Discriminating Fluvial and Deltaic Channel Indonesia Case (Payenberg, 2003)

7/28/2019 Discriminating Fluvial and Deltaic Channel Indonesia Case (Payenberg, 2003)

http://slidepdf.com/reader/full/discriminating-fluvial-and-deltaic-channel-indonesia-case-payenberg-2003 1/16

IPA03-G-112

PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATIONTwenty-Ninth Annual Convention & Exhibition, October 2003

DISCRIMINATING FLUVIAL FROM DELTAIC CHANNELS – EXAMPLES FROM INDONESIA

Tobias H.D. Payenberg*

Simon C. Lang*

Bintoro Wibowo**

ABSTRACT

The successful exploration and development of hydrocarbons from channel reservoirs requiresdetailed knowledge of the external geometry and

internal architecture of the reservoir body. Thisknowledge becomes even more critical whenexploring for stratigraphic traps. A classification intofluvial (meandering, braided, anastomosing), deltaic(distributary) or incised valley (fluvial or estuarine),sets strict boundary conditions that constrain theexternal geometry and internal architecture of thechannel deposit.

In the past, distributary channels have beenincorrectly compared with fluvial channels. In mixed-load systems, deltaic distributary channels are

frequently rectilinear channel segments located on thedelta plain between the head of passes and thedepositional mouthbars. Fluvial channel reservoirs aremost commonly sandstone deposits of meander

pointbars or braided sheets. Deltaic distributarychannel reservoirs, on the other hand, are mosttypically elongate sandy channel sidebars attached tomorphologically rectilinear channel walls. Channelsidebars form by both lateral and downstreamaccretion resulting from flow in a confined, low-sinuous thalweg, and may be filled with organic mudfollowing channel abandonment. A deltaic

distributary channel therefore will often show as aslightly sinuous feature on 3D seismic and can easily be mistaken for a meandering channel belt.

To avoid misclassification, it is important to beconscious of relative scales. Deltaic distributarychannels are usually thinner and shallower compared

* NCPGG & APCRC, The University of Adelaide

** NCPGG, The University of Adelaide

to their updip fluvial channel belts, and cannot bethicker than their depositional mouthbars. Width-thickness ratios for fluvial distributary channelreservoirs are on average 50:1, while meanderingfluvial channel reservoirs have width-thickness ratios

of typically >100:1, and braided river reservoirs showratios of 500:1 or higher.

INTRODUCTION

Fluvial distributary channel sandstones (found in thedeltas of fluvial systems as opposed to submarinedistributary channels) are common reservoirs for hydrocarbons, including in many parts of Indonesia.With increasing maturity of many hydrocarbon basinsthat depend largely on structural traps, searching for stratigraphic traps gains significance. Although

recoverable hydrocarbons are often fewer than thosefound in, for example, structurally trapped sheetsandstones, existing infrastructure and higher oil andgas prices, as well as contract obligations, makesmaller accumulations economically viable. Manyaccumulations already have some stratigraphictrapping component, and determining this isimportant in developing correct and efficient flowsimulation models.

Distributary channels of mixed-load systems, such asthey are common around Indonesia, often constitute

ideal stratigraphic traps because they form isolatedsand ribbons encased in delta plain and channelabandonment mudstone. Despite being channels, their internal reservoir geometry is very different fromriver channels of meandering and braided rivers aswell as incised valleys. This paper highlights theinternal architecture of distributary channels fromdeltas and their potential variations and reservoir geometries. Examples from the mixed-load MahakamDelta system, Kutei Basin, East Kalimantan and the

© IPA, 2006 - 29th Annual Convention Proceedings, 2003sc Contents

Contents

Search



Wanda/Gita system, Sunda Basin Southeast Sumatrawill be used to highlight these issues and implicationsfor exploration and development are discussed.

