editing oral#015 paper-tidal husein-iagi2006

PROCEEDINGS PIT IAGI RIAU 2006 The 35th IAGI Annual Convention and Exhibition

Pekanbaru – Riau, 21 – 22 November 2006

TIDAL INFLUENCE ON SEDIMENTATION PROCESSES OF THE MAHAKAM DELTA, EAST KALIMANTAN

Salahuddin Husein

Geological Engineering Department, Universitas Gadjah Mada, Yogyakarta 55281

ABSTRACT The modern Mahakam Delta has long been classified as a mixed fluvial and tide-dominated prograding delta. The delta plain has a lobate, fan-shaped morphology with a network of distributaries and estuaries. Previous studies which were mainly focused on sedimentary characteristics indicate that tidal features are not restricted to the channel mouths. This study focuses on the dynamics of the tide and its influence on the sediment distribution. Hydrodynamic measurements were systematically taken at 22 locations for complete spring and neap tidal cycles, along with 328 bottom grab sediment samples and a few shallow cores. Sand covers the bottom of the distributaries at the delta apex and gradually fines seaward but does not extend to the channel mouths. Mud dominates the offshore, the estuaries and the distal reaches of the distributaries. Sand and mud couplets are common upstream to at least the delta apex. An abundant, diverse assemblage of benthonic marine organisms was recovered as much as 20 km upstream in the distributaries. The hydrodynamic measurements indicate that tidal processes strongly control sedimentation throughout the distributaries and even upstream of the delta apex. Tidal stratification occurs dynamically and influences the bedload transport that commonly takes place during spring tide but not during neap tide, which later suggests that the sand and mud couplets reflect spring-neap variations. This study suggests that the Mahakam Delta is indeed a mixed fluvial and tide-dominated system but has been recently transgressed. Fluvial dominance is constrained to the upper reaches of the active distributaries and tides are the most important processes on the delta. The tidal processes control the distribution of the potential reservoir in the delta plain and significantly decrease the reservoir quality.

INTRODUCTION Since the early 1970’s, several sedimentological studies have been carried out on the modern Mahakam delta by a number of workers. The general sedimentology and modern processes of the delta were studied by Allen et al. (1976), Gastaldo et al. (1995), Allen and Chambers (1998) and Storms et al. (2005). Those studies indicate that the delta exhibits characteristic features of both fluvial and tidal processes, that tidal features are not restricted to the channel mouths. This study focuses on the dynamics of the tide and its influence on the sediment distribution.

Delta Morphology The delta plain of the Mahakam Delta has a lobate, fan-shaped morphology and comprises an area of

about 2800 km2, which is about 60 % subaerial and 40 % subaqueous (Figure 1). It extends about 50 km from the delta apex to the coastline and exhibits an extremely gentle slope of about 0.06 m/km. It is densely vegetated with tropical rain forest in the supratidal areas near the delta apex and Nipah palm and mangroves on the intertidal areas (Allen et al., 1976). The delta plain is dissected by numerous distributaries and estuaries. The distributaries are relatively straight channels and exhibit 7 to 20 m deep and 400 to 1300 m wide. The estuaries have similar depths to the distributaries at 8 to 23 m deep and are characterized by sinuous and flaring channels. Those channels can be grouped into 3 distinct geographic areas (Allen et al., 1976): a northern area that consists of 3 distributaries and 3 estuaries; a central area that consists only of 2



estuaries; and a southern area that consists of 4 distributaries and 3 estuaries (Figure 1). Intertidal bars generally occur near the channel mouths; they are commonly perpendicular and attached to the shoreline, although some are detached and exhibit a triangular shape. The gentle slope persists offshore as a subaqueous delta plain (Orton and Reading, 1993) to a water depth of 5 m where it abruptly increases to 1.0-2.5

o on the delta front. There are several channels on the seabed that are contiguous to adjacent distributaries (Figure 1). The delta front appears to be very stable with no evidence of gravity-driven mass-movement features (Roberts and Sydow, 2003). The subaqueous delta plain, delta front and prodelta comprises an area of about 5500 km2.

