Sediment supply systems of the Champion “Delta” of NW Borneo: Implications for deepwater reservoir sandstones

Journal of Asian Earth Sciences 76 (2013) 356–371

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Sediment supply systems of the Champion ‘‘Delta’’ of NW Borneo: Implications for deepwater reservoir sandstones Joseph J. Lambiase a,⇑, Andrew B. Cullen b a b

Petroleum Geoscience Program, Department of Geology, Faculty of Science, Chulalongkorn University, 254 Phyathai Road, Bangkok 10330, Thailand Chesapeake Energy, Oklahoma City, OK, USA

a r t i c l e

i n f o

Article history: Available online 20 December 2012 Keywords: Sediment supply Deepwater NW Borneo

a b s t r a c t Middle Miocene to Pliocene sedimentation on the NW Borneo margin has been interpreted as the product of one relatively large deltaic system, the Champion Delta. However, several lines of evidence indicate that the Champion system was not a simple, large delta; its drainage basin was too small, fluvial outcrops indicate multiple, relatively small rivers and outcrop studies indicate the same facies associations as the diverse, modern depositional systems. The number and location of rivers reaching the shoreline changed as rapidly subsiding footwall synclines, episodically active inversion anticlines and growth faults created an evolving structurally-generated topography that not only controlled drainage pathways, but also segregated Champion strata into thick, wave-dominant and tide-dominant successions. Although the principal rivers within the Champion system, the Limbang, Padas and Trusan Rivers, transport significant loads of coarse sediment, the intermittent proximal ponding of sand in local basins, as is currently occurring in Brunei Bay, resulted in a variable delivery of sand to the shelf edge. The number and distribution of shelf edge canyons also changed with time. Consequently, the spatial and temporal distribution of deepwater sand accumulations sourced from the Champion system are not solely related to relative sea level fluctuations; such accumulations should be smaller and more scattered than those sourced from a large shelf edge delta. Because the catchments of the Champion system’s principal rivers represent different provenances, the system’s deepwater sands may carry the signal of specific rivers. For example, mineralogical contrasts between in the main reservoir sands of the deepwater Gumusut and Kikeh fields suggest that the relative contributions of the principal rivers shifted with time with the Trusan and Limbang Rivers dominating sand supply for the youngest reservoirs at Gumusut. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction NW Borneo is a foreland basin with an active deepwater fold and thrust belt (FTB, Fig. 1; Hamilton, 1979; Hinz et al., 1989; Hutchison, 1996). Seaward of the FTB, undeformed sediments in the NW Borneo Trough onlap the rifted crust of the Dangerous Grounds and represent the under-filled, distal part of the foreland basin (Fig. 1; Franke et al., 2008; Hesse et al., 2009; Cullen, 2010a). The present-day shelf and coastal plain of NW Borneo is a prolific petroleum province dominated by oil and gas fields that produce from middle Miocene to Pliocene shallow marine clastic reservoirs that were deposited when denudation of Borneo’s rising highlands shed a tremendous volume of sediment into the basin (Levell, 1987; Petronas, 1999; Sandal, 1996). Structural traps formed along gravity-driven normal faults generated by sediment loading that

⇑ Corresponding author. Tel.: +66 84 538 0541. E-mail addresses: [email protected] (J.J. Lambiase), abcullen@hotmail. com (A.B. Cullen). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.12.004

was a product of the rapid sedimentation. In addition, many of the fields have experienced structural inversion. Early last decade, the NW Borneo petroleum system has been proven to extend to the coeval deepwater turbidites trapped in structures of the NW Borneo FTB. However, following major deepwater oil discoveries at Kikeh (536 mmboe in place) and Gumusut (620 mmboe in place) in 2002 and 2003 respectively, there have been no significant discoveries announced; the recently announced Guenunggon-1 discovery appears to be an extension of the Gumusut Field. One of the critical subsurface factors determining the commercial success in deepwater exploration ventures is the presence of thick, porous and permeable reservoirs capable of sustaining high production rates. The linkage between the shelf and deepwater depositional systems is a critical consideration when risking the presence of reservoir and predicting reservoir quality with respect to connectivity and sandstone framework mineralogy. Published work to date (e.g. Grant, 2006; Cullen, 2010a) and the distribution of deepwater discoveries suggest that the Kikeh–Gumusut and Kebabangan–Ubah areas (Fig. 1) represent two different

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200

Pacific Ocean

Borneo

00

Indian Ocean 1200

1000

shelf edge

UB

KM KB

MK

KI GU

Klias Peninsula Berakas Syncline

Brunei Bay

Limbang River Luconia-Balingian Basin

Baram Delta

Baram River

Padas River

N Trusan River

50 km

Fig. 1. Location of the NW Borneo study area. Locations of deepwater oil fields are shown with gray circles: GU-Gumusut, KB-Kebabangan, KM-Kamunsu, KI-Kikeh, MKMalikai, UB-Ubah.

depositional systems. This paper considers the nature of the up-dip sediment sources and delivery system for the prolific late Miocene reservoirs of the Kikeh and Gumusut oil fields, the Pink, Kamunsu, and Kinarut fans (Fig. 2). We retain Levell’s (1987) nomenclature for the unconformities on the shelf, but use the slightly younger ages for those unconformities assigned by Krebs (2011) in a paper that synthesized extensive multi-discipline biostratigraphic studies of the shelf and deepwater sediments by various operators, principally Shell and Murphy. 1.1. Geological framework The present-day NW Borneo margin has a relatively straight coastline and shelf slope break. Whilst the trends of the paleo-shelf edges in the middle and late Miocene show a deflection relative

those in the latest Miocene to Recent (Fig. 3), the limited range of the shelf edge trajectory (ca. 50 km) attests to very high subsidence rates. These observations, and the fact that the oceanographic setting of NW Borneo in relation to the South China Sea’s monsoonal cycles, tides, and waves extends back to at least the early Miocene, suggests the persistence of broadly similar depositional systems (Petronas, 1999; Sandal, 1996). Across NW Borneo’s FTB, the widespread and repeated succession of mass transport deposits overlain by turbidite sandstones is interpreted as massive failure at the shelf-slope break followed by turbidity flows funnelled into the slope scar (Algar et al., 2011; Gee et al., 2007; Morley et al., 2003). With respect to delivering reservoir-quality sand across the shelf to feed the turbidity currents, regional depositional models for NW Borneo typically invoke two major deltaic systems, the Baram Delta to the SW and the

