Scope of Future Exploration in Andaman Basin, Genetically Linked with Producing Provincesof South-East Asia.

Subir Das*, R.K.SrivastvaE & D Directorate. Oil and Natural Gas Corporation Limited

E-mail: subiromi@gmail.comKeywords

Back arc, syn-rift, genetically linked, morpho-tectonic.

SummaryIn Andaman Sea basin though gaseous hydrocarbon hasflowed in two wells, one each in the fore arc and outerstructural high (shallow water), commercial hydrocarbonpool is still eluding. Exploration for hydrocarbons in abasin is driven by understanding of its evolution throughgeological times and linked with its sedimentation historyand tectonics. Newly acquired seismic data in Andamanback arc areas provided new insight in understanding. Thetectonics and geological history of the Andaman Sea cannotbe separated from the tectonics and geological histories ofMyanmar (Burma) on the north, and Sumatra on the south.Hydrocarbon occurrences in the North Sumatra and MerguiBasins typically exhibit rift-related petroleum systemswhich are proven to be quite prolific in parts. Since the EastAndaman Basin is a virgin exploration area with no wellcontrol, this paper looks at the possibilities of futureexploration in the potential syn-rift sequence of Andamanback-arc setup, which is genetically linked with theproducing Mergui and North Sumatra basin.

IntroductionThe Andaman Sea Basin is still considered to be a frontierarea with respect to hydrocarbon exploration due to paucityof drilled data, complex geology, remote location andoperational constraints (water depth hovering around2500m or more). But the basin has always attracted focusdue to its proximity to mature hydrocarbon provinces in theregion viz. North Sumatra, Mergui and Irrawaddy inseemingly similar geological setup. In last ten years, a lotof work has been done to understand the tectono-sedimentation model, sequence stratigraphy, petroleumsystem and hydrocarbon potential of the basin. Since 2011,additional seismic data has been acquired in the NELPBlocks and seven deepwater wells have been drilled, six inforearc and one in backarc. Andaman basin extends fromMyanmar in the north to Sumatra, Indonesia in the southand from Thailand and Malaysia in the east to Andamanand Nicobar group of Islands in the west (Fig-1). Tertiarysediments are exposed on the Andaman-Nicobar Islandsand preserve a record of the tectonic evolution of the Basinas a whole including the evolution and closure of theTethys Ocean.

Fig.1 Regional map showing the location of Andaman Basin andadjacent producing fields.

However, main Andaman Basin is a North-South elongateddepression narrowing to the south against the tip ofSumatra Pluton-an elevated Paleozoic-Mesozoic arc massif.The basin is surrounded by prolific hydrocarbon producingfields like North Sumatra Basin towards south, MerguiBasin in the east and Moattamma /Irrawaddy-Delta in thenorth (Fig-1). The basin is segmented into many sub basinsdue to presence of several prominent morpho-tectonicelements. In a west to east transect, the main tectonic zonesare the subduction zone i.e. trench, the Accretionarycomplex, forearc subbasin, magmatic/volcanic arc, CentralAndaman sub Basin (CAB) and the back arc basin (Fig-2).The Andaman basin has two major depo-centres. One is inthe fore-arc ponded low part and another is in the mainback arc part. There are two volcanic Seamounts namelyAlcock and Sewell within the back-arc basin and that wereprobably segmented later. East Andaman Basin in thebackarc, bordering Mergui Ridge is time equivalent ofnearby petroliferous North Sumatra and Mergui Basins andexhibits similarity in basin evolution and tectono-stratigraphy. A probable petroleum system in rift-relatedplay model (like North Sumatra) is expected in the EastAndaman Basin.

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Fig.2 Tectonic map of Andaman Basin showing different tectonicelements with study area.

Tectonic setting and sedimentationThe Andaman Sea Basin extends approximately 1,250 kmfrom Myanmar to Sumatra and was formed by the obliqueconverging plate boundaries of the Indian Oceanic andSouth East Asian Tectonic plates, which was initiated in theearly Cretaceous and has continued to the present day. Theeffects of the easterly subduction of the Indian Ocean Platebeneath the South East Asian Plate created a classic islandarc system, with the formation of six discreet geotectonicunits which (from west to east through the Andaman area)include; foredeep (Andaman Trench); innerslope/accretionary prism; island arc/outer structural high;fore-arc basin; volcanic-arc; and back-arc basin (Fig.2).This arc basin accommodated pelagic, turbidities andlocally derived fan sediment due to base level changes.These sedimentary successions constitute a majorSequence. In the back arc, extensional forces generatedNorth Sumatra, Murgui and east Andaman basins; whichreceived younger sediments from Sumatra pluton (south)and Malaya orogeny (east). Rise of volcanic arc in eastcreated a fore arc ponded low, which accommodated thickNeogene sediments. During Late Neogene, intra volcanicarc rifting due to NW-SE extension and local cauldronsubsidence accommodated fan and pelagic sediments.Further in the upper part, sediments deposited underdeepening bathymetry due to Andaman sea rift. Fore-arcsub-basins in east of Middle Andaman and in ponded lowaccommodated 3-5 Km of Neogene sediments suppliedfrom local accretionary arc and Irrawadi delta – fan system.