Hydrodynamic Setting The modern Mahakam delta formed under conditions of low wave energy, low to medium tide ranges, and a large but non-flooding fluvial discharge (Allen and Chambers, 1998). The delta is fed by the Mahakam River which has a 75,000 km2 drainage basin that extends into the central Borneo highlands (Figure 1). The climate is equatorial, with only a slight monsoonal effect, and temperatures remain between 26 and 300C (Allen and Chambers, 1998). The annual rainfall varies between 4000-5000 mm in the central highlands and 2000-3000 mm near the coast (Schuettrumpf, 1986). Large seasonal variations in river discharge occur, with peak flow in the months of March to June (SE monsoon) and November to December (NW monsoon) when the inland rainfall is high. Floods apparently never occur on the delta, even at peak fluvial discharges, because a large lake system upstream of the delta absorbs any excess flow (Allen and Chambers, 1998). Wave energy is low due to the limited fetch within Makassar Strait, with the largest waves approaching from the SE. Fourteen km offshore, observed average wave height is 0.3 m with a period of 6 seconds and maximum wave height is 0.6 m (Total, 1986). Littoral drift is minimal (Allen and Chambers, 1998) and storms are rare events (Roberts and Sydow, 2003). Tides in the Strait of Makassar are semi-diurnal

with a marked diurnal inequality. Tidal range varies from less than 0.5 m during neap tides to 2.9 m during spring tides, with an average range of 1.2 m. Tides affect the entire delta plain and delta front, and normally influence the river as much as 160 km upstream from the coastline (Schuettrumpf, 1986). During extremely dry periods, tidal fluctuations were observed as far as 360 km upriver.

Sedimentary Facies In general, sand covers the bottom of the distributaries and gradually fines seaward, but does not extend to the channel mouths, and the subaqueous delta plain is dominated by mud (Figure 2). The southern area is sandier than the northern area, while mud is predominant in the central area. The seaward limit of sand distribution approximately matches the seaward limit of mixed fresh-water hardwood and palm forest in the subaerial delta plain (Figure 2). Downstream, sand is gradually replaced by mud and Nipah palms become the dominant vegetation. Five sedimentary facies comprise the sediment distribution, which are Medium Sand, Muddy Fine Sand, Bioturbated Muddy Fine Sand, Sandy Mud, and Mud (Husein and Lambiase, 2005). In addition to these five facies, detrital organic debris is also widely distributed along the high tide shoreline. These deposits occur as peat beaches and beach ridges (Figure 2) that may be up to 2.5 m thick. They are observed 3 km upstream and may cover a total surface area up to 50 km2 (Gastaldo et al., 1995).

BEDLOAD TRANSPORT PATTERNS AND

TIDAL CURRENT STRATIFICATION

Bedload Transport Patterns Hydrodynamic data were collected at 22 stations in the distributaries and estuaries (Figure 3). Vertical profiles of current speed and direction, salinity and turbidity were recorded at 30 minutes intervals for complete spring and neap tidal cycles. Current measurements were used to calculate bed shear stress velocity (u*), which was used to infer the bed-shear stress (τ0) (van Rijn, 1990; Brown et al., 1997). The latter was employed to compute sediment motion (θs), which was charted against



