(gulf of tonkin), vietnam cenozoic song hong basin ... · petromod software to obtain a more...

167Journal of Petroleum Geology, Vol. 28 (2), April 2005, pp 167-184

The northern offshore part of the Cenozoic Song Hong Basin in the Gulf of Tonkin (EastVietnam Sea) is at an early stage of exploration with only a few wells drilled. Oil to source rockcorrelation indicates that coals are responsible for the sub-commercial oil and gas accumulationsin sandstones in two of the four wells which have been drilled on faulted anticlines and flowerstructures. The wells are located in a narrow, structurally inverted zone with a thick predominantlydeltaic Miocene succession between the Song Chay and Vinh Ninh/Song Lo fault zones. Thesefaults are splays belonging to the offshore extension of the Red River Fault Zone.

Access to a database of 3,500 km of 2D seismic data has allowed a detailed and consistentbreak-down of the geological record of the northern part of the basin into chronostratigraphicevents which were used as inputs to model the hydrocarbon generation history. In addition, seismicfacies mapping, using the internal reflection characteristics of selected seismic sequences, hasbeen applied to predict the lateral distribution of source rock intervals. The results based on Yükler1D basin modelling are presented as profiles and maturity maps. The robustness of the results areanalysed by testing different heat flow scenarios and by transfer of the model concept to IESPetromod software to obtain a more acceptable temperature history reconstruction using theEasy%Ro algorithm.

Miocene coals in the wells located in the inverted zone between the fault splays are present inseparate intervals. Seismic facies analysis suggests that the upper interval is of limited arealextent. The lower interval, of more widespread occurrence, is presently in the oil and condensategenerating zones in deep synclines between inversion ridges. The Yükler modelling indicates, however,that the coaly source rock interval entered the main window prior to formation of traps as a resultof Late Miocene inversion.

Lacustrine mudstones, similar to the highly oil-prone Oligocene mudstones and coals which areexposed in the Dong Ho area at the northern margin of the Song Hong Basin and on Bach LongVi Island in Gulf of Tonkin, are interpreted to be preserved in a system of undrilled NW–SEPaleogene half-grabens NE of the Song Lo Fault Zone. This is based on the presence of intervalswith distinct, continuous, high reflection seismic amplitudes. Considerable overlap exists betweenthe shale-prone seismic facies and the modelled extent of the present-day oil and condensategenerating zones, suggesting that active source kitchens also exist in this part of the basin. Recentlyreported oil in a well located onshore (B10-STB-1X) at the margin of the basin, which is sourced

1 Geological Survey of Denmark and Greenland (GEUS),Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.2 Vietnam Petroleum Institute (VPI), Yen Hoa, Cau Giay,Hanoi, Vietnam.

*Author for correspondance: [email protected]

DISTRIBUTION OF SOURCE ROCKS ANDMATURITY MODELLING IN THE NORTHERNCENOZOIC SONG HONG BASIN(GULF OF TONKIN), VIETNAM

C. Andersen1, A. Mathiesen1, L. H. Nielsen1, P. V. Tiem2,H. I. Petersen1* and P. T. Dien2

mainly from “Dong Ho type” lacustrine mudstones supports the presence of an additional Paleogenesourced petroleum system.

168 Source rocks and maturity modelling, Cenozoic Song Hong Basin, Vietnam

INTRODUCTION

The Song Hong Basin (Bac Bo/Yinggehai/Red RiverBasin) is one of a number of Cenozoic basins locatedalong the western margin of the East Vietnam Sea(South China Sea) (Fig.1a). It was formed by left-lateral transtension along the Red River Fault Systemand is a deep, NW–SE elongated basin, whoseoffshore portion stretches more than 500 km acrossthe Gulf of Tonkin and the northern part of the centralVietnamese shelf (Fig.1a).The depth to the base ofthe Cenozoic fill in the main depocentre is well belowthe base of conventional seismic sections and mayexceed 15 km (Hao et al., 1995). The onshore part isknown as the Hanoi Trough (Fig.1b).