Analogous Prospectivity comparisons with adjoiningproducing Fields of Asia/South East Asia

Fore Arc Prospectivity:

Regional tectono-sedimentation model of Forearc suggeststhat Cretaceous to Early Paleogene sedimentation tookplace in deepwater open marine condition, which isunfavorable for preservation of organic matter. Another keyissue in the forearc prospectivity is paucity of reservoir. Asper the sedimentation model main source of sediments inforearc is Irrawaddy delta which mainly contributed finerclastics. However, clastic input in forearc has also beenprovided by Andaman Island itself (Andaman flysch).Hence it is likely to have reservoirs close to and all alongthe eastern Margin of the Andaman Islands. Therefore,exploration for Neogene reservoir and suitable entrapmentclose to the island may be thought of. Drilled result showthat the early post-rift (Early to Mid-Miocene) carbonatebuild-ups may not be interesting in the area as they werenever exposed to sub-aerial conditions for development ofsecondary porosity, contrary to the Yadana field ofMyanmar (Fig.3).

Fig.3 Carbonate build-ups drilled in Fore-Arc Basin without anyhydrocarbon show in tight lime stones of Miocene age. The bottomsection shows the analogous Multi TCF Yadan Field of Myanmarproducing from carbonate platform.

Source rock studies suggest that Eocene and oldersediments, in general, have marginal organic matterrichness. Neogene sedimentation in the forearc took placein somewhat restricted environment due to the rise ofAndaman Islands and volcanic arc by this time. As a result,Neogene sediments have better organic matter richness. Inone deepwater well reported TOC is 4.3%. But thesediments are immature due to low heat flow in the regionbecause of the presence of oceanic crust (Fig.4). Five wellshave been drilled in fore arc to probe different playsavailable in the area. Only in one well biogenic gas at threedifferent depth points within Pliocene was recorded (Fig.5).

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The identification of suitable reservoir facies remained themain emphasis of carrying out exploratory activities in forearc setup. But so far our exploration has been restrictedonly up to upper part of Neogene. Petroleum systemanalysis on the basis of 2D modeling also suggests thepresence of the thermogenic petroleum system in the forearc basin in the deepest part of the fore arc i.e in the pondedlows (Fig.6). Prospective structural and stratigraphic playswithin these ponded lows at a comparatively deeperstratigraphic level than the already drilled plays at shallowlevel are the potential targets for future exploration.

Fig.4 Geometry of Sunda plate around Andaman Sea area showingoceanic and continental crusts in the area. (Modified after Bird.P,2003).

Fig.5 Inline showing Gas bearing zones in a well of Fore Arc setup

Fig.6 Petroleum system analysis on the basis of 2D modellingsuggests presence of the thermogenic petroleum systems in thedeepest part of the ponded lows of fore arc.

Back Arc Prospectivity:

Back Arc of East Andaman Basin is time equivalent ofnearby petroliferous North Sumatra and Mergui Basins andexhibits similarity in basin evolution and tectono-stratigraphy. Although the available geoscientific data ofnorthern part of Andaman Sea does not show anypronounced rift related play, but drilled well in the northernpart of back arc can be compared with the adjacentShwepyithy gas field of Myanmar located in Andaman Sea.The objective of the drilled well was to probe the Miocenetargets characterized by strong seismic amplitudes alongwith clear inversion features. But due to very high bottomhole temperature (indicating high geothermal gradient)identified objects could not be tested. Hydrocarbon show(MLU gas data) was observed at places during drilling, andconsiderable clastic facies have been encountered (Fig.7).

Fig.7 Sections showing the drilled well of (A) Andaman and (B)gas well SHWE-1 of Myanmar having similar structural setup andplay.