time and the threshold of motion (θcr) (Soulsby, 1997) to get the direction and duration of bedload transport at a particular site. In general, most bedload sediment transport occurs during spring tide. It also occurs during transitional tides, as long as the tidal range is large enough (≥ 1.5 m) to produce significant bed shear stress. There is a significant downstream decrease in the duration and rate of bedload transport as well as a change in the direction of net transport (Figures 3). Seaward, or downstream, bedload transport dominates the delta apex and the upper to middle reaches of distributaries, particularly in the southern area (Figure 3). On the delta apex, downstream bedload transport persists for 4 hours, whilst in the upper reaches of distributaries it lasts for about 3 hours and in the middle reaches of distributaries for about 1 hour (Figure 3). In the lower reaches of distributaries and in the estuaries, landward, or upstream, net bedload transport is dominant with a duration of less than 1 hour (Figure 3). In the upper to middle reaches in the northern area, upstream bedload transport occurs as a subordinate phase to downstream bedload transport and has a duration of less than 1 hour (Figure 3). The bedload transport rate in the Mahakam River is 6.46x10-6 m3/m/s or 5.81x10-3 m3/s (Table 1), or 0.093 m3/m per tidal cycle. On average, about 28 tidal cycles per month or 336 tidal cycles per year, have tidal ranges larger than 1.5 m. Therefore the annual bedload transport rate in the Mahakam River is estimated at 31.24 m3/m/year or 28,118 m3/year (Table 1). About 21% of the Mahakam River bedload enters the northern area with a rate of 1.35x10-6 m3/m/s (Table 1). The other approximately 79% goes to the southern area. In the upper to middle reaches of the northern area, the subordinate upstream bedload transport reduces the net downstream bedload transport rate (Figure 1). Averaged over a tidal cycle, the net downstream bedload transport rate is 9.69x10-3 m3/m per tidal cycle in the upper reaches which decreases to 2.24x10-3 m3/m per tidal cycle in the middle reaches (Table 1). This implies that bedload is deposited in the upper to middle reaches of the northern area at a rate 7.45x10-3 m3/m per tidal cycle or 2.50 m3/m per

year. All downstream bedload transport ceases further seaward, causing deposition in the lower reaches. Upstream bedload transport contributes 2.73x10-9 m3/m per tidal cycle in this area, yielding a net deposition of 2.24x10-3 m3/m per tidal cycle or 0.75 m3/m per year. In the southern area, bedload transport from the Mahakam River enters multiple distributaries. On average, sediment transported as bedload is deposited at a rate 2.70x10-2 m3/m per tidal cycle or 9.08 m3/m per year in the upper to lower reaches of the distributaries (Table 1). The upstream bedload transport deposits 3.69x10-9 m3/m per tidal cycle or 1.24x10-6 m3/m per year in the lower reaches (Table 1). In the estuaries of the central area, upstream bedload transport yields a net depositional rate of 8.21x10-8 m3/m per tidal cycle or 2.76x10-5 m3/m per year (Table 1). This number is extremely small compared to the fluvial input from the Mahakam River, which is 31.24 m3/m per year.

Tidal Current Stratification Tidal influences on sediment transport are best illustrated by a downstream distribution of current profiles along a distributary, which indicates the occurrence of tidal current stratifications from the delta apex to channel mouth (Figures 4a to 4d). Generally they occur during both spring and neap tides, but more pronounced during neap tides. They produce bottom upstream, subordinate reversal currents, mainly at 2 m height from the bottom at the delta apex to 4 m height near the channel mouth. As described in the previous sub-chapter, the presence of the tidal current stratification reduces bedload transport capacity and promotes sedimentation of the fluvial sediment-laden along the middle to lower reaches of the distributaries.

THE ROLE OF TIDES IN THE MODERN DEPOSITIONAL PROCESSES

The sediment transport patterns and the facies distribution indicate that tides are the most important process on the Mahakam Delta, although fluvial processes and waves have some influence. Tidal processes dominance arguably extends further tens of kilometres upstream above



the delta apex up to where the fluvial sedimentation prevails, as evidenced by the presence of sand and mud couplet in the Medium Sand facies which was previously observed by Gastaldo et al. (1995). Tidal processes control sediment transport in the delta as indicated by bedload transport that only occurs during spring tides, which suggests that the sand and mud couplets reflect spring-neap variations. Tidal processes also cause fluvial influence to decrease downstream as evidenced by the decrease in the downstream magnitude of the net bedload transport and the fining seaward sediment distribution (Figures 2, 3). Concurrently, tidal dominance increases seaward as indicated by the dominantly upstream net bedload transport in the lower reaches of the distributaries and in the estuaries, increasing mud drapes and more abundant brackish to marine organisms (Figures 2, 3). Turbidity maxima and a lack of sand cause mud to dominate the lower reaches of distributaries, where tidal currents rework relic sand bars (Figures 2, 3). The bedload transport patterns and the facies distribution indicate that fluvial influence extends further downstream in the southern area than in the northern area (Figures 2, 3), which supports the contention of Allen and Chambers (1998) that the distributaries in the southern area are more active than those in the northern area. The downstream net bedload transport lasts longer in the southern area and it transports medium sand to the middle reaches of the distributaries where the upstream net bedload transport starts to dominate. Meanwhile, in the northern area those processes occur in the upper reaches of the distributaries where the upstream bedload transport exists as a subordinate of the shorter net downstream bedload transport. Waves have a significant influence mainly on the southern coastline near the delta front, where they winnow mud and slightly modify the geometry of intertidal sand bars (Figure 2). Although their energy is mostly attenuated on the broad subaqueous delta plain, waves are still able to concentrate detrital organic debris along the upper part of intertidal mud flats to form peat beaches and beach ridges.