RESERVOIR ARCHITECTURE OF

DISTRIBUTARY CHANNELS

Fluvial, tidal and wave currents are the three maintypes of currents responsible for moving sand on adelta. Of those, only fluvial and tidal currents areactive within distributary channels. Wave action ismainly operational at the delta front, and does notreach far up the distributary channels. The degree towhich fluvial and tidal processes are active and/or

predominant on a delta depends on the fluvialdischarge of the river and upon the tidal range withinthe adjacent sea. The two end member distributarychannels resulting from this interaction are fluvial-

dominated distributary channels, and tide-influenceddistributary channels, with an endless range of thetwo in-between. It needs to be stressed, that there isno such thing as a “tidal distributary channel”,

because fluvial processes have to be active totransport sand from the hinterland to the delta front.Therefore tides will never be the predominant forcemoving sand into a distributary channel (Allen et al,1979). If there are no fluvial processes active within adistributary channel a funnel-shaped estuary results(Figure 1). Tidal channels do exist on a delta, wherethey serve to inundate and drain the delta plain with

the tides (Figure 1). Tidal channels have a highlymeandering morphology, are dominantly mud filledand highly bioturbated. This is in strong contrast totypical meandering fluvial channels on the alluvial

plain.

The external morphology of any river channel has a profound impact on the distribution of sand within thechannel. Common channel morphologies found inriver channels are braided, anastomosing, highsinuous (meandering) and low sinuous (straight).Distributary channels almost always fall into the low-

sinuosity class. They commonly have rectilinear channel segments, which mean that the channelmargins are both relatively straight between points of

bifurcation. Bifurcations form on a delta due tochannel diversions around a subaqueous mouthbar,which accretes vertically and seaward as delta plaindeposits prograde over delta front deposits. Thiscontrasts with bifurcation due to avulsion eventstypically formed during flood events. A river channelis defined as a distributary channel when it is located

on the delta plain, between the head of passes (thefirst major channel bifurcation) and the depositionalmouthbars (Allen et al, 1979).

Fluvial-Dominated Distributary Channels

a. Lithofacies

The productive Miocene interval of the MahakamDelta, Indonesia and in the Sunda Basin, SoutheastSumatra contain, among others, ancient fluvial-dominated distributary channels. They commonlyexhibit a fining-upwards profile and comprisedominantly trough cross-stratified (St ) and planar tabular cross-stratified (Sp), medium to coarsesandstone (Figure 2, Table 1). These sandstones weredeposited by unidirectional traction currents in thefluvial-dominated portion of the distributary channels

with overall low current variability. Another lithofacies observed is massive sandstone (Sm), whichis interpreted as bank collapse in conjunction withliquefaction (Jones and Rust, 1983; Turner andMonro, 1987) and, if present, occurs close to the baseof a distributary channel-fill. Ripple cross-laminatedlithofacies (Sr ) and bioturbated ripple cross-laminatedlithofacies (Srb) are also commonly present (Figure2). Both are found in the upper part of the channeldeposit, with the bioturbated lithofacies Srb onlyrarely encountered. Lastly lithofacies Fl , representinginterbedded fine-grained, laminated sand, silt and

mudstone, can occasionally be found within a channel(Table 1, Figure 2).

b. Architecture

The internal architecture of a fluvial-dominateddistributary channel is illustrated by the Mioceneoutcrops around the Mutiara Field, East Kalimantan,Indonesia (Figure 3). Although the channel exposedin Figure 3 is only five metres thick, aspect ratios areconsistent with thicker and wider channels foundwithin the section (Payenberg, 1998; Payenberg et al,

2003). Ratios are also consistent with the modernMahakam Delta distributary channels, as shown inFigure 4. The erosional base of the channel marks the

bottom of the channel succession (Figures 2 and 3).At the southern end of the channel (left in Figure 3) alateral and downstream accretion complex is welldeveloped, interpreted as a channel sidebar. Thissandy complex fingers laterally (northerly, to theright) into a muddy abandoned channel plug. Thelateral accretion surfaces dip toward the north, but the



trough and tabular cross-bedding, which indicate theflow direction, trend west into the outcrop (Figure 3).Good reservoir quality within the channel is onlyfound in the lateral and downstream accretioncomplex, the channel sidebar, while the remainder of the channel is mud filled and effectively a lateral seal.

A depth profile through this palaeo-distributarychannel would show a very similar shape to one of the fathometer (depth) profiles across a modernMahakam Delta fluvial-dominated distributarychannel, as shown in Figure 4. Profile A shows anasymmetric thalweg morphology with a steep slopeon the right (east) and a gentler dipping slope on theleft (west), similar to the outcrop example in Figure 3.The grab samples plotted in Figure 4 onto the profilesconfirm the existence of sand within the gentler dipping channel banks, indicating the existence of

sandstone in channel sidebars. Depth profiles B and Cin Figure 4 show the transition between two sidebarsalternating on either side of the distributary channel.The profile spacing indicates the length of eachchannel sidebar to be approximately five kilometres,although this is highly variable and depends on thelength of the rectilinear channel segment. A model of sand distribution within a typical distributary channeland its sinuous mud plug is shown in Figure 5.