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Lingan Yellow

Ma

3.1

Pink SRU 8.5 Kamunsu

2.6

Kinarut Middle

15

3.4

Pliocene Late

Miocene

10

Fans / Reservoir/ Unconformities Unconformities shelf

5

TB

3.3

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+ sea level -

Ma

relation to folding

Synkinematic Early Synkinematic Pre-kinematic

UIU 10.4 LIU 11.6

2.5

Age

2.4

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DRU 15.5

Fig. 2. Stratigraphic nomenclature for the Kikeh and Gumusut reservoirs in relation to the shelf unconformities; modified from Krebs (2011). Unconformities are those from Levell (1987): DRU-Deep Regional Unconformity, LIU-Lower Intermediate Unconformity, UIU-Upper Intermediate Unconformity, SRU-Shallow Regional Unconformity.

Champion Delta to the NE (Fig. 4A), as point sources with the potential to deliver large volumes of sediment to the shelf edge during falling base level (Koopman, 1996). The Baram and Champion depositional systems represent different periods of progradation as defined by a combination of chronostratigraphy and unconformities. The Baram system was named for the modern river that has deposited a relatively large delta (Fig. 1) and the Champion system was named for one of the major oil fields. Hiscott (2001) interpreted the depositional history of the Pleistocene – Recent succession of the Baram Delta in southwestern Brunei from high resolution seismic data. During that time, a number of delta lobes prograded to the edge of the modern shelf and beyond during lowstands. Sandy sediment was supplied to the deep sea by a variety of processes including slumping and major incision that led to shelf bypass and sediment supply directly into the Baram Canyon, although the succession is interpreted to include a substantial amount of muddy sediment (Hiscott, 2001). It appears likely that the Baram River and Delta system had a similar history prior to the Pleistocene and that it has supplied significant quantities of sediment to the deep sea in Brunei and Sarawak since its inception. Hiscott (2001) also noted that growth faults trap sediment and increase the thickness of individual stratigraphic units by a factor of 2–5. However, Nolira (2002) demonstrated that episodic movement along a growth fault results in a response that varies from bypass to ponding. Consequently, a fault that traps sediment and prevents sand supply to the shelf edge during one lowstand may have no influence during a subsequent lowstand. Generally, the depositional processes of the Baram system appear to be efficient in supplying sediment to the deep sea. Proprietary sea-floor images indicate that there is a large, thick wedge of sediment on the slope and base of slope in front of the Baram Delta (Abdullah Ibrahim, pers. comm.; Gee et al., 2007; Morley et al., 2003). The surface topography of the wedge is relatively smooth and, generally, the wedge appears to be the product of ongoing, quasi-continuous and relatively rapid sedimentation. The Champion Delta, as defined by James (1984), Schreurs (1996) and Koopman (1996), is an important element in the

literature on the NW Borneo petroleum system (e.g. Petronas, 1999; Sandal, 1996). The core of this system, as well as a noticeable clustering of five major oil fields (>250 mmboe) on the shelf and in deepwater (Cullen, 2010a), coincides with a boundary between two of NW Borneo’s transverse structural domains (Fig. 4B). That boundary is essentially a large-scale accommodation zone, defined by the change from counter-regional to down-to-the-basin growth faults on the shelf, which may have focused drainage systems towards the Gumusut and Kikeh area. There is a basic problem with the Champion Delta concept; there is no modern Champion River nor is there any evidence of a paleo-river that could have built the Champion Delta. The basic elements of NW Borneo’s presentday geological setting have been in place since the middle Miocene following the Sabah Orogeny (Hutchison et al., 2000) and cessation of sea floor spreading in the South China Sea (Briais et al., 1993) and it appears unlikely that there ever was a Champion River. This paper presents evidence that the Champion ‘‘Delta’’ represents a prograding clastic coastline fed by the predecessors of several river systems that presently flow into Brunei Bay, but which followed different courses in the Miocene.

2. The Champion ‘‘Delta’’ The sedimentary succession that is mapped as deposits of the Champion ‘‘Delta’’ exhibits a delta-shaped bulge that occupies approximately 100 km of the NW Borneo margin that has been interpreted as a wave-dominant delta (Koopman, 1996; Fig. 4A). However, several lines of evidence indicate that the Champion ‘‘Delta’’ was not a simple delta but a complex system that included a variety of depositional settings as indicated in Fig. 12 of Back et al. (2008). The Brunei Bay drainage basin now occupies the same part of the margin as the former Champion Delta. Most of the uplift in the hinterland that determined the southeastern margin of the drainage basin was associated with an early Miocene regional unconformity and had occurred by the middle Miocene (Hutchison, 1996), suggesting that the overall size of the drainage basin has not changed

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1.6 3.0 3.4 5.3 7.2 8.5 10.6 12.2

14.2

N 16.2

40 km Fig. 3. Shelf edge trajectory from the middle Miocene to the Holocene in mybp. (after Cullen, 2010a).

significantly since then. The Brunei Bay drainage basin is considerably smaller than the Baram River drainage basin and appears incapable of depositing a delta anywhere near as large as that interpreted for the Champion Delta (Fig. 4A).