In Myanmar offshore of Andaman Sea, the multi-TCF gasfields of Yadana in a limestone reservoir and Yetagun in adeltaic sand reservoir are two mature hydrocarbon

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provinces. These trends can be extrapolated into backarcAndaman region because of the similar geological setup. G& G analysis shows that thicker sedimentary sequence ispresent in the deeper parts of back-arc sub-basin whichmight have acted as source for effective hydrocarbongeneration analogous to nearby Myanmar basin (Fig.8).The seismic data reveals a thick Cenozoic rift packagewithin the Central Andaman Basin which will haveimplications on basin modeling and direct impact on thepetroleum systems of the deep water Myanmar and Indian-Andaman blocks (Fig-8B). Petroleum system analysisbased on 2D modelling also suggests presence of majorthermogenic, induced thermogenic system in back arcregion. Because, in island arc tectonic setup like AndamanSea, high geothermal gradient (Fig.9B) with high heat flowregime is prevalent in back arc region due to its proximityto spreading centre. This kind of set up is responsible forenhanced maturation (Fig.9A) and in turn hydrocarbongeneration. Exploration history of adjoining producingbasins in Myanmar and North Sumatra basin of Indonesiahas indicated the fact.

Fig.8 (A) Structural cross section from Yadana field to Yetagunfield of Myanmar. The huge low in between the two fields is thekitchen area for both the producing fields. (B)Thick sediment-filled (> 4 Km) graben shows the similar setup in Andaman backarc basin which is expected to be the source pod.

Fig.9 (A) Areas of matured source rock in and around Andamanarea (B) Geothermal gradient in and around Andaman area.

In the southern part of Andaman, recent G & G interpretedseismic data revealed the presence of a completelyunexplored rift set-up, similar to the nearby North Sumatraand Mergui rift basins situated on the eastern side ofMergui Ridge (Fig.10). Therefore, a probable petroleumsystem in rift-related play model (like Mergui & NorthSumatra) is expected in the East Andaman Basin, whereearly synrift sequences of lacustrine to marine shalesprovide the source.

Fig.10 Regional Section through East Andaman and Mergui Basinclearly depicts presence of Syn-rift play in East Andaman basingenetically related with Mergui basin.

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Due to greater depth of burial and proximity to the volcanicarc and spreading center the area will have better thermalregime favouring maturity compared to Mergui basin.Fluvial coarser clastics within early synrift phase and basinmargin deltas of late synrift phase provide the reservoirs.Transgressive shales of late synrift/early postrift phase areacting as regional seal (Fig.11).

Fig.11 Seismic section in Andaman back arc basin showingexpected hydrocarbon system elements.

Analogous Yetagun Field of Myanmar is producing fromMiocene deltaic sands, where Miocene to Recent sandshave been brought in by the Irrawaddy Delta. Similarly, theback-arc basin south of the spreading centre is in afavourable position to receive sediments shed from theMalay Peninsula, where granites are common, so thepresence of Neogene-aged sands can be postulated in thesouthern part of the back-arc basin.

Nature of crust has got an important role in the basal heatflow of any basin which in turn determines the geothermalgradient in a basin to a great extent. Analyzing all thetectonic aspects of Andaman basin it appears that a majorpart (Fore Arc & Volcanic Arc) of Andaman Basin hasoceanic crust at bottom. Many workers (Metcalfe.I, 2006,Longley, Ian, 1997, Bird.P, 2003) have shown this in theirtectono stratigraphic models (Fig.4). Continental crustcovers a portion of the southern part of Andaman backarcbasin (Fig.12), which is marked by higher geothermalgradient (Fig.9B).

Fig.12 Schematic diagram showing presence of continental crust inback arc of East Andaman basin in continuation with NorthSumatra/Mergui basin.

In addition, volcanic activities in Paleogene time suppliedadditional heat in the basin. In the southern part, Andamanback arc basin is bounded in the west by Sewell Rise and inthe east by Mergui ridge which separates it from Merguibasin further east (Fig.10). Mergui basin is the north wardcontinuation of the North Sumatra basin which is a typicalback arc basin. Mergui basin can be considered as a part ofthe Andaman back arc basin. The typical syn-riftarchitecture found in Mergui basin are well pronounced insouthern part of Andaman back arc (Fig.13, 14, 15).

Fig.13 Sections showing typical Rift play of Mergui Basin

Fig.14 Sections showing typical Rift play in back arc of EastAndaman Basin.

Mergui and North Sumatra Basins are known to holdsignificant hydrocarbon reserves in its rift related plays.This in fact adds confidence about the presence of aplausible working petroleum system in East AndamanBasin in a similar rift-related play model.

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Fig.15 North-South regional line of East Andaman basin (back arc)exhibiting typical Syn-rift play.