Tide-Influenced Distributary Channels

a. Lithofacies

Tide-influenced distributary channels typically occur within the lower delta plain, where tidal processesinfluence water circulation up the distributarychannels. A vertical profile through such a channel isshown in Figure 6. Lithofacies comprise trough cross-stratification (St ), planar tabular cross-stratification(Sp), ripple cross-lamination (Sr ), parallel-stratifiedsandstone (Spl ) and fine-grained laminated ( Fl )lithofacies (Figure 6, Table 1). Bioturbation iscommon, creating bioturbated ripple cross-laminated

(Srb) and laminated sand, silt, and mudstone facieswith bioturbation ( Flb). Cross-stratified sandstone (St and Sp) occurs mainly at the base of the successionand provides evidence of larger bedforms migratingdown the channel, as is typical of a fluvial-dominatedchannel deposit. As the channel becomes an inactivedistributary channel, tidal processes increasinglydominate. Sedimentation changes from traction tosuspension deposition and vice versa as flowvelocities within the channel decease and increase

with the tides (neap – spring velocity variations or by periodic flood-generated flow events). The tidal portion of the channel-fill therefore constitutesinterbedded sandstone with finer grained siltstone andmudstone. Also important are changes in the

bioturbation style and intensity, which generally

increase as velocities decline.

b. Architecture

The internal architecture of tide-influenceddistributary channels is very different to that of thefluvial-dominated distributary channels. Becausefluvial discharge is only strongly active during theinitiation of the distributary channel, reservoir qualitysandstone is limited to the channel floor or to verysmall channel sidebars (Figures 6 and 7). Thinsandstone beds are commonly present within the tidal

portion of the channel-fill, but their limited lateralextent and poor vertical connectivity means their reservoir potential is very limited.

DISTINGUISHING CHARACTERISTICS OF

DISTRIBUTARY CHANNELS, FLUVIAL

CHANNELS AND INCISED VALLEY FILLS

Micropalaeontology (foraminifers or ostracodsadapted to brackish or estuarine conditions) or

palynology (the presence of abundant and diversedinoflagellates) are the best tools for determining if a

channel deposit was formed on a delta plain or in analluvial environment. It is important to differentiatethese two settings, because of different reservoir geometries and effects on reserves. A carbonaceousmud plug that fills the low or high sinuosity thalwegwithin a channel is commonly visible on 3D seismicdata as high-amplitude features (Figures 5, 8 and 9).However, because of the slight sinuosity of adistributary thalweg, a distributary channel can easily

be mistaken for a meandering river (Figure 9).Drilling into the visible mud plug often results inmissing the sandstone contained in the channel

sidebars of the distributary channel, or the pointbarsof a meandering river. To improve explorationsuccess, it is therefore important to verify thedepositional setting of the channel deposit.

Despite the possibility of differentiating fluvial fromdistributary channels through the identification of thedepositional setting, fluvial-style channels in the formof incised valley-fills also occur within delta plaindeposits. In an example from the Miocene Mahakam



Delta, fluvial-dominated channels tend to appear intwo different configurations. One comprises anaverage sandstone thickness of 10 to 15 metres (after compaction) with only one major erosional surface atthe bottom of the succession. Such a channelrepresents the deposit of a ribbon-like, single storey

fluvial-dominated distributary, deposited in the upper delta plain environment. The second configurationrepresents multistorey channel-fills with numerousinternal scour surfaces indicating channel lobeswitching. This sandstone deposit is significantlythicker than distributary channels, typically exceeding30 metres. Although vertical truncations and cross-cuttings of individual distributary channels areencountered in the delta plain setting, multistoreychannels are typically the result of channel clusteringdue to geomorphologic constrictions, commonlyinduced by a relative sea level fall. Therefore,

multistorey channel successions in a deltaicsuccession typically represent the filling of an incisedvalley. Palaeoflow analysis through either direct

palaeocurrent analysis on cores (Payenberg et al,2000; Payenberg, 2003) or through the use of microresistivity logs will aid in the differentiation

between cross-cutting distributary channels andincised valley-fills, as will facies analysis, sequencestratigraphy and careful analysis of 3D seismic data(Figures 8 and 9).