2.1. Strata of the Champion ‘‘Delta’’ Middle Miocene – Pliocene outcrops that lie within the Champion ‘‘Delta’’ consist primarily of tidal and shoreface successions but their stratigraphic architecture is much different than that of outcropping deltaic successions or the modern Baram Delta. The Champion ‘‘Delta’’ strata are segregated into successions at least hundreds of meters thick, and often as much as 2–3 km thick, that can be either progradational or retrogradational. Sediments are nearly all tidal- or all wave-dominated within one succession. Fluvial sandstones form a minor, but important, part of the outcropping Champion succession, with outcrop evidence indicating multiple fluvial systems. It should be noted that relatively few fluvial systems are preserved and/or crop out because of the extensive tropical vegetation, so it is difficult to generalise about them. Generally, wave-dominated successions comprise upper shoreface sandstones, lower shoreface sandstones, interbedded

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sandstones and mudstones of the offshore transition, and shelfal mudstones. The most common facies association is shallowing and coarsening upward, progradational parasequences, although fining-upward, retrogradational parasequences are also common. Parasequences typically are tens of meters thick and lie within parasequence sets that are hundreds of meters thick. In outcrop, many wave-dominated successions do not display a deltaic morphology; they are laterally continuous for up to tens of km and follow the basin contours, suggesting that they are non-deltaic shoreline sands (Back et al., 2001; Lambiase and Ovinda, 2006; Ovinda, 2005; Fig. 5) and that the long, straight segments of the Brunei and Klias Peninsula coastlines that face the South China Sea are modern analogues for the depositional setting of the Champion ‘‘Delta’’ shoreface successions (Fig. 4A). These coasts are wave-dominated, despite the small waves that have persisted in the fetch-limited South China Sea since at least the middle Miocene. Three facies comprise the tidal successions of the Champion ‘‘Delta’’, tidal channel and bar sandstones, tidal flat sandstones and mudstones and brackish water mudstones. Tidal channel and bar sandstones commonly combine with tidal flat sandstones and mudstones to form fining upward, progradational parasequences that range in thickness from a few meters to several tens of meters due to a variable amount of aggradation. The parasequences are commonly capped by coals or coaly shales up to 2 m thick that contain almost exclusively mangrove pollen. The Champion tidal successions are interpreted to be deposits of tide-dominated deltas within structurally-controlled coastal embayments, of which Brunei Bay is a modern example (Fig. 4B). There, tidal currents dominate the hydrodynamics because most waves cannot enter the semi-enclosed bay and wave action is minimal. Most sandy sediments within Brunei Bay are accumulating on tide-dominated deltas. The Trusan River Delta was studied in detail by Damit (2001) and is generating the same facies succession and stratigraphic architecture that is recorded in the Champion ‘‘Delta’’ tidal outcrops (Lambiase et al., 2003). Consequently, the Brunei Bay deltas can be viewed as modern analogues for the Champion Delta tidal successions. Fluvial sediments crop out in three locations within the Champion ‘‘Delta’’, in the axis of the Berakas Syncline, on Labuan Island and on the Klias Peninsula (Fig. 6). Outcrops of fluvial conglomerate in two areas on the Klias Peninsula, previously designated as early Miocene deepwater deposits (Wilson, 1964), are now assigned to the late Miocene based on microfossils in the underlying and overlying marine strata (Drahaman, 1999; Tan, 2010). They also have been re-interpreted as braided stream deposits because they are primarily clast-supported conglomerates with pebble to boulder-sized imbricated clasts that occur as the lenticular deposits of relatively small channels (Drahaman, 1999; Tan, 2010; Fig. 7A–D). The successions also include cross-bedded pebbly sandstones with local coal clasts, cross-bedded medium-grained sandstones (Tan, 2010) and thin interbedded mudstone lenses with exclusively fresh water pollen and no marine microfossils (Azmi Yakzan, pers. comm.). The pebbles and conglomerate clasts are composed of multiple rock types, including older, reworked chert pebble conglomerates (Fig. 7E), suggesting a complex provenance. The conglomerate outcrops towards the NE end of the Klias Peninsula are tightly silica-cemented and cut by a steeply dipping shear zone (N60°W) in which slickenslides have an average dip of 30°SE (Fig. 7F–H). An underlying structural influence on the development of the braided stream deposits is suspected, but cannot be demonstrated. The late Miocene to early Pliocene Berakas Syncline exposure has cross-cutting channel sands that range in width from 10 to approximately 100 m. Most are 5–6 m thick, although they are commonly stacked into multi-storey sands up to 15 m thick

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(A)

(B) KI GU SM KN

Champion Delta

D

Padas River

CP

C

Baram Delta

Trusan River

Limbang River

B

Champion Drainage Basin A

Klias Syncline Brunei Bay Regional normal fault

Baram Drainage Basin

Counter-regional normal fault Reverse Fault (teeth upthrown)

N

Deepwater fold-thrust belt A Transverse Domain

N

50 km

Major oil field

80 km Fig. 4. (A) The Champion and Baram deltas and their present-day drainage basins. (Adapted from Schreurs, 1996 and Koopman, 1996). Major oil fields are shown in gray circles: KI-Kikeh, GU-Gumusut, SM-Samarang, KN-Kinabalu, CP-Champion. (B) regional fault patterns and structural domains. (adapted from Cullen, 2010a).

5 km

N

Jalan Sungai Akar outcrop

Berakas Syncline

prominent topographic ridge comprising parasequence sets of wave-dominated shoreline sandstones

Fig. 5. Satellite image showing outcropping parasequence sets comprising wave-dominated sandstones that are laterally continuous along basin contours.

(Fig. 8). The channel sands are dominantly fine sandstone and commonly have thick, conglomeratic lag deposits at their base that pinchout laterally toward the channel margins. Most lags are tightly-cemented, texturally mature, fine-grained sandstone, although a few are composed of mud pebbles. The sands exhibit

a variety of large and small-scale cross-bedding and have northerly paleo-flow indicators (Ibrahim, 1998). Large coalified wood fragments occur locally. Interbedded mudstones are thin and uncommon, except as rare abandoned channel fill, and marine microfauna are absent (Ibrahim, 1998).