The exploration history of the Song Hong Basinand adjoining areas was reviewed by Nielsen et al.(1999), but new data from relevant oil and gasexploration wells has become available. In the HanoiTrough and in the NE Song Hong Basin north of theSong Lo Fault Zone (Fig.1b), Anzoil has drilled tenexploration and appraisal wells with TD in the depthrange 1,200–3,800 m. The B10-STB-1X well on themargin of the Hanoi Trough (Fig.1b) encountered oil,and the D14-STL-1X well on the Tien Hai C structureto the SW (Fig.1b) encountered sub-commercialvolumes of gas and condensate in Miocene sandstonesat a depth of c. 3,300 m. PetroVietnam has drilledthree exploration wells in the Hanoi Trough with TDranging from 1,180–2,000 m, and one well had a gasshow at a depth of about 1 km. So far, only five wellshave been drilled in the study area, which is locatedin the northernmost offshore part of the Song HongBasin north of the 19th parallel (Fig.1). Four wellswere drilled during the early 1990s to test the Miocene,predominantly deltaic section on faulted anticlinesformed by late Miocene inversion movementsconfined to a rather narrow zone between the SongChay and Vinh Ninh/Song Lo fault zones. Gas andminor quantities of condensate and oil were recoveredfrom stacked, thin Lower Miocene deltaic sandstonesby Total in the 103-TH-1X well. Later, Idemitsuencountered gas shows and recovered traces of oil onRFT from a Miocene deltaic succession with coal bedsin the 102-CQ-1X well, located to the north on theup-dip trend from 103-TH-1X (Fig.1b). The Paleogenesyn-rift section has only been penetrated by the Totalwell 107-PA-1X. This was spudded on the crest of adomal anticline caused by inversion of a NE–SWtrending syn-rift segment to the east of the Song LoFault (Fig.1b). The well was dry with onlyinsignificant traces of gas in spite of the presence of awell developed seismic anomaly interpreted as a directhydrocarbon indicator (DHI).

It is the aim of the present paper to describe thepetroleum systems in the northern part of the Song

Hong Basin and the Hanoi Trough. The paper is acontinuation of the work presented by Nielsen et al.(1999), but a much larger database has been availablefor the present study both with respect to seismic andsource rock data. The latter include new analyses ofOligocene lacustrine mudstones and coals exposed inthe Dong Ho area immediately to the north of the studyarea and on Bach Long Vi Island, the sub-aerial partof a large inversion structure in the Gulf of Tonkin(Dien et al., 1999; Dau et al., 2000; Petersen et al.,2001, 2004, 2005). The studied immature rocksconstitute analogues to possible terrestrial oil-pronesource rocks offshore. The seismic database comprisesabout 3,500 km of 2D data acquired by the formeroperators and by Geco-Prakla as a speculative survey.This allowed a more detailed and consistent break-down of the geological record into chronostratigraphicevents used as inputs to model the hydrocarbongeneration history. Furthermore, seismic faciesmapping using the internal reflection characteristicsof selected seismic sequences has been applied to mapthe lateral distribution of possible source rockintervals. The objective is to assess hydrocarbongeneration and to evaluate remaining explorationpotential. The results are presented as profiles andmaturity maps.

GEOLOGICAL SETTINGAND BASIN DEVELOPMENT

The structural framework of the study area has beendescribed in some detail by Rangin et al. (1995), andbasin development in a regional context has beensummarised by Nielsen et al. (1999). So, only a briefdescription of the geological setting is given below.

Cenozoic development of the Gulf of Tonkin canbe divided into a number of specific tectonic phasesseparated by basin-wide unconformities. Paleogeneextension led to the formation of widespread grabensand half-grabens followed by a Late Oligocene toEarly Miocene post-rift subsidence phase. During theMiddle–Late Miocene, the tectonic regime changed,and basement inversion took place leading to theformation of conspicuous compressional structuresover former depocentres delimited by the splays ofthe Red River Fault Zone. Basin inversion culminatedin late Late Miocene times with the formation of asignificant unconformity that deeply truncates theinversion structures. Renewed basin subsidence isindicated by southward–thickening, mostlyundisturbed cover of latest Miocene to Pliocene–Quaternary sediments.

NE–SW Paleogene extension occurred in a zonemore than 100 km wide in the Gulf of Tonkin, whichis related to left-lateral motion along Red River FaultZone (Rangin et al., 1995). As a result, a number of

169C. Andersen et al.

Fig.1. (a) Location map of basins offshore Vietnam. Boxed area corresponds to Fig.1b. (b) Structural outlineof the northern part of the Song Hong Basin with major fault trends. Location of wells and seismic profilesdiscussed in the text are shown. Boxed area corresponds to Figs. 3, 8 and 9. Dashed area corresponds toFig.10. TNG: Thuy Nguyen Graben.

NW–SE oriented grabens and half-grabens weredeveloped such as the Kien An Graben (Fig.2). Thedominant NW–SE trend is replaced by NE–SWoriented structures eastward of this graben segmentconnecting with the trend of Paleogene grabens in theGulf of Beibu in Chinese waters (Fig.1). Both sets ofgraben structures are filled with coarse-grained andconglomeratic clastics of Paleocene–Eocene ageoverlain by Eocene–Early Oligocene lacustrine andalluvial deposits. A number of internal unconformitiesare observed within the graben-fill successions (Fig.2)

and so is lateral migration of depocentres along strike.These features suggest a pull-apart origin for thegrabens in a transtensional tectonic regime withrapidly changing fault patterns controlling palaeo-topography and deposition.