IMPLICATION TO RESERVOIR The morphology, facies distribution and hydrodynamics of the Mahakam Delta all suggest that the delta is presently being transgressed and modified by marine processes (Husein and Lambiase, 2005). Bedload transport patterns indicate that most, if not all, fluvially-derived sand is being stored onshore in the distributaries, which is a feature of transgressive systems according to sequence stratigraphic models (e.g. Shanley and McCabe, 1993). In present-day setting where the fluvial sand is deposited along the distributaries, the lateral bars are possibly to become continuous reservoirs. Their geometry commonly are narrow and rectilinear belts up to 5-10 m thick and 0.5-1.0 km wide. The range of width/thickness ratio of individual lateral bar varies from 50 to 100 and is quite similar to that observed in the Miocene distributaries (Duval et al, 1992). There is also possibility to have stacked channel sand bodies reservoir. Prasetyo (2003) observed a 50 m thick stacked distributaries sand bodies in the Nilam Field at the Mahakam Delta province. It filled an incised valley that has eroded the overlying delta plain deposit and has a width/thickness ratio of 1:200. The lateral bars have erosional bases which overlay prodelta mud and exhibits laterally accreting deposits (Allen and Chamber, 1998). A typical succession fill is cross bedded medium sands to flaser bedding followed by mud-sand couplets and lenticular bedding, which reflects gradual changing from river-dominated distributary channel sand to tide-dominated distributary channel sand. The sand/mud ratios are highly variable, ranging from 90:10 to 15:85, depends on their location with respect to the bars (Gastaldo et al., 1995). This certain amount of small scale internal reservoir heterogeneities could form localized permeability barriers and result in a lower values of vertical permeability than in the more massive and cleaner fluvial-dominated sand facies. Tidal signatures also significantly decrease reservoir quality. Trevena et al (2003) observed in the Attaka Field at the Mahakam Delta province that the river-dominated distributary channel sand has average porosity 24 % and median permeability 665 md, while the



tide-dominated distributary channel sand has average porosity 26.1 % and median permeability 256 md.

CONCLUSIONS The main conclusions of the present study are: 1. The Mahakam Delta is indeed a mixed fluvial

and tide-dominated system but tidal processes are the most important and control sedimentation throughout the delta plain.

2. Tidal stratification occurs dynamically and influences the bedload transport that commonly takes place during spring tide but not during neap tide, which later suggests that the sand and mud couplets reflect spring-neap variations.

3. The delta is being transgressed. Fluvially-supplied sand is being stored onshore in the distributaries and is not reaching the shoreline and rich in tidal signatures.

4. The tidal processes control the distribution of the potential reservoir in the delta plain and significantly decrease the reservoir quality.

ACKNOWLEDGEMENTS The author wish to thank Total E&P Indonésie, Total E&P Borneo BV and Universiti Brunei Darussalam for sponsoring the research, and Universitas Gadjah Mada for the study leave.

REFERENCES

Allen, G.P. and Chambers, J.L.C., 1998. Sedimentation in the modern and Miocene Mahakam Delta. Indonesian Petroleum Association, 236 pp. Allen, G.P., Laurier, D., and Thouvenin, J.M., 1976. Sediment distribution patterns in the modern Mahakam Delta. Proceedings Indonesian Petroleum Association, 5th Annual Convention, 159-178. Brown, J., Colling, A., Park, D., Phillips, J., Rothery, D. and Wright, J., 1997. Waves, tides and shallow-water processes. In: G. Bearman (ed.). Volumes on oceanography. The Open University, Oxford, 187 pp.