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25 km N

Champion Delta Padas River

Brunei Bay

Limbang River

abandoned channels projected paleo-drainage

Trusan River

fluvial outcrop Miocene tide-dominated delta

Fig. 6. Location of the Champion ‘‘Delta’’ relative to the modern and paleo-drainage around Brunei Bay plus fluvial outcrop locations, illustrating Pliocene capture of the Limbang and Padas Rivers by a rapidly subsiding Brunei Bay. See the text for an explanation.

Madon (1997) described the 85 m thick middle Miocene fluvial succession on Labuan Island. Generally, it is very similar to the Berakas Syncline succession in that it is mostly stacked fine to medium grained channel sandstones, some of which are pebbly (Fig. 9). However, the pebbles include several rock types (Chong, 2008), unlike the Berakas Syncline succession. Trough and planar cross-bedding is abundant and coalified wood fragments are common; paleo-flow indicators suggest a northerly to northwesterly paleocurrent direction (Madon, 1997). Individual channel sands are up to 6 m thick and 30 m wide, although complete lateral exposures of channels are rare. Burrows occur in some of the channel sands, indicating at least occasional marine influence. The outcropping fluvial sandstones of the Champion Delta succession were deposited by braided and meandering rivers. Madon (1997) interpreted an upward transition from braided to meandering environments in the Labuan succession and the Berakas Syncline succession is dominantly meandering fluvial channel and point bar deposits (Ibrahim, 1998); the Klias Peninsula exposures are limited to braided stream deposits (Tan, 2010). One common feature of the 3 successions is that channel width is small, ranging from a few tens of meters to approximately 100 m, although there is considerable uncertainty about the maximum channel width because of the limited outcrop. This is considerably less than the approximately 300 m width of the modern Baram River upstream of any tidal influence, suggesting that the river systems which constructed the Champion Delta were relatively small; there is no evidence that a large river ever existed within the Brunei Bay drainage basin.

All aspects of depositional systems of the Champion ‘‘Delta’’ were virtually identical to the modern systems that occupy the same region today. It appears that factors controlling sedimentation have remained unchanged since the middle Miocene and that the Brunei Bay and surrounding systems can be used as analogues for interpreting the Champion succession. 2.2. Tectono-stratigraphic model for the Champion ‘‘Delta’’ Succession Structurally-generated topography determines shoreline geometry and segregates tidal from wave-dominant facies in the modern system. The back-thrust that uplifted the NW-facing shoreline of the Klias Peninsula extends offshore to create a bathymetric sill across the western margin of Brunei Bay; limiting wave penetration and promoting tidal deposition across most of the peninsula (Fig. 6). Topography and shoreline geometry change with time because inversion anticlines grow and subside episodically. The amount and rate of growth is related to the rate of shale withdrawal from adjacent synclines and is therefore linked to the subsidence rate in those synclines (Van Rensbergen et al., 1999). The rate, amount and duration of each episode determine which synclines form embayments, and which do not, at any given time. Structural inversion also affected surface topography occasionally, especially along inversion anticlines (Morley et al., 2003). The influence of local subsidence and uplift on stratigraphic development is illustrated by the middle Miocene to late Miocene/early Pliocene succession in the Berakas Syncline (Fig. 10). The basal part of the succession consists of nearly 2 km of

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Fig. 7. Fluvial sandstone outcrops on the Klias Peninsula. (A and B) pebbly sandstones, (C) cross-bedded, medium-grained sandstone, (D) polymictic, clast-supported conglomerate, (E) Cobble of reworked chert-pebble conglomerate, (F) SE-dipping slickenslide in a shear zone, (G) sheared, silica-cemented cobble comglomerate, and (H) outcrop scale photo of the high angleshear zone (A–D are after Tan, 2010).

sediments that were deposited within a semi-enclosed coastal embayment formed between the shale-cored Jerudong Anticline to the west and Kota Batu – Muara Ridge to the east and are nearly all tidal, although there are wave-dominant units (Figs. 5 and 10A). Following later subsidence, the Jerudong Anticline was submerged, which opened the basin to the west and allowed deposition of an approximately 3 km thick succession that is strongly wavedominated (Fig. 10B). The transition from tidal to wave-dominance was abrupt and is exposed in the Jalan Sungai Akar outcrops. There, the approximately 400 m thick retrogradational parasequence set that forms

the top of the tidal succession is overlain by 30 m of marine mudstone followed by upper and lower shoreface sandstones that also are part of a retrogradational succession (Fig. 10C and D). The subsidence that generated a retrogradational architecture in the tidal succession eventually caused the Jerudong Anticline to submerge and then continued during deposition of the wave-dominated succession. A second episode of growth on the Jerudong Anticline closed the basin to the west, forming an embayment that accumulated another 2 km of mostly tidal sediments on top of the wavedominated succession and persisted until the latest Miocene or early Pliocene.

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A

A A

channel boundaries

large foresets 0.5 m

B

coalified wood

B

lag deposits

0.5 m

Fig. 8. Fluvial sandstones in the axis of the Berakas Syncline. (A) Multi-storey, finegrained channel sands and (B) cobble-sized lag deposits of tightly-cemented, finegrained sandstones with coalified wood.