In the Mergui Basin (offshore Thailand) the mid-Mioceneinterval overlying the limestone has a low TOC (<1%) but,according to published sources, is prognosed to be thesource that has charged the successful oil and gas wells inthat basin. By analogy, the same source could be present inthe back-arc of the Andaman Sea Basin; where TOC is 1.5to 2.0 (Fig.16).

Fig.16 TOC values in and around East Andaman Basin.

Extensive work has been done on the techtono-stratigraphyand evolution of backarc basins of Indonesia includingNorth Sumatra basin (Doust and Nobel, 2008). TheAndaman back arc basin experienced more or less similarpolyphase tectonic history. Though no well has been drilledin the southern part of the Andaman back arc basin, on thebasis of the seismic data and analogy to North Sumatra andMergui basin, it can be said that these basins opened duringEocene to Early Oligocene and display syn-rift, early post-rift and late post-rift sedimentation. The syn-rift ischaracterized by fluvio-lacustrine/ fluvio-marinesedimentation with different clastic plays, early post-rift(Late Oligocene to Early Miocene) by shallow

marine/shelfal and the late post-rift phase (Late Miocene toRecent) is represented by deepwater sedimentation in theform of channel-fan system (Fig.17).

Fig.17 Geological cross section of East Andaman Back Arcshowing the main play types of Syn-rift and Post-rift phases.

2D Petroleum system analysis suggests hydrocarbongeneration probably started as early as latest Oligocene inAndaman back arc basin and hydrocarbon expulsion startedin Early Miocene and continued till recent (Fig.18)

Fig.18 (A) Petroleum system elements clearly depicts the probablesource and reservoir in Early Neogene/ Paleogene level in back arcsetup.(B) Petroleum system chart showing the Paleogene –Paleogene petroleum system in the back arc basin based onmodeling results.

Many indirect hydrocarbon indicators have been eitherreported as being present in Andaman Sea area orinterpreted from the reprocessed seismic data by differentauthors. Available satellite seep surveys indicate that anumber of hydrocarbon seeps are recognized andcatalogued over the Andaman area, some reputedlyappearing near or on some seismic lines. Otherhydrocarbon indications include sea-floor pock marks andvent mounds, gas clouds or chimneys, bright spots and flatspots (high amplitude reflector lengths and/or flatanomalous features) and bottom simulating reflectors

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(BSRs), generally accepted to be caused by the presence ofgas hydrates in the sedimentary section (Fig.19). All ofthese strongly favors the presence of hydrocarbon in eastAndaman Basin.

Fig.19 Sections of East Andaman basin showing strongBottom simulating reflectors (BSRs).

ConclusionEast Andaman Basin with its different morpho-tectonicunits is a part of the Andaman-Sumatra subduction megacomplex. Recently acquired seismic data in Andaman backarc areas provided new insight in understanding; depictingthe fact that the basin in every possible way is time-equivalent to the well explored, mature hydrocarbonprovinces of North Sumatra and Mergui Basins. 2D & 3Dseismic data interpretation convincingly point out a closeresemblance of tectono-stratigraphy amongst them.Presence of a completely unexplored rift set-up in thesouthern part of East Andaman basin, similar to geneticallyrelated North Sumatra and Mergui rift basin is emerging asa hotspot for future exploration. Presence of Continentalcrust similar to North Sumatra and Mergui basin increasesthe chance for entrapment of Potential amount ofhydrocarbons in strati-structural combinations within thisrift play of Andaman back arc and destined to become amajor focus for future exploration.

References

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Doust, H. & Sumner H.S., 2007, PetroleumSystems in Rift Basins – a Collective Approachin Southeast Asian Basins. PetroleumGeoscience, vol. 13.

Roy. T.K (1983): geology and Hydrocarbonprospects of Andaman – Nicobar Basin

Tectono-sedimentary model and hydrocarbonprospectivity of Andaman basin: KDMIPEReport 2007

Evaluation of petroleum system in Andaman inconjunction with North Sumatra-Myanmar areas:KDMIPE Report 2008

Metcalfe, I., 1984. Late Paleozoicpaleogeography of SE Asia: some stratigraphicpaleontological and paleomagnetic constraints,Fifth Regional Congress Geology MineralResources. SE Asia, Kuala Lumpur.

A relook into structural and depositional Modelsof the Andaman Basin by integrating 2D/3Dseismic data and recently drilled well data:KDMIPE Report 2014.

Unpublished Reports of ONGC

AcknowledgementThe authors express their sincere thanks to the managementof Oil and Natural Gas Corporation Limited for giving theopportunity to write this paper. The views of the paper areof the author’s only. Organization is not responsible for anyof the views.

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