Duval, B.C., de Janvry, G.C. and Loiret, B., 1992. Detailed geoscience reinterpretation of Indonesia’s Mahakam Delta scores. Oil and Gas Journal, 90, 67-72. Gastado, R.A., Allen, G.P., and Huc, A.Y., 1995. The tidal character of fluvial sediments of the modern Mahakam River Delta, Kalimantan, Indonesia. In: B.W. Flemming and A. Bartholona (eds.). Tidal signatures in modern and ancient sediments. International Association of Sedimentologists. Special Publication 24, 171-181. Husein, S. and Lambiase, J.J., 2005. Modern Sediment Dynamics of the Mahakam Delta, Proceedings Indonesian Petroleum Association, 30th Annual Convention, 367-379. Orton, G.J. and Reading, G.H., 1993. Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size. Sedimentology, 40, 475-512. Prasetyo, B., 2003. Facies mapping and reservoir potential of the G58 interval using 3D seismic data in Nilam Field, Sanga-sanga PSC, Indonesia. Universiti Brunei Darussalam, unpubl. M.Sc. thesis, 86 pp. Roberts, H. and Sydow, J., 2003. Late Quaternary stratigraphy and sedimentology of the offshore Mahakam Delta, East Kalimantan (Indonesia). In: F.H. Sidi, D. Nummedal, P. Imbert, H. Darman and H.W. Posamentier (eds.). Tropical deltas of Southeast Asia, sedimentology, stratigraphy and petroleum geology. SEPM Special Publication 76, 125-145. Schuettrumpf, R., 1986. Hydrological monography of the Mahakam River. Technical Cooperation for Area Development, Kutai District, East Kalimantan. Shanley K.W. and McCabe P.J., 1993. Alluvial architecture in a sequence stratigraphic framework: a case history from the upper Cretaceous of southern Utah. In: S.S. Flint and I.D. Bryant (eds.). The Geological Modelling of Hydrocarbon Reservoirs and Outcrop Analogues, IAS Special Publication 15, 21-56.



Soulsby, R., 1997. Dynamics of marine sands: a manual for practical applications. Thomas Telford, London, 249 pp. Storms, J.E.A., Hoogendoorn, R.M., Dam, R.A.C., Hoitink, A.J.F. and Kroonenberg, S.B., 2005. Late-Holocene evolution of the Mahakam delta, East Kalimantan, Indonesia. Sedimentary Geology, 180, 149-166. Total, 1986. Metereological and oceanographical campaign. Total internal report, 67 pp.

Van Rijn, L.C., 1990, Principles of fluid flow and surface waves in rivers, estuaries, seas and oceans, Aqua Publications, 335 pp. Trevana, A.S., Partono, Y.J. and Clark, T., 2003. Reservoir heterogeneity of Miocene-Pliocene deltaic sandstones at Attaka and Serang fields, Kutei Basin, offshore East Kalimantan, Indonesia. In: F.H. Sidi, D. Nummedal, P. Imbert, H. Darman and H.W. Posamentier (eds.). Tropical deltas of Southeast Asia: sedimentology, stratigraphy and petroleum geology. SEPM Special Publication 76, 235-254.