Another effect of the shifting topography was that the number and locations of rivers reaching the shoreline changed with time. The Pliocene – Recent history of the Brunei Bay drainage basin exemplifies the process. Of the rivers presently flowing into Brunei Bay, the Padas, Trusan and Limbang are the most important (Fig. 4). All three rivers drain mountainous regions and it is unlikely that the location at which they exit the highlands has changed significantly in recent times. However, the lower course of both the Padas and Limbang Rivers apparently changed dramatically since the early to mid-Pliocene. The Limbang River probably flowed northward along the axis of the Berakas Syncline and, after depositing a middle Miocene tidedominated delta at the SW corner of the syncline (N. Singh, pers. comm.) and the late Miocene to early Pliocene Berakas Syncline succession of fluvial sandstones described above, turned sharply to the east because it was captured by the onset of the latest, ongoing episode of subsidence in Brunei Bay (Fig. 6). The abandoned channel pattern of the Padas River suggests that it previously flowed northwestward across the Klias Peninsula from where it enters the coastal plain but has since been deflected to the south and captured by Brunei Bay (Fig. 6). The fluvial conglomerates and pebbly sandstones now exposed on the Klias Ridge and described above also are directly onshore from an east–west orientated Miocene channel system (Hoggmascall et al., 2012); they almost certainly were deposited along older watercourses of the Padas River. We believe capture of the Limbang and Padas Rivers was initiated by structurally-driven subsidence of the Klias syncline in the footwall beneath the back-thrust that uplifted the Klias Ridge (Fig. 4B). Mud volcanoes on Pulau Tiga and the southern end of the Klias Peninsula suggest that shale withdrawal from the

Fig. 9. Fluvial channel sands at Kg. Ganggarak, Labuan. (A) Cross-bedded, pebbly sandstones near the base of the succession as indicated on the inset and (B) polymictic pebbles at the location of the red box in A. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

syncline amplified the amount of subsidence and demonstrates the positive feedback of consequent depositional loading. A prominent Bouguer gravity low beneath Brunei Bay and 2D seismic data show that recent subsidence in Brunei Bay is superimposed upon an early Miocene sub-basin, which suggests an earlier episode of shale mobilization may have occurred. Brunei Bay serves as an excellent example of an ‘‘out-of-sequence’’ mini-basin that developed inboard from older depositional sequences. 3. Champion sand supply to the deep sea The effect of tectonically-driven topography on fluvial systems in the Champion depocenter has important implications for the amount and distribution of sand that is supplied to the shelf edge, and ultimately the slope and basin. Shifting sediment supply routes results in the potential for a highly variable sand supply to the shelf edge during sea level lowstands. At times when all the rivers in the drainage area coalesced before reaching the shore of the South China Sea, a delta somewhat smaller than the modern Baram Delta supplied moderate amounts of sand to one location on the shelf edge (Fig. 11A). At other times, multiple, but even smaller, deltas distributed an equivalent volume of sand over a wider area (Fig. 11B; Back et al., 2008). During some lowstands very little, if any, sand reached the deep sea because subsiding depocenters trapped most of the sediment on the shelf (Fig. 11C). The Champion system is affected by both inversion anticlines and growth faults (Back et al., 2001; Demyttenaere et al., 2000;

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A

B

C 60

110

55 6

50 105 45 40

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kilometres

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Dominanlty Tidal

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stratigraphic extent of sections C and D

15 10 5

10 0

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Mudstone Tidal Flat Sand Tidal Channel and Bar Sand Shoreface Sand

Fig. 10. Stratigraphic development of the Berakas Syncline. (A) Schematic maps of the changing shoreline geometry with time. The black indicates the location of the Jalan Sungai Akar outcrop; see Fig. 5 for its location. (B) Schematic stratigraphy of the major tidal and shoreface successions in the syncline. (C) The upper part of the dominantly tidal succession exposed on Jalan Sungai Akar. It lies immediately below the lower tidal – shoreface boundary in B, and (D) The dominantly shoreface succession that directly overlies the succession in C.

Hodgetts et al., 2001; Masri, 2002; Tromp et al., 2000). The ponding effect of large growth faults and shale ridges is twofold; sediment supply routes are blocked and the actively subsiding depocenters form sinks for large volumes of sediment. Mobile shale and growth faults occur across the entire width of the shelf and part of the slope within the Champion system so that sediments can be trapped almost anywhere along their path from the highlands to the deep sea. One very large counter-regional growth fault, the Frigate or Perdana Fault, operated from the late Miocene and has up to 5 km of sediment in its hangingwall (Fig. 9 of Cullen, 2010a). This implies that there is not necessarily a one-to-one relationship between periods of relative sea level fall and fan development because sediment sinks on the shelf can prohibit sand supply to the shelf edge during some lowstands. Conversely, local uplift can result in delivery of sand to the outer shelf, especially considering NW Borneo’s relatively narrow shelves. For example, deposition of the Kamunsu reservoirs in deepwater and development of the Upper Intermediate Unconformity on the shelf occurred during a time of high eustatic sea level (Fig. 2). The independent movements of different blocks at different times produced a varied pattern of localized uplift and subsidence in Sabah (Levell, 1987) and points to a complex history for the supply of sediment to the outer shelf for ultimate delivery to deepwater. The Klias Ridge/Brunei Bay system is an excellent modern example of proximal ponding that has persisted for at least several 1000 years. With its present configuration, coarse sediment from the Limbang, Trusan, or Padas Rivers cannot reach the shelf edge until the Brunei Bay depocenter is filled (Fig. 11C). Brunei Bay is subsiding rapidly enough to maintain water depths exceeding 50 m and remains under-filled despite the high sediment influx from the combined discharge of the Padas, Limbang, and Trusan Rivers, which exceeds that of the Baram River (Cullen, 2010b). Subsidence was sufficient to outpace sedimentation during the latest Holocene lowstand when a large wedge of sand, now lying in 20 m of water, accumulated in the western side of the bay (Damit,