The syn-rift prisms are generally separated fromthe overlying post-rift sediments by a pronouncedunconformity, marked CaU300 on Fig.2. By analogywith the neighbouring Beibu Wan Basin, Rangin etal. (1995) suggested an age for this unconformity ofabout 30 Ma. Sedimentation following this

102-HD-1X


unconformity was mainly concentrated in a rathernarrow zone between the Song Chay and Vinh Ninh/Song Lo fault zones (Fig.1b). The area between thesefault zones is dominated by east-west trending normalfaults (see Fig.10), which suggests the continuation ofthe left-lateral tectonic regime. Elsewhere, on adjacentfoot wall blocks and in the main depocentre of the SongHong Basin, fault-controlled subsidence and depositionplayed a minor role. The depositional environmentduring Late Oligocene–Early Miocene times variedfrom fluvial, estuarine, and deltaic to offshore marinewith deposition of sandstones and mudstones. The firstrecorded marine incursion is dated NP 25 (Chattian).

A distinct near base-Middle Miocene unconformity,which in places shows deep channel incision, andmarking a conspicuous lateral (eastward) shift indepocentres, can be mapped in the study area. It iscoeval with the cessation of sea-floor spreading in theEast Vietnam Sea and is associated with a change to acompressional strike-slip regime in the NE part of theGulf of Tonkin. This change in the stress field led tothe formation of a number of distinct NW–SE orientedcompressional domes and flower structures in the zonebetween the Song Chay and Vinh Ninh/Song Lo faultzones. Prograding deltaic units of sandstones,mudstones and coals were deposited in Middle–LateMiocene times in the Hanoi Trough and the northernpart of the study area.

The strike-slip activity causing reversal of faults,formation of inversion structures and several localunconformities culminated in Late Miocene times withthe formation of a significant and widespreadunconformity, which deeply truncates thecompressional structures. Renewed subsidence of theSong Hong Basin is indicated by a thick, draping and

virtually undisturbed latest Miocene to Pliocene–Quaternary section. This is dominated by shelf toshallow-marine mudstones, siltstones and sandstoneswith minor proportions of lagoonal and fluvialdeposits. In the easternmost part of the study area,uplift and basin inversion continued over former NE–SW oriented grabens into the Pliocene and possiblyeven Quaternary. These late movements areresponsible for the exposure of Oligocene strata onBach Long Vi Island (Fig.1b).

SOURCE ROCKS

Possible and proven source rock intervals in thenorthern part of the Song Hong Basin are reviewedbelow and their areal distribution is predicted basedon new seismic facies mapping.

1. Paleogene lacustrine shalesPalaeogene lacustrine mudstones, coals and coalymudstones constitute highly oil-prone source rocksin many places in SE Asia. Carbonaceous mudstonesdeposited in rift lake systems and to a lesser extentin floodplain lakes are the principal oil source rocksin a number of prospective Cenozoic basins alongthe northern and western margin of the East VietnamSea (Fig.1a). These include the Beibu Wan and PearlRiver Mouth Basins on the Chinese shelf (e.g. Wangand Sun, 1994; Zhu et al., 1999; Huang et al., 2003),and the Cuu Long Basin located on the Vietnameseshelf off the Mekong River delta (Todd et al., 1997;Lee et al., 2001). In the Vietnamese Nam Con SonBasin further to the SE, paralic carbonaceousmudstones and coastal plain coals are considered tobe the primary source rocks; however, contributions

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Fig.2. SW–NE oriented seismic section (90-1-20) crossing the Paleogene Kien An Graben (location in Fig.1b).Note the interval bounded by marker horizons Ca380 and Ca350 characterized by continuous high amplitudereflections between 1.1 and 1.5 sec. TWT, possibly representing Lower Oligocene lacustrine mudstone facies.


from lacustrine source rocks have been suggested byCanh et al. (1994) and Lee et al. (2001).

The presence of Paleogene oil-prone lacustrineshales and coals in offshore syn-rift prisms in thenorthern Song Hong Basin is predicted based onintervals with continuous, high amplitude reflectionsbetween 1.1 and 1.5 sec. bounded by seismic markerhorizons Ca350 and Ca380 in the graben structuredisplayed in Fig.2. These reflections may originatefrom lacustrine shaly units embedded in an otherwisecoarse-clastic dominated syn-rift succession byanalogy with other SE Asian basins, e.g. the PearlRiver Mouth Basin and the Gulf of Thailand (Leo,1997). This shale-prone facies in the interval boundedby seismic markers Ca350 and Ca380 is widespreadin the undrilled NW–SE oriented graben NE of theSong Lo Fault Zone in blocks 102 and 106 (Fig.3).The predicted offshore extension of rich Paleogenesource rock intervals is further substantiated by subseaand subaerial outcrops of thermally immature,organic-rich lacustrine shales with minor coals onBach Long Vi Island. These rocks are similar to the

Dong Ho Formation resting on Triassic sedimentsexposed in the Dong Ho area on the Vietnamesemainland (locations in Fig.1b). Bach Long Vi Islandis located on a NE–SW trending inversion ridge, andthe partly transparent seismic facies in this invertedgraben is most probably an artefact caused by strongstratigraphic dips and defocusing of seismic energy.