Station Channelwidth (m) a1 a2 b1 b2 a1 a2 b1 b2 c1 c2

1. Mahakam River (Mhk) 900 6.46E-06 5.81E-03 9.30E-02 83.69 9.30E-02 28,118Northern area:3. Bru-2 1000 2.12E-12 2.12E-09 7.63E-09 7.63E-06 -7.63E-09 2.56E-034. Kli-1 500 1.35E-06 6.73E-04 9.69E-03 4.85 4.82E-10 2.41E-07 8.67E-07 4.34E-04 9.69E-03 16286. Kli-3 1100 1.96E-13 2.15E-10 3.53E-10 3.88E-07 -3.53E-10 1.30E-048. Ilu 800 1.06E-13 8.45E-11 1.9E-10 1.52E-07 -1.90E-10 5.11E-059. Ptn-1 400 6.21E-07 2.49E-04 2.24E-03 0.89 2.23E-11 8.91E-09 4.01E-08 1.60E-05 2.24E-03 30110. Ptn-2 550 1.14E-13 6.29E-11 2.06E-10 1.13E-07 -2.06E-10 3.81E-0511. Ptn-3 1100 1.14E-13 1.26E-10 2.06E-10 2.27E-07 -2.06E-10 7.61E-05Central area:12. Byr-1 550 4.56E-11 2.51E-08 8.21E-08 4.52E-05 -8.21E-08 1.52E-02Southern area:14. Byt-1 400 1.67E-06 6.66E-04 1.50E-02 6.00 1.50E-02 201415. Byt-2 500 7.62E-10 3.81E-07 2.74E-06 1.37E-03 2.74E-06 4.61E-0116. Byt-3 700 1.97E-12 1.38E-09 7.1E-09 4.97E-06 -7.10E-09 1.67E-0317. Ulu-1 450 8.13E-07 3.66E-04 5.85E-03 2.63 5.85E-03 88519. Ulu-3 1400 8E-14 1.12E-10 2.88E-10 4.03E-07 -2.88E-10 1.35E-0420. Pdg 700 3.10E-06 2.17E-03 3.91E-02 27.35 3.91E-02 9189

Upstream Downstream Net Transport

TABLE 1: Summary of bedload transport rates. a1: rate in m3/m/s; a2: rate in m3/s and is obtained by multiplying a1 with the channel width; b1: rate in m3/m per tidal cycle; b2: rate in m3/tidal cycle and is obtained by multiplying b1 with the channel width; c1: net transport rate in m3/m/s (a negative sign and

light blue colour indicates upstream direction); c2: annual net transport rate in m3/year. Station numbers refer to Figure 3 for locations.



525

50

50

5

25

10 km

N

10 km

N

Tropical lowland rain forest

Mixed hardwood and palm forest

Nipah swamp

Mangrove swamp

100 km

KutaiLakes

MahakamDeltaMahakam River

Drainage Basin

Southern area

Northernarea

Central area

Delta plain

Deltafront

Subaqueousdelta plain

dd

d

d

ee e

e

e

d

d

e

e

e

d

FIGURE 1: Location and morphology of the Mahakam Delta, showing the morphological zonations (delta plain, subaqueous delta plain and delta front) as well as distributaries (d) and estuaries (e).



-5 m

-25

mN

10 k

m

Med

ium

san

dFi

ne s

and

Bio

turb

ated

fine

sand

San

dy m

udM

udP

eat b

each

esLa

ndw

ard

limit

of b

enth

ic m

arin

e or

gani

sms

-5 m

-25

mN

10 k

m

Med

ium

san

dFi

ne s

and

Bio

turb

ated

fine

sand

San

dy m

udM

udP

eat b

each

esLa

ndw

ard

limit

of b

enth

ic m

arin

e or

gani

sms

FIGURE 2: Block diagram of sedimentary facies on the Mahakam Delta. Bathymetry was compiled from echo sounding profiles; the offshore break in slope is in approximately 5 m of water. The red dotted

line indicates the upstream limit of benthic marine organisms.



10 km

N10 km

N10 km

N

1

3

4

68

9 10

11

12

14

15

16

17

19

20

1 hour

Scale

5

2

7

13

18

21

22

FIGURE 3: Bedload transport patterns. The blue arrows indicate net downstream bedload transport and the red arrows indicate net upstream bedload transport. Arrow length is proportional to transport duration.

Numbers in yellow circles refer to the the measurement stations in Table 1.



FIGURE 4a: Bed shear stress and current velocity profiles at the delta apex, location is Station #1 (see Figure 3). Red box indicates bedload transport occurrence.



FIGURE 4b: Bed shear stress and current velocity profiles at the upper reaches, location is Station #14 (see Figure 3).

FIGURE 4c: Bed shear stress and current velocity profiles at the middle reaches, location is Station #15 (see Figure 3).



FIGURE 4d: Bed shear stress and current velocity profiles at the lower reaches, location is Station #16 (see Figure 3). Note that the bedload transport (red box) also occurs during neap tide since the

measurement was done at near-neap condition or transition from spring to neap.

editing oral#015 paper-tidal husein-iagi2006

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