2001). Consequently, until there is a significant decrease in subsidence rate or uplift in the hinterland, Brunei Bay will continue to sequester all the sand supplied by the Padas, Limbang, and Trusan Rivers. The combined effects of smaller fluvial supply systems and varied, robust mechanisms for trapping sediment on the shelf dramatically reduce the amount of sandy sediment that reaches the shelf edge during sea level lowstands. This is illustrated by the present day seafloor configuration where, in contrast to the Baram system, proprietary seafloor images indicate much less modern and submodern sediment at the base of the slope in the vicinity of Brunei Bay (Abdullah Ibrahim, pers. comm.). No sedimentary wedge is present and there are a variety of pronounced topographic features that cover the slope, including inversion anticlines that intersect the seafloor, slump scars and channels; many of the channels originate in the mid-slope and appear to be transporting slumped sediment exclusively (Masri, 2002; Gee et al., 2007; Morley et al., 2003; Morley, 2009). This apparently is caused by a relatively slow sedimentation rate that allows topographic features to remain prominent. 3.1. Earliest Miocene to Pliocene shelf edges and shelf-edge canyons Multiple deep incisions have been mapped on 2D and 3D marine seismic data and tied back to biostratigraphically-dated horizons in offshore wells (Fig. 12). These incisions narrow to an up-dip nick point and resemble the drowned Holocene canyons observed on the present-day shelf. The position of these incisions relative to their coeval shelf edge lead us to interpret the incisions as shelf edge canyons cut by separate, relatively small rivers, which represent the feeder systems of the turbidites of NW Borneo’s deepwater fold and thrust belt. The limited advance of the 10.6 mybp to present-day shelf edges (ca. 30 km) highlights the strongly aggradational nature of the Champion succession and implies a relatively steep slope

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A

shelf edge

Champion Delta

N 25 km

B

shelf edge

Champion Delta

N 25 km

C

365

records sediment supply pathways that shifted with time, canyons associated with the modern Padas, Trusan, and Limbang Rivers persistently occupied one segment of the margin (Fig. 12). Contrasts in the bedrock lithologies of the catchments of these rivers suggest that, even in a limited area, compositionally different sands could be delivered to the deepwater by the different rivers. Bedrock in the Padas, Trusan and Limbang catchments is mainly Palaeogene to early Miocene clastic sediments from which little petrographic data is available, although their composition ranges from texturally very mature, quartz-rich Meligan Sandstone to the texturally immature West Crocker Formation with abundant rock fragments of variable lithology (Petronas, 1999; Sandal, 1996). Seismic mapping indicates that submarine canyons were cut into the shelf at least three different ages (Fig. 12). In our study area, the oldest canyons correspond to the earliest late Miocene Upper Intermediate Unconformity and appear to be related to deposition of the Kinarut fan (Figs. 2 and 13). Generally, the older Miocene canyons are bigger than either the latest Miocene or Pliocene canyons and appear to represent the entry points of three separate rivers within the Champion system (Fig. 12). A second set of canyons corresponds to the Shallow Regional Unconformity, ca. 8.5 my; these canyons appear to be related to deposition of the Pink fan (Figs. 2 and 13). Although the Shallow Regional Unconformity corresponds to a third order eustatic sea level lowstand (Haq et al., 1987; Kominz et al., 1998), the timing also corresponds to the intrusion of the Mt. Kinabalu pluton (Cottam et al., 2010) and inversion along the St. Joseph-Bunbury Ridge (Cullen, 2010a), strongly indicating the lowstand was tectonically amplified. The youngest set of canyons also occurs at a regional unconformity that is close in age to the Miocene – Pliocene boundary (Fig. 14). However, age dating in NW Borneo is only accurate to ±1 myr and it is highly likely that the unconformity represents a slightly younger, early Pliocene tectonic inversion event (Morley et al., 2003). The Pliocene canyons, which occur as two small, geographically-separate groups seaward of the Miocene canyons (Fig. 12), are relatively shallow, suggesting minimal local erosion (Fig. 14). 3.2. The Padas, Trusan and Limbang River sediment supply systems

shelf edge

The modern Limbang, Padas and Trusan and their antecedent watercourses have been the three most important rivers in the Brunei Bay/Champion drainage basin since at least the middle Miocene. They are similar sized rivers with respect to channel width and drainage basin area (Fig. 3), yet their stream gradients, sediment sources and watercourse histories vary considerably so that the volume, distribution and mineralogy of the sands that they have delivered to the shelf edge probably also varies significantly with respect to reservoir potential. There is no direct evidence for the volume of coarse sediment carried by any of the three rivers; however, the relative size of their Pliocene to present-day Brunei Bay-margin deltas probably reflects their relatively coarse sediment discharge.

Muara Channel

N 25 km Fig. 11. A generalized map-view model illustrating how evolving topography results in variable sediment supply to the shelf edge so that at different times: (A) multiple relatively small rivers reach the shelf edge, (B) a single, larger river reaches the shelf edge, and (C) coarse sediment is trapped on the shelf during lowstands.

similar to the present-day slope (Steffens et al., 2003). Although the areal distribution of middle Miocene to Pliocene shelf canyons

3.2.1. Limbang River It appears that much of the Limbang River has been a relatively low gradient stream for its entire history. Upon exiting the Borneo highlands much further from the sea coast than either the Padas or Trusan Rivers, the Limbang watercourse followed the axis of the Berakas Syncline from at least the earliest late Miocene until the early Pliocene when it was captured by Brunei Bay (Fig. 15). It has the smallest Brunei Bay margin delta of the three rivers, although its exact dimensions are not as easily defined as those of the Trusan and Padas deltas (Fig. 16).

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K G

5.2 Ma 8.5 Ma 10.6 Ma Kikeh Gumusut

modern shelf edge

3

K

G 2

1

Padas River

Trusan River

Baram River

Limbang River

N 50 km

Fig. 12. Location of the major rivers and late Miocene to Pliocene shelf edge canyons of NW Borneo. The numbered middle late Miocene shelf edge canyons (ca. 8.5 Ma) are interpreted to have been cut by the (1) Limbang, (2) Trusan and (3) Padas rivers.

TWT WSW (Sec.)

water bottom

ENE

1.0 8.5 SRU

2.0

10.4 UIU

N 3.0

2 km Fig. 13. Interpreted seismic line across stacked submarine canyons cut at approximately 10.4 Ma and 8.5 Ma. Clinoforms filling the 10.4 Ma incision are interpreted to represent the prograding highstand systems tract of the Kamunsu shelf.

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N 25 km meters

SW

NE

meters

near base Pliocene

5 km Fig. 14. Seismic line across two approximately 5.2 Ma shelf edge canyons. Canyon bases are outlined in white (after Cullen and Phillip, 2006).

