling waves and currents. Routine core analysis indicated excellent reservoir quality with porosities up to 30% and permeabilities in the multi-Darcy range (Fig. 14).
Deep-water architectural elements
Coalescing fans were the primary deep-water reservoir architectural elements (Fig. 9). Basinward, the sandy fan deposits evolved onto muddy basin floor deposits that had polygonal faults. The polygonal faults were associated with dewatering and compaction of fine-grained, mud-dominated sediments (Lonergan et al., 1998). The fan architecture was delineated by numerous well penetrations (Fig. 15).
Core data from four wells provided insight in the sedimentological characteristics and variability of the fans. Axial deep-water deposits were sand-prone and fine-upwards in a “box-car” well-log motif. Near the fan margins, the “box-car” pattern was replaced by a serrated well-log motif. The inner fans were composed of coarse-grained to pebbly sandstones and had faint parallel laminations and fluid escape structures that were deposited by high-density sediment gravity flows (sensu Lowe, 1982) (Fig. 16).
Occasionally, debrites and low density turbidites have been found interbedded within the inner fan succession. At the fan margins, homogenous high-density sediment gravity flows have thinned and evolved into debrites that in some instances have been interpreted as linked debrites as described by Haughton et al., 2003 (Fig. 17). More commonly, the distal debrites were found as isolated beds. At the fan margins there was a paucity of sandy low density turbidites. Routine core analysis of the deep-water cores indicates multi-Darcy reservoirs at the fan core that degraded slightly towards the fan margin owing to the incorporation of mudstones and siltstone within the debrites (Fig. 18).
Facies and Property Modeling
To calculate GIIP and EUR, the architectural elements were replicated in geocellular models. A method to define the facies in analogous wells, without core data, was implemented. Facies were defined by a multiresolution graphical clustering (MRGC) approach. MRGC used a multidimensional recognition method based on nearest neighbor and graph data representation of petrophysical data that analyzed the underlying natural forming data groups (Shin –Ju Ye and Rabiller, 2000). Data groups were then clustered into sets that were merged into electrofacies. The electrofacies were further simplified into groups that reflected the important geologic variability and heterogeneities expected to affect the dynamic delivery of the reservoir. These electrofacies groups were then propagated to all analogous wells. Reservoir properties were tied to these facies (Fig. 19).
The approach quantified the ranges in reservoir properties and uncertainties in facies distribution through the architectural elements by including wells from the discovered fields and analogous wells. Uniform electrofacies were used in the facies modeling for fields with similar architectural elements and to define ranges in reservoir properties. This methodology was increasingly important for characterizing GIIP and EUR ranges for fields with a single well penetration.
The Lower Barrow Group consisted of a wave-influenced delta and contemporaneous deep-water fans that were organized into clinoform seismic stratigraphic units. Delta-top and deep-water depositional elements form important reservoirs in gas-condensate accumulations in the WA-390-P concession area.
Clinoform geometries were oblique, complex oblique-sigmoid and sigmoidal. Oblique clinoforms possessed well-developed toesets formed during periods when the shelf-slope break had a uniform to falling trajectory. At these times, sediment supply exceeded shelf accommodation and was input into the deep-water. Sigmoidal clinoforms were associated with a rising shelf-slope break that indicated sediment supply was keeping up with or exceeding shelf accommodation. These clinoform geometries have well-developed topsets, indicating deposition on the shelf, and foresets that converge into a smaller number of toeset reflectors, indicating low sediment input into the deep-water.
Oblique clinoform foreset were demarcated by gullies. These gullies formed a line source for feeding sediment into the deep-water. Occasionally, a gully developed into a larger canyon by capturing subordinate gullies.
Delta-top architectural elements consisted of a wave-influenced, coarsening-upward, shoreface successions that were organized into arcuate and cuspate lobes with excellent reservoir properties.
Deep-water architectural elements were sands-rich coalescing fans. The cores of the fans were primarily high-density turbidites of excellent reservoir quality that graded into debrites and linked debrites at the fan margins with moderate reservoir quality.
Architectural elements were constructed in geo-cellular models using facies derived from an MRGC electrofacies approach. The electrofacies were propagated to all analogous wells and related to the block-wide data set of conventional core, sidewall core, biostratigraphic information, and wireline logs to define a holistic range in the reservoir properties throughout the architectural elements. This methodology facilitated a robust understanding of the uncertainties in facies model and reservoir properties of the architectural elements used in the calculation of GIIP and EUR.
The author thanks Hess Australia Corporation for permission to publish the work on the depositional systems of the Lower Barrow Group. This work includes substantial contributions from numerous geoscientists that have worked WA-390-P from exploration, technology and development during the early days identifying the fields through to the appraisal and characterization of the gas-condensate accumulations. I thank Phil Seligmann, Steve Massie, Steve Carney, Mark Whelan, Tim Kirst, Dean Griffin, Paul Owen, Yohan Kusumangera, Phil Cox, Woody Prescott, Pat Boss, John Smallwood, Richard Harmer, Toni Munckton, and Antonie du Toit for discussions and insights on the Lower Barrow Group.
Figures & Tables
New Understanding of the Petroleum Systems of Continental Margins of the World
- abyssal plains
- Carnarvon Basin
- clastic rocks
- coastal environment
- continental shelf
- continental slope
- deep-water environment
- deltaic environment
- erosion features
- Exmouth Plateau
- geophysical methods
- geophysical profiles
- geophysical surveys
- gravity flows
- Indian Ocean
- Lower Cretaceous
- marine environment
- natural gas
- petroleum exploration
- reservoir rocks
- sedimentary rocks
- seismic methods
- seismic profiles
- shallow-water environment
- source rocks
- submarine fans
- well logs
- Western Australia
- Barrow Group
- Mungaroo Formation
- northern Carnarvon Basin