Lacustrine shales with a contribution from coalsare considered to be the source of oil found in wellB10-STB-1X, located onshore at the margin of the NESong Hong Basin. This confirms the existence of anactive lacustrine-coal sourced petroleum system NEof the Song Lo Fault Zone with a kitchen area probablylocated in the Thuy Nguyen Graben (Petersen et al.,2004). Likewise, well GK63 located in the HanoiTrough encountered oil, which was generated from alacustrine source with some contribution from higherland plant material (Petersen et al., 2004).

Lacustrine shales of source rock quality were notpenetrated in well 107-PA-1X, the only offshore wellin the NE Song Hong Basin, which penetrated thePaleogene syn-rift succession. Massive conglomerates

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Fig.3. Map of the area NE of the Song Lo Fault Zone, showing the dominant reflection characteristics ofseismic interval Ca350–Ca380 (model event 4). The location of the map is indicated in Fig.1b.


and sandstones in the basal part of the succession hereare overlain by alternating sandstones, siltstones, andclaystones deposited in a mixed alluvial-fluvial andlacustrine environment. The seismic reflection patternat the well location is characterised by low to moderateamplitudes (Fig. 3). The TOC content is low, mostlybelow 0.5 wt%, except for a few scattered high valuescaused by the occasional presence of thin coals. HIvalues are low, and S2 less than 2 kg HC/ton rock,classifying the drilled section as a poor source rock(S2 and TOC cut-off values from Bordenave et al. 1993).

The thermally immature mudstones exposed atDong Ho and on Bach Long Vi Island have TOCcontents of 4–20 wt%, and HI values are generallyabove 450 mg HC/g TOC, but HI may reach 700 mgHC/g TOC (Traynor and Sladen, 1997; Dien et al.,1999; Petersen et al., 2001, 2004, 2005). S2 yields aregenerally in the range of 20–70 kg HC/ton rock, butmay reach 116 kg HC/ton rock. Petrographic analysesshow that the organic matter is of terrestrial origin andis mainly composed of fluorescing amorphous organicmaterial, detrital liptinite, and alginite withBotrycoccus-morphology (dominantly Type Ikerogen). Coal samples are dominated by huminite(Type III kerogen) derived from the woody tissues ofhigher land plants, and they yield HI values of 200–360 mg HC/g TOC (Petersen et al., 2001, 2004, 2005).

Significant proportions of oleanenes confirm theirCenozoic age.

The coals have broad activation energy (Ea)distributions, while the mudstones have Eadistributions characterised by a pronounced principalvalue. Artificial maturation by progressive hydrouspyrolysis shows that the lacustrine mudstones willgenerate large quantities of paraffinic oil within arelatively narrow oil window, whereas the oil windowfor the coals is broader (Petersen et al., 2001, 2004;Petersen, 2002). For the mudstones from the DongHo Formation, the start of the oil window is estimatedto occur at a maturity corresponding to a vitrinitereflectance of 0.72–0.80%Ro, while the expulsionthreshold for the coals with an HI of about 200 is1.03–1.15%Ro. More oil-prone coals with HI valuesabove 300 may expel oil at a lower maturity (Petersenet al., 2005).

2. Miocene coaly source rocksOil-source rock correlations suggest that coals withhigh Hydrogen Indices (HI) in Miocene deltaicdeposits are responsible for the hydrocarbon showsrecorded in wells 103-TH-1X and 102-CQ-1X.Miocene coal and coaly shale source rocks are knownfrom various parts of SE Asia, e.g. offshore Bruneiand the Kutei Basin (Mahakam Delta), Indonesia

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Fig. 4. Pyrolysis parameters from well 102-CQ-1X (location in Fig.1b). Analytical results from Geochem GroupLimited (1994). Ties of key seismic markers, distribution of coal beds and shows are indicated on the depthcolumn to the left.


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Fig.5. Seismic well correlation and chronostratigraphic scheme used in maturity modelling (well locations inFig. 1b). Identified foraminifera and nannoplankton biozones are indicated along the GR log traces.