Comparison of Fluvial-Dominated and Tide-

Influenced Distributary Channels

Generally, fluvial-dominated distributary channelsexhibit a much higher sand/shale ratio than tide-influenced distributary channels (Figures 2 and 6).The clay content within the fluvial-dominatedchannels in the Miocene Mahakam Delta is estimatedto be around 10%, while it is highly variable in tidalenvironments and often exceeds 50%. The grain sizeis also typically finer in the tide-influenced channelscompared to fluvial-dominated distributary channels.Trough and tabular cross-stratification is abundant in

the fluvial distributary channels, whereas it is lesscommon in the tide-influenced channels. Ripplecross-lamination is more common in the tide-influenced deposit. Due to the high clay contentwithin the tide-influenced channel-fill, the vertical

permeability would be very low compared to thefluvial distributary channel-fill, while lateral

permeability should be similar if the clay content inthe tidal deposit is not too high. Fluvial-dominateddistributary channels and tide-influenced distributary

channels generally have similar dimensions of theexternal channel geometry (the container) within thesame deltaic system. However, tide-influenceddistributary channels contain much less sandstonethan do fluvial distributary channels in their sidebars,and often have a funnel-shaped opening towards the

sea (Figures 1, 5 and 7). Because of the high claycontent, tide-influenced channels typically constitute

poorer reservoirs.

Width-Thickness Ratios

Fluvial-dominated distributary channels are usuallynarrower and shallower compared to their updipfluvial channel belts, because the water discharge of the fluvial channel belt is dispersed by a series of

bifurcating distributary channels to the delta front.Distributary channels on a delta commonly decrease

in width and depth towards the delta front as moreand more bifurcations reduce the amount of water transported by each channel. The amount of sandstored in channel sidebars decreases with proximity tothe delta front. Therefore, the sandbody width-thickness ratios vary considerably: ratios for fluvialdistributary channel sidebar reservoirs are on average50:1 (range 15:1 to 100:1) based on data from theMiocene Mahakam Delta (Duval et al, 1995; Sidi,1998). Alluvial channels with no relationship to adelta are typically much wider in comparison todistributary channels. Meandering fluvial channel

reservoirs typically exhibit width-thickness ratios of >100:1, while braided river reservoirs are the mostsheet-like of the channel reservoirs and thereforeshow ratios of 500:1 and higher.

DISCUSSION AND CONCLUSIONS

Delta successions formed by mixed-load systemstypically have relatively straight, bifurcatingdistributary channels feeding time-equivalentmouthbars that comprise the overall coarsening-upward delta front. Distributary channels are of

comparable depth to the height of their depositionalmouthbars, with width-thickness ratios on average50:1 (range 15:1 to 100:1). Fluvial channels aretypically more sinuous, broader and deeper (width-thickness ratios of >100:1), lying within broader channel belts, and are significantly wider than their rectilinear distributary channels. Both fluvial anddistributary channels have fining-upward profilesassociated with abandonment, but distributarychannels should be shoestring-like bodies rather than



sheet-like bodies, and they should be interbeddedwith coarsening-upward delta mouthbars rather thanaggradational floodplain deposits. Distributarychannels typically show some brackish influence asthey lie on the delta plain which is subject to marineinfluences, especially tides. Abandoned distributary

channels often show more tidal influence, includingincreasing bioturbation intensity, in the upper part,leading to poorer reservoir quality.

Given an average width to thickness ratio for mouthbars of >1000:1, distributary channels can be

predicted near the well at a given chronostratigraphicinterval because each mouthbar will be associatedwith a distributary channel. On 3D seismic data,isolated distributary channels may be obvious asslightly sinuous features with high-amplitude fill, but

commonly this reflects carbonaceous or muddy fill,with the sand-prone reservoir section confined to therelatively narrow sidebars within the rectilinear distributary channel. Incised valleys have thicker fluvial channel-fill than typical distributary or fluvialchannels, and are often associated with transgressiveestuarine fill. In some cases the incised dendritictributary drainage can be detected on 3D seismicattribute maps.