The present-day mineralogy of the Limbang’s sediment load is unknown. However, the fine-grained sandstones that dominate its early Pliocene Berakas Syncline deposits appear to have been derived from mechanical breakdown of the tightly-cemented, texturally mature fine-grained sandstone clasts that comprise associated conglomeratic lags; the mineralogy and textural maturity of the conglomeratic clasts and the fluvial sands are nearly identical and there are virtually no intermediate grain sizes present in the outcrops. On the basis of their lithological similarity, we interpret the conglomeratic clasts to be derived from the middle Oligocene – early Miocene Meligan Formation shallow marine sandstones that crop out widely in the Limbang catchment area (Fig. 14). 3.2.2. Padas River Pebbly sandstones and conglomerates deposited by the Padas River are exposed in two separate localities approximately 15 km apart on the Klias Peninsula (Fig. 6). Microfossil analysis confirms a late Miocene age for both areas but cannot determine the age more precisely (Drahaman, 1999; Tan, 2010). Limited exposure and possible structural discontinuities make it impossible to confirm the exact stratigraphic relationship between the two outcrop areas but the northern area is almost certainly much younger than the southern exposures, suggesting that each area was deposited during a different relative sea level lowstand.

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Although the mineralogy of the present-day bedload of the Padas River is unknown, the diverse rock types represented in the pebbles and conglomerates of the Klias Peninsula outcrops and appears to support the contention that they are Padas deposits. The Padas is the only Champion system river whose catchment area extends beyond the Crocker Range and, therefore, it drains more formations, with a wider range of lithologies, than the other rivers. The Padas is a high gradient river and has a larger drainage basin than the Trusan or Limbang Rivers (Figs. 3 and 15). It also has deposited the largest delta of the three rivers (approximately 945 km2), indicating that it transports a relatively large amount of coarse sediment; the delta is ponded behind the shale-cored inversion anticline that forms the western edge of the Klias Peninsula (Fig. 16). 3.2.3. Trusan River The present-day Trusan River has the highest gradient and smallest drainage basin of the three rivers (Figs. 3 and 15). However, at relative sea levels lower than today and prior to the Pliocene capture of the Limbang and Padas Rivers, it had a considerably larger drainage basin as the modern Lawas, Temburong and Mangalong Rivers were almost certainly tributaries of the Trusan. The Trusan Delta has an area of approximately 630 km2, suggesting that its coarse sediment discharge is about 33% less than that of the Padas River (Fig. 16). There are no data about the mineralogy of the modern Trusan’s sediment load, nor from any of middle Miocene to Recent deposits. However, the Trusan drains a large area of Meligan Formation sandstone so that its deposits probably are lithologically similar to those of the Limbang River (Fig. 15). 3.3. Kikeh and Gumusut reservoir sands The major Kikeh and Gumusut reservoirs sands are late Miocene in age, which suggests that one or more of the late Miocene shelfedge canyons was the entry point for the sands and the orientation, size and geographic proximity to the fields makes the southernmost late Miocene canyon in the Champion system the most likely candidate (Figs. 12 and 15). However, the Kikeh and Gumusut reservoir sands are considerably different; the thick Pink sand, which is the main reservoir at Gumusut, is a high porosity (ca. 30%), high permeability (ca. 800 md) quartz-rich arenite (Ingram et al., 2004; Grant, 2006; Cullen, 2010a; Algar et al., 2011). At Kikeh, the Pink sand is poorly developed; the primary reservoirs are litharenites of the Kamunsu and Kinarut fans which are thinner bedded and have lower porosities and permeabilities (Fig. 17). These data strongly suggest that, despite the geographical proximity of the fields (Fig. 1), various reservoirs may reflect supply by different sediment sources that correspond to different river systems. The composition of coarse clasts in the late Miocene fluvial outcrops in the Berakas Sycline (quartz pebbles) and on the Klias Peninsula and Labuan (reworked litharenites) indicate derivation from different provenances; most likely Meligan sandstone and Crocker formation, respectively. This observation suggests that evolving patterns of uplift and subsidence on the shelf could result in compositional differences in the supplied sand to the outer shelf and hence influence deepwater reservoir quality. Our interpretation of the late Miocene watercourses (Fig. 15) is that during the early late Miocene (10.4 mya) all three principal rivers of the Champion system delivered sediment to the shelf up-dip of Gumusut and Kikeh. In the middle late Miocene (8.5 mya) the Padas River appears to have changed its course to the north, leaving the Trusan and Limbang Rivers as the main depositional systems supplying sediment to the outer shelf. Thus, it appears likely that the Pink sands in the Gumusut Field were deposited by late Miocene watercourses of the Limbang and/or

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Fig. 15. The interpreted watercourses of the Limbang, Padas and Trusan Rivers: (A) earliest late Miocene, (B) middle Miocene, (C) early Pliocene, (D) present-day configuration. The areal distribution of the Meligan Formation is from Koopman (1996).

Trusan River which supplied quartz-rich material derived from the Meligan sandstone. Earlier in the late Miocene, the Padas, Limbang, and Trusan Rivers probably all delivered sediment to the shelf to be homogenized by wave and tidal processes.

4. Implications for deepwater reservoir sandstones Deepwater sandstones derived from the Champion system are expected to differ from sands delivered to the shelf edge during

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369

25 km

N

Padas Delta Limbang Delta

Trusan Delta

Fig. 16. Relative size of the modern Padas, Trusan and Limbang Deltas.