(Curiale et al., 2000; Peters et al., 2000). The UpperOligocene–Miocene mudstones in thick, generallydeltaic-paralic successions penetrated in the fouroffshore wells between the Song Chay and Song Lo faultzones (Fig.1b) are generally lean in TOC and have lowHI and S2 values, classifying them as low potential gas-prone source rocks. However, two separate intervals withcoals attract attention. The coal-bearing intervals areclearly recognised on seismic sections as a set of parallelvery high amplitude reflections.

The lower coal-bearing interval is found in all fourwells close to the assumed Oligocene–Mioceneboundary. The number and thickness of individual bedsdecrease from north to south, with few coals all lessthan 1 m thick in well 103-TH-1X. They exhibit highHI values (above 300 mg HC/g TOC) and high S2 yieldson pyrolysis. The same applies for the upper, thermallyimmature, coal-bearing interval only recorded between850–1,300 m in well 102-CQ-1X. Pyrolysis data fromthis well is displayed in Fig.4 together with indicationsof coals as evidenced from wireline logs and the ties ofseismic marker horizons. The upper coal-bearing intervalin well 102-CQ-1X is believed to have a rather restrictedareal extent. The high seismic amplitudes of the intervalgradually dim southward, consistent with the observationthat coals have not been recorded in the southern wells.

Oil, condensate and gas samples recovered from

DSTs in well 103-TH-1X all indicate a terrestrialhigher land plant source (e.g. suggested by a highPr/Ph ratio and the presence of oleanane) (Total,1990). The sterane distribution shows a slightpredominance of C27 suggesting a marine orlacustrine input to the kerogen (Ha, 1998). Showsof waxy, early mature oil recorded in well 102-CQ-1X correlate with coals drilled in the basal LowerMiocene section in well 102-HD-1X located some25 km further to the south (Fig.1b) (Geochem GroupLimited, 1994).

BASIN MODELLING

The hydrocarbon generation history from thepotential source kitchens in the study area, asdelineated from structural mapping and seismicfacies analysis and calibrated to well information,has been assessed. Yükler 1D basin modelling(Yükler et al., 1978) has been used to continue thework by Nielsen et al. (1999), and in order to beable to compare it with the new model conceptdescribed below. Subsequently, the model conceptwas moved to IES Petromod software to ensure amore acceptable temperature history reconstructionusing the Easy%Ro algorithm (Petersen, 2002 forType III and Petersen et al., 2004 for Type I). A


total of 31 simulations at well and pseudo-welllocations were performed to evaluate both the provenpetroleum system in the structurally inverted areabetween the Song Chay and Song Lo fault zonesinvolving Upper Oligocene–Miocene paralic sourcerocks, and the proven petroleum system further to theNE relying on lacustrine shales and coals depositedin Palaeogene syn-rift prisms. The present-daymaturity of Type I (lacustrine) and Type III (humiccoal) kerogens were modelled.

Model event definitionThe Cenozoic evolution of the northern Song HongBasin has been divided into 32 model events. Theevent splitting scheme comprises, in addition to amodel basement, 21 depositional model events and10 erosion or non-deposition model events. Lower andupper boundaries are defined solely by seismicmarkers. The age of each model event follows thechronostratigraphic scheme shown in Fig.5. Knownor inferred ages have been transferred into absoluteages (Ma) using the time-scale of Harland et al.(1989).

Well ties for the seismic markers used to delimitthe depositional model events are shown in Fig.5. Theseismic markers were named informally with a numberand the prefix “Ca”. It is noted that unconformitiesCaU300, CaU200, and CaU100 are identical to majortectono-stratigraphic boundaries described by Ranginet al. (1995). The same applies to seismic markerCa260, the presumed Oligocene–Miocene boundary.

Several mismatches occur between wellcorrelations based on interpretation of seismic markersand correlations based on existing sparsebiostratigraphic data. In order to overcome theseinconsistencies, the seismic well correlations weregiven first priority, although the seismic databaseavailable did not allow a unique interpretation in thestructurally complex inverted zone between the SongChay and Song Lo fault zones. In addition, datingsbased on planktonic foraminifera and nannofossilswere ranked above datings established onpalynological evidence in non-marine intervals, andanalyses based on core material and side-wall coreswere given priority over analyses carried out oncuttings samples. As an example, the ages establishedfrom palynology in the heavily-caved Miocene deltaicsections in the 102-wells tend to be younger than theseismically correlating intervals in the 103-wells.Here, datings were based on nannofossils recorded inside-wall cores. The foraminifera and nannofossilzones identified in the wells are shown in Fig.5.