Drilling high-amplitude, low-sinuosity, channel-likefeatures, rather than the relatively narrow sidebars

could result in missing the reservoir at a givenchronostratigraphic interval because the features areoften only visible due to the impedance contrastassociated with the carbonaceous mud plug. Isolatedfluvial channel belts, on the other hand, have moresinuous mud plugs (sometimes oxbow lakes), and theidentification of the sand-prone pointbars thatdelineate the bounds of the fluvial channel belt aremore obvious if there is sufficient impedance contrastwith the surrounding floodplain sediments. In-channelfeatures that lie within a low accommodation interval,together with lateral and vertical amalgamation, can

make both the distributary and fluvial channels hardto recognise on 3D seismic attribute maps.As with all geological models, caution has to beexercised when using the presented data. Althoughthe presented results are generally applicable, localtectonic setting, source and composition of the fluvialsystem, catchment size, local oceanographicconditions, etc. all play pivotal roles in the resultingfluvial and deltaic morphology and architecture.Therefore, the presented data should always be

integrated with and adjusted for local condition andnot be applied blindly.

ACKNOWLEDGEMENTS

The authors would like to thank the late George P

Allen for introducing us to the Mahakam Delta andthe intricacies of deltaic sedimentation. VICOIndonesia is acknowledged for providing support andlogistics for the field and subsurface work while bothauthors were at the Queensland University of Technology. THDP and SCL acknowledge theAPCRC for supporting the research on reservoir analogues as part of the Reservoir CharacterisationProgram. We would also like to thank DirectorateGeneral Migas, Departemen Energi Dan SumberdayaMineral Indonesia, BP Migas, Pertamina andCNOOC Indonesia (formerly Repsol YPF Maxus

SES) for providing the 3D seismic data from OffshoreSoutheast Sumatra and their permission to use themin this publication. This paper is a modification fromPayenberg and Lang (2003), and the authors thank APPEA for allowing us to reuse some of the material.

REFERENCES

Allen, G.P., and Chambers, J.L.C., 1998.Sedimentation in the Modern and Miocene MahakamDelta. Indonesia Petroleum Association, 236 p.

Allen, G.P., Laurier, D., and Thouvenin, J., 1979.Etude sédimentologique du delta de la Mahakam.

Notes et Mémoirs, TOTAL, Companie Franaise desPétroles, 15, 156 p.

Apt, J., Helfert, M., and Wilkinson, J., 1996. Orbit: NASA astronauts photograph the Earth. InRessmeyer, R. (ed). National Geographic Society,223 p.

Duval, B.C., Allen, G.P., Madaoui, K., Gouadain, J.,

and Kremer, Y., 1995. Applications of explorationtechnologies to reservoir prediction and management

– field examples of South-East Asia. SPE Bulletin,90i32, p. 1-10.

Galloway W.E., 1975. Process framework for describing the morphologic and stratigraphicevolution of the deltaic depositional systems. InBroussard, M. L. (ed) Deltas, models for exploration.Houston Geological Society, Houston, p. 87-98.



Jones, B.G., and Rust, B.R., 1983. Massive sandstonefacies in the Hawkesbury Sandstone, a Triassic fluvialdeposit near Sydney, Australia. Journal of Sedimentary Petrology, v. 53, p. 1249-1259.

Miall, A.D., 2002. Architecture and sequence

stratigraphy of Pleistocene fluvial systems in theMalay Basin, based on seismic time-slice analysis.AAPG Bulletin, v. 86, p. 1201-1216.

Payenberg T.H.D., 1998. Paleocurrent and reservoir analysis of the Miocene channel deposits in MutiaraField, Kutai Basin, East Kalimantan, Indonesia.Masters Thesis, Queensland University of Technology, Brisbane, Australia, 1, 235 p,unpublished.

Payenberg, T.H.D., 2003. Paleocurrents from FMS

and scribe-oriented core - a comparison. In Lovell, M.A. and Parkinson, D. N. (eds) GeologicalApplications of Well Logs. AAPG Methods inExploration Series, Number 13, p. 185-198.

Payenberg, T.H.D. and Lang, S.C., 2003. Reservoir geometry of Fluvial Distributary Channels – implications for northwest shelf Deltaic Successions:APPEA Journal, v. 43, Part 1, p. 325-338.