Kikeh

Gumusut ~ 40 km

“

”

Kamunsu

~ 100 m

Kinarut

lowstands by a single deltaic system with respect to their spatial distribution and reservoir quality. Subsiding synclines, episodically active inversion anticlines and growth faults created an evolving topography that controlled drainage pathways so that the number of rivers reaching the shoreline, and their locations, changed relatively frequently. Tectonic control of river location is clearly demonstrated by the distribution of shelf edge canyons, which formed during discrete tectonic events and whose number and location changed with time (Fig. 12). In addition, growth faults, rapidly subsiding synclines, inversion anticlines and shale ridges trapped sand on the shelf, resulting in a variable sand supply to the shelf edge. Although reservoir quality within Champion system deepwater sands is largely a product of depositional process (Algar et al., 2011), mineralogy and textural maturity also are important. This is highlighted by the mineralogical and corresponding porosity/ permeability differences in Gumusut and Kikeh that probably reflect sand supply from different river systems, a relationship that is likely to hold throughout the Champion system. Indeed, identifying the sediment supply system could be a significant factor for high-grading exploration targets and optimizing field development. 5. Conclusions

arenite litharenite mudstone

Fig. 17. Diagramatic correlation of the principal reservoir sandstones in the Kikeh and Gumusut fields, which are approximately 40 km apart. Figure is drawn from various published and unpublished sources including Grant (2003), Ingram et al. (2004), and Algar et al. (2011).

The Champion ‘‘Delta’’ is not a delta; it is the product of multiple diverse, shallow marine depositional systems that have been accumulating thick, discrete shoreface and tidal successions in a variety of deltaic and shallow marine environments since the middle Miocene. Stratigraphic architecture is controlled by evolving topography generated by episodic structural inversion that causes depocenters to shift location as well as change from tidedominated embayments to wave-dominated straight coastlines. Anticlinal ridges and rapidly subsiding synclines related to inversion and shale withdrawal significantly alter sediment dispersal patterns by deflecting multiple, relatively small fluvial systems.

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Predecessors of the Padas, Limbang and Trusan Rivers apparently are important sources of sand within the Champion system. All three rivers probably contributed to the main earliest Miocene reservoir sands in the deepwater Kikeh and Gumusut fields, respectively. The younger, quartz-rich, Pink reservoir in Gumusut appears to be sourced from the Limbang and/or Trusan Rivers owing to avulsion of the Padas River to the north in the middle late Miocene. Sediment supply to the shelf edge and the deep sea is strongly controlled by sedimentary processes on the shelf as growth faults, shale ridges and inversion anticlines are capable of trapping a significant amount of sediment on the shelf. Pliocene to Recent capture of the Trusan, Limbang, and Padas Rivers by Brunei Bay offers a modern analogue for the sequestration of sediment in an actively subsiding minibasin. As a consequence of these processes, the spatial and temporal distribution of deepwater sand accumulations sourced from the Champion system probably is not related solely to relative sea level fluctuations; deepwater sand accumulations are expected to be smaller and more scattered than those sourced from a large shelf edge delta. The reservoir quality of deepwater sands is also expected to vary because the Champion system’s rivers transport different mineralogical assemblages. Acknowledgements We wish to acknowledge the many colleagues and former students who contributed to the research on which this paper is based. Among them are Stefan Back, Jim Booth, Alec Bray, Abdul Razak Damit, Herman Darman, Vincent Drahaman, Angus Ferguson, Abdullah Ibrahim, Miskiah Masri, Chris Morley, Mike Simmons, Navpreet Singh and Tan Chun Hock. Journal reviewer Howard Johnson made many helpful suggestions that improved the manuscript significantly. References Algar, S., Milton, C., Upshall, H., Roestenburg, J., Crevello, P., 2011. Mass – Transport Deposits of Deepwater Northwestern Borneo Margin—Characterization from Reflection-Seismic, Borehole, and Core Data with Implication for Hydrocarbon Exploration and Exploitation. Society of Economic Mineralogists and Paleontologists, pp. 351–366 (Special Publication 96). Back, S., Morley, C.K., Simmons, M.D., Lambiase, J.J., 2001. Depositional environment and sequence stratigraphy of Miocene high-frequency deltaic cycles exposed along the Jerudong Anticline, Brunei Darussalam. Journal of Sedimentary Research 71, 913–921. Back, S., Strozyk, F., Kukla, P., Lambiase, J.J., 2008. Three-dimensional restoration of original sedimentary geometries in deformed basin fill, onshore Brunei Darussalam. NW Borneo: Basin Research 20, 99–117. Briais, A., Patriat, P., Tapponnier, P., 1993. Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea: implications for the Tertiary tectonics of Southeast Asia. Journal of Geophysical Research 98, 6299–6328. Chong, K.W., 2008. Sedimentology, sequence stratigraphic and reservoir characteristics of the Belait and Temburong Formations in Kg.Ganggarak, Labuan, Sabah, Malaysia. Unpublished M.Sc. thesis, Universiti Brunei Darussalam, 99p. Cottam, M., Hall, R., Sperber, C., Armstrong, R., 2010. Pulsed emplacement of layered granite: new high-precision age data from Mount Kinabalu, North Borneo. Journal of the Geological Society of London 176, 49–60. Cullen, A.B., 2010a. Transverse segmentation of the Baram-Balabac Basin, NW Borneo: refining the model of Borneo’s tectonic evolution. Petroleum Geoscience 16, 3–29. Cullen, A.B., 2010b. The Klias Peninsula and Padas River, NW Borneo: An example of drainage capture in an active tropical foreland basin. American Association of Petroleum Geologists, Search and Discovery, vol. 50294, 7p. Cullen, A.B., Phillip, G., 2006. Structural and stratigraphic controls on the distribution of hydrocarbons in the greater Kinabalu Field, Sabah, Malaysia. In: American Association of Petroleum Geologists International Conference, Perth, Australia. Search and Discovery, vol. 90061. Damit, A.R., 2001. Depositional systems in a tropical embayment, Brunei Bay, NW Borneo.Unpublished Ph.D. thesis, University of Aberdeen, 440p. Demyttenaere, R., Ibrahim, A., Tromp, J.P., Zulkifli Ahmad, 2000. Brunei deep water exploration: from sea-floor images to depositional models in a slope turbidite setting. American Association of Petroleum Geologists International Conference, Bali, Abstracts, p. A21.

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