The model event splitting scheme is illustrated inFig.6. The oldest syn-rift deposits in the model areassigned a Middle Eocene age (43.5 Ma),corresponding to a major plate reorganisation in the

SE Asian realm and the beginning of extension in theEast Vietnam Sea (Longley, 1997). Five depositionalmodel events (2 through 6) are defined in the mappedPaleogene syn-rift prisms east of the Song Lo FaultZone, and seven depositional model events (9 through14 and 16) are defined in the inverted Late Oligocene–Early Miocene depocentre between the Vinh Ninh andSong Chay fault zones. Most of these model eventsonlap the platform to the east or are thin below thelimits of seismic resolution. Finally, three model events(18–20) encompass the Middle and Late Miocene, anddeposition post-dating late Late Miocene inversion iscontained within six model events (25–32). Theproven and potential source rocks described aboveare located in model event 4, 11 and 13.

The event splitting scheme differs from the schemeapplied by Nielsen et al. (1999) (Fig.6). The previousscheme was primarily based on the establishedlithostratigraphic subdivision of the Cenozoicsuccessions in the onshore Hanoi Trough, which wasextended to cover the offshore areas. The ages of themajor unconformities, however, remain unchanged.

Modelling strategy and optimisationThe basic strategy for the modelling is based on theassumption that basal heat flow development has beensimilar throughout the basin with an increase at theonset of Paleogene rifting reaching 1.1 heat flow units(HFU) during model events 2 through 6. These valuesare much reduced compared to the values of Nielsenet al. (1999) (Fig.6). The lower heat flow scenario isbelieved to reflect more accurately the generallyaccepted pull-apart nature of the Paleogene grabens,which formed in an extensional strike-slipenvironment along the offshore extension of the RedRiver Fault Zone, rather than in a classical “McKenzie-type” extentional rift basin (Andersen et al., 1998).During the post-rift phases characterised by moreuniform subsidence patterns, the heat flow was keptconstant at 1.0 HFU. From the latest Miocene, it wasallowed to increase gradually to a present-day valueof 1.4 HFU in order to match the present-day highgeothermal gradient which has been recorded, andwhich locally exceeds 100 mW/m2. This value is basedon calculations using thermal conductivities measuredon core samples from the onshore Hanoi Trough andon temperatures measured in wells (Dao and Huyen,1995). The high heat flows may be related to Mioceneto Recent volcanic activity in the region and thepossible presence of a mantle plume below Indochina(Ru and Pigott, 1986).

The model has been optimised using control datafrom the five wells in the study area. This data includesavailable vitrinite reflectance values, sterane andhopane isomerisation ratios, and corrected bottom-hole temperatures from log runs and DSTs. The


Fig.6. Event splitting scheme used in this study defined from seismic markers. Erosional events are indicatedin grey. Potential source intervals are confined to model events 4, 11 and 13. The standard heat flow scenarioobtained through optimisation from four wells in blocks 102 and 103 is shown to the right (HF). Thealternative heat flow scenario used for simulations to the east of the Song Lo Fault Zone is annotated HFa.Where nothing is indicated, the heat flow value has been kept constant at 1 HFU. The event splitting schemeand heat flow scenario used by Nielsen et al. (1999) are shown to the left.

Present study

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Nielsen et al., 1999 Dura Time HF HFa [Ma] [Ma]

32 Pleistocene-2 0.5 0.5 1.4 1.4 31 Pleistocene-1 1.0 1.5 1.3 1.5 30 LPE1 0.5 2.0 1.3 1.5 29 LPE2 0.5 2.5 Top Pliocene 1.3 1.5 28 LPE-Ca50 1.0 3.5 1.2 1.5 27 Ca50-Ca60 0.5 4.0 1.2 1.5 26 Ca60-Ca90 0.5 4.5 Top Late Miocene 1.2 1.5 25 Ca90-CaU100 0.5 5.0 1.1 1.3 24 CaU100-4 0.5 5.5 23 CaU100-3 0.5 6.0 22 CaU100-2 1.0 7.0 21 CaU100-1 1.0 8.0

20 CaU100-CaU120 2.0 10.0 Top Mid Miocene

19 Ca120-Ca170 2.5 12.5

18 Ca170-CaU200 2.5 15.0 17 CaU200 1.0 16.0 Top Early Miocene 16 CaU200-CaU205 1.5 17.5 15 CaU205 0.5 18.0 14 CaU205-Ca207 1.0 19.0 13 Ca207-Ca210 3.0 22.0 12 Ca210-Ca230 1.0 23.0 11 Ca230-Ca260 0.5 23.5 Top Oligocene

10 Ca260-Ca280 1.5 25.0 9 Ca280-CaU300 5.0 30.0 8 CaU300-2 1.0 31.0 7 CaU300-1 1.0 32.0 Top ?Early Oligocene 1.05 1.05 6 CaU300-Ca340 1.0 33.0 1.1 1.1 5 Ca340-Ca350 2.0 35.0 1.1 1.1 1.1 1.1 4 Ca350-Ca380 3.0 38.0 1.1 1.1 3 Ca380-Ca400 3.0 41.0 1.1 1.1 2 Ca400-CaBsm 2.5 43.5 1.05 1.05