Payenberg, T.H.D., Lang, S.C. and Koch, R., 2000. Asimple method for orienting conventional core usingmicroresistivity (FMS) images and a mechanicalgoniometer to measure directional structures on cores.Journal of Sedimentary Research, v. 70, p. 333-336.

Payenberg, T.H.D., Sidi, F.H. and Lang, S.C. 2003.Paleocurrents and Reservoir geometry of middleMiocene channel deposits in Mutiara Field, KuteiBasin, East Kalimantan, In Sidi, F.H., Nummedal, D.,Imbert, P., Darman, H., and Posamentier, H.W. (eds)Tropical Deltas of Southeast Asia - Sedimentology,Stratigraphy, and Petroleum Geology. SEPM SpecialPublication, No. 76, p. 255-266, in press.

Sidi, F.H., 1998. Sequence stratigraphy, depositionalenvironments, and reservoir geometry of the Middle-Miocene fluvio-deltaic succession in Badak & Nilam

Fields, Kutai Basin, East Kalimantan, Indonesia.Masters Thesis, Queensland University of Technology, Brisbane, Australia, 110 p, unpublished.

Turner, B.R. and Monro, M., 1987. Channelformation and migration by mass-flow in the Lower Carboniferous fluviatile Fell Sandstone Group,

Northeast England. Sedimentology, v. 34, p. 1107-1122.



Lithofacies code

Lithofaciesdescription

Grain sizeCharacter and stratification Interpretation

Pc Cross-stratifiedpebblysandstone

Very coarsesand topebble

Beds, commonly less than 1 metre thick,stratified and graded. Contact tounderlying beds is strongly erosional.Forms base of thicker (cm – metres)sandy succession.

Cross-stratified pebbly beds formed atthe base of a channel or a larger barform (?flood event).

St Trough cross-stratifiedsandstone

Fine tocoarse sand

Small, 5 – 20 cm thick sets of troughcross-stratification. Often with well-rounded mudstone, siderite or coalpebbles at the base of the trough.Commonly interbedded with Sp.

Sinuous-crested and linguoid dunesformed under varying upper and lower flow conditions primarily in proximalparts of a distributary channel withhigh flow turbulence.

Sp

Planar tabular cross-stratifiedsandstone

Very fine tocoarse sand

Set thicknesses around 5 to 50 cm withmore planar foresets and non-erosionalbases. Commonly interbedded with St .Backflow ripples present on foresets.

Migrating , linguoid bars, traverse barsand sand waves form under low flowconditions with little flow turbulence.

SmMassivesandstone

Fine tocoarse sand

Massive, non-stratified sandstone. Abundance of coalified plant fragmentsand occasional rounded siderite andmudstone pebbles. Thicknesses rangefrom 10s of cm to 5 metres.

Sediment gravity flow in fluvial-dominated distributary channels as aresult of bank collapse and/or water-escape liquefaction.

Sr & Srb

Ripple cross-

laminated(Sr ) andbioturbatedripple cross-laminatedsandstone(Srb )

Very fine to

fine sand

Ripple cross-lamination, thickness less

than 5 cm, occurs as sets, interbeddedwith, or on top of other bedforms.

Forms under lower flow regime

conditions as traction deposits of 3Dripples in fine grained sediments.

Fl & Flb

Laminatedvery finesand-, silt-andmudstone (Fl )andbioturbated

(Flb )

Very finesand to clay

Thin laminated beds with occasionalbioturbation, soft sediment deformation or siderite nodules.

Suspension and weak traction currentsedimentation in floodplains,abandoned channels and mouth-bars.

FmMassivemudstone

Clay Massive, black to brown mudstone withoccasional siderite or calcareous nodules.Bioturbation possible. Black mudstoneusually thin bedded; brown mudstone canbe very thick (few metres).

Black mudstone is the result of quiet,overbank and channel abandonmentdeposits, brown mudstone is distaldelta front to prodelta mudstone.

TABLE 1

CHARACTERISTIC FEATURES OF THE LITHOFACIES FOUNDIN MIOCENE MAHAKAM DELTA DISTRIBUTARY CHANNELS



Figure 1 - Tripartite delta classification after Galloway (1975) showing satellite images of three end-member deltas dominated by fluvial (Mississippi), tidal (Ganges) and wave processes, as well as the mixedfluvial- and tide-dominated Mahakam Delta. Regardless of the dominant processes, distributarychannels typically have a rectilinear morphology between points of bifurcation. Visible also arenumerous highly-meandering tidal channels in deltas influenced by tides. These tidal channelsinundate and drain the delta plain and typically connect to a distributary channel. Satellite images of the Mississippi, the wave-dominated and the Ganges Deltas from the NASA web site, satelliteimage of the Mahakam Delta from Apt et al. (1996).