Dura Time HF [Ma] [Ma]

27 Kien Xuong Fm 0.5 0.5 1.4 26 Hai Duong Fm 1.0 1.5 1.2 25 LPE1 0.5 2.0 1.2 24 LPE2 0.5 2.5 1.2 23 Vinh Bao Fm. Up 1.0 3.5 1.2

22 Vinh Bao Fm. Lo 1.5 5.0 1.1

21 L.Mio. Erosion 1.5 6.5 20 L.Mio. Erosion 1.5 8.0 19 Tien Hung Fm. Up 1.0 9.0 18 Tien Hung Fm. Mi 1.0 10.0 17 Tien Hung Fm. Lo 1.5 11.5 16 Late Mid Mio. Ero. 0.5 12.0 15 Phu Cu Fm. Up 1.0 13.0 14 Phu Cu Fm. Mi 1.0 14.0 13 Phu Cu Fm. Lo 1.0 15.0 12 E.Mid Mio. Ero. 1.0 16.0

11 Phong Chau Fm 7.5 23.5 10 L.Oligo. Erosion 0.5 24.0 9 Thuy Anh/Ph C 6.0 30.0

8 E.Oligo. Erosion 2.0 32.0 1.05 7 Dinh Cao Fm. 2 2.5 34.5 1.05 6 Dinh Cao Fm. 1 2.5 37.0 1.1

5 L.Eocene Erosion 1.0 38.0 1.15 4 Phu Tien Fm. 2 1.5 39.5 1.2 3 Phu Tien Fm. 1 1.5 41.0 1.2 2 Pre-Early Rift Depo. 3.0 44.0 1.1 1 Model Basement 1.0 45.0

lithology was derived from sample descriptionstogether with the interpretation of wire-line logs.The stratigraphic column from TD to modelbasement for each well was extrapolated usingseismic information.

An optimisation plot from well 102-CQ-1X usingthe above heat flow scenario is given in Fig.7. Thereis a good fit between calculated and measured valuesof sterane- and hopane isomerisation ratios, vitrinitereflectance, and temperatures. Similar good fits areobtained in wells 102-HD-1X, 103-TG-1X, and 103-TH-1X. However, a less favourable match is obtainedin well 107-PA-1X. The fit is improved if the heat flowis allowed to increase to 1.5 HFU during model events26 through 31. This alternative high heat flow scenario(marked HFa on Fig.6) is therefore used to model thematurity levels in Palaeogene grabens mapped eastof the Song Lo Fault Zone. The present-day maturityobtained by the older TTI method and the newerEasy%Ro method is compared below, together with

the most critical parameters affecting maturity andhydrocarbon generation.

DISCUSSION OF RESULTS

The modelled present-day maturity of Type I and IIIkerogens in the three model events (4, 11, and 13)representing proven or possible source rock intervalsare displayed as a pseudo-2D profile along a SW–NEoriented depth-converted seismic profile (Fig. 8). Theprofile crosses the structurally complex, inverted zonewest of the Song Lo Fault Zone and two narrowPaleogene grabens to the east, and is based onmodelling results from 11 pseudo-wells.

Paleogene source rocks (model event 4)The proposed presence of source rocks in the Ca350–Ca380 interval is based, in addition to the seismiccharacter, on analogy with the immature Oligocenelacustrine mudstones and coals exposed onshore at


Dong Ho and on Bach Long Vi Island, and theoccurrence of similar Paleogene source rocks inseveral basins in SE Asia (e.g. the Adjuna and CentralSumatra Basins, Indonesia, and the western PearlRiver Mouth Basin, China) (Noble et al., 1991; Katzet al., 1993; Huang et al., 2003). Possible Paleogenesource rocks in the Ca350–Ca380 interval (modelevent 4) show a wide range of modelled maturity levelsreflecting variations in depth of burial and kerogentype (Fig.8). The modelled maturity at the base of theinterval is displayed on an iso–maturity map (Fig.9).The basal part of the interval is mostly within the oilwindow in the Kien An Graben in the east, the upperpart being immature. The interval is much thicker inthe more deeply buried Thuy Nguyen Graben, andconsequently the basal part is in the dry gas generatingzone. Most of the interval is post-mature in thesouthern part of the Thuy Nguyen Graben as a functionof burial depth. However, comparing the iso–maturitymap with the seismic facies map of the Ca350–Ca380interval (Fig.3), it can be seen that considerableoverlap exists between the inferred shale-proneseismic facies and the extent of present-day oil andcondensate generating zones. This suggests that anactive source kitchen exists in parts of block 106 and inthe adjacent NE part of block 102, which is supportedby the lacustrine-coal sourced oil encountered in wellB10-STB-1X (Fig.1b) (Petersen et al., 2004).

In addition to the selection of the maturitymodelling approach (see below), the most criticalparameter affecting the calculation of hydrocarbongeneration is the inferred heat flow history. Thesensitivity has been tested at a number of pseudo-welllocations in the Paleogene half-grabens by use of boththe “standard” and the “high” heat flow scenariosduring the Pliocene–Pleistocene (Fig.6), and by usingdifferent assumptions about uplift and erosion at theunconformity separating the syn-rift and post-riftsuccessions. The calculated maturity at the base ofthe Ca350–Ca380 interval is more sensitive to thehigh heat flow scenario than to additional uplift anderosion of 400 m at unconformity CaU300 using thestandard heat flow scenario.

The heat flow values during the Paleogene appliedin the present study are lower than the values in themodel applied by Nielsen et al. (1999) (Fig.6). Thelower heat flow values result in peak hydrocarbongeneration at a considerably later point in time. Thisdifference is important, as it allows a longer time forpost-rift structures to be formed and buried hills to besealed prior to hydrocarbon expulsion and migration,and therefore increases the play area for a petroleumsystem relying on Paleogene source rocks.

The hydrocarbon generation history in both theapplied Yükler model and the IES Petromod model isdetermined from a kinetic model based on Tissot and

Espitalié (1975). Basin modelling of possible sourcerocks in the Ca350–Ca380 interval assumes both aType I kerogen and a Type III kerogen (Figs.8 and 9).

Type I kerogen: lacustrine mudstonesOnset of early oil generation is normally based onType II kerogen and is defined to occur at a vitrinitereflectance of 0.5%Ro. This is a low maturitycompared to estimates of hydrocarbon generation fromlacustrine source rocks where the onset of oilgeneration generally occurs at >0.75%Ro (e.g. Baskinand Peters, 1992; Tegelaar and Noble, 1994). Basedon artificial maturation by hydrous pyrolysis oflacustrine mudstones dominated by Type I kerogenfrom Dong Ho, the start of the oil window for theserocks was determined to occur at a vitrinite reflectanceof between 0.72%Ro and 0.80%Ro (average~0.75%Ro) by Petersen et al. (2004). This value isthus in agreement with published thresholds for theonset of oil generation. The vitrinite reflectance of0.75%Ro corresponds roughly to the middle of themodelled early oil generating zone using the Yüklermodel (Fig.8a). The modelling of the Type I kerogenyields a broad, early oil generating zone, implying aninitial limited change in maturity during oil generation(Fig.8a). This is in agreement with the activationenergy distributions of Type I kerogens, which aretypically dominated by a large principal Ea value (e.gTissot et al., 1987; Tegelaar and Noble, 1994; Petersenet al., 2001, 2002). During maturation, these sourcerocks reach a point (corresponding to the principalactivation energy) at which they generate largequantities of hydrocarbons associated with a veryminor increase in maturity. The modelled maturity atthe base of the source rock interval shows that thelacustrine mudstones at present are oil-generating inlarge parts of the Kien An and Thuy Nguyen Grabens(Fig.9a).

Petersen et al. (2004) used 0.75%Ro as the start ofoil generation for the lacustrine mudstones. Byapplying the kinetic Easy%Ro model (Burnham andSweeney, 1989; Sweeney and Burnham, 1990), it wasshown that the lacustrine mudstones are mature in theThuy Nguyen Graben but only in the deepest part ofthe Kien An Graben. This contrasts with the resultsobtained using the Yükler model, in particular for theKien An Graben, because this maturity model yieldsa more rapid increase of vitrinite reflectance withdepth. Calculation of vitrinite reflectance in the Yüklermodelling programme is based on the TTI-%Rocorrelation (Waples, 1980), which has been shown toyield higher vitrinite reflectance values compared toother methods (Morrow and Issler, 1993; Throndsenet al., 1993; Yalcin et al., 1997). However, it is notablethat both models yield a mature Type I kerogen in theThuy Nguyen Graben. This is consistent with the


occurrence of oil in well B10-STB-1X, which issuggested to have been generated in a principallylacustrine kitchen area in the Thuy Nguyen Graben(Petersen et al., 2004) (Fig.1b). It thus seemsreasonable to assume that an active Paleogene sourcekitchen generating oil at present exists in parts of block106 and adjoining parts of block 102 (Fig.9a).

Type III kerogen: humic coalsModelling of a Type III kerogen (coal) yields a thin,early oil generating zone and a thicker main oilgenerating zone in the Kien An and Thuy NguyenGrabens (Fig.8b). In the Thuy Nguyen Graben, a thickzone of condensate generation occurs, whereas thedeepest part is in the dry gas zone