Figure 2 - Typical fluvial-dominated distributary channel-fill as found in the Miocene Mahakam Delta. Thefining-upwards succession comprises lithofacies St, Sp, Sr and Fl. The sandstone body is topped bya massive mudstone (Fm). This fluvial-dominated channel-fill is 11.5 metres (38 feet) thick. SeeTable 1 for lithofacies classification.



Figure 3 - Photo mosaic and interpretation of a fluvial-dominated distributary channel-fill in the Miocene Mahakam Delta, Indonesia. The channel base is crossing an older channel system. A channel sidebar with lateral and downstream accretion surfaces is visible in the southern endof the channel, fingering into channel abandonment mudstone in a northerly direction. The channel sidebar with its lateral anddownstream accretion surfaces consists of dominantly medium-grained sandstone with millimetre-thin mud interbeds on the accretionsurfaces. Trough and tabular cross-stratification are the dominant bedforms in the sidebar, indicating a palaeoflow direction into theoutcrop. Note the upstream or downstream end of an opposing sidebar in the northern end of the channel exposure.



Figure 4 - Fathometer profile of a fluvia l-dominated distributary channel located in the upper delta plain of theMahakam Delta. The bathymetry of the river channel clearly shows the lateral bars (channelsidebars), creating a sinuous thalweg morphology. Channel sidebars in this part of the channel areapproximately 5 kilometres long. Compiled with new data after Allen and Chambers (1998).



Figure 5 - Model of the typical external geometry and internal architecture of a fluvial-dominated distributarychannel based on modern and ancient examples from the Mahakam Delta, Indonesia. The channelmodel is assembled to reflect the channel morphology exposed in the outcrop shown in Figure 3.The channel sidebar deposit is limited in lateral and downstream extent, indicating a low-sinuosityriver morphology. Modified after Allen and Chambers (1998).



Figure 6 - Tide-influenced distributary channel-fill from the Miocene Mahakam Delta. Note the lower part of the channel-fill shows fluvial-derived sandstone with subsequent marine dominance, as evident bythe high percentage of fine-grained facies. Bioturbation is common in the marine-influenced fill.Lithofacies include St, Sp, Sr/Srb and Fl/Flb. This tide-influenced distributary channel has athickness of 8.5 metres (28 feet). See Table 1 for lithofacies classification.



Figure 7 - Model of the typical external geometry and internal architecture of a tide-influenced distributarychannel based on modern and ancient examples from the Mahakam Delta, Indonesia. Note the smallamount of reservoir sand compared to the fluvial-dominated distributary fill (Figure 5).



Figure 8 - Hypothetical 3D seismic amplitude map (1) and interpreted amplitude map (2) of a non-marine to

shallow marine interval. High amplitude areas (red), often mistaken for the channels, are actuallychannel plugs filled with low-density carbonaceous mudstone and show the location of a channelthalweg before channel abandonment. These high amplitude mud plugs can indicate the width of channel belts and define a meander belt (2 a) and a distributary channel (2 b) in this interval. The

bifurcation of the thinner channel [belt] clearly indicates a distributary channel. Because of thedifference in sandbody geometry, the identification and differentiation between a distributarysystem and a meander channel belt system is crucially important for hydrocarbon reserve estimatesand development. Note the incised tributaries of the lower left corner of the map (2 c) indicating themargin of an incised valley complex (see also Miall, 2002). Map area is 5 kilometres x 5 kilometres.



Figure 9 - 3D seismic amplitude maps from offshore southeast Sumatra showing A) a low-sinuosity distributarychannel with rectilinear channel margins, approximately 1.5 km wide, and B) a meandering channelwith a channel (belt) margin width of at least 6 km. While sand in the distributary channel is stored inelongate sidebars, scroll bars of the pointbar deposit indicate more extensive sand sheets.

Discriminating Fluvial and Deltaic Channel Indonesia Case (Payenberg, 2003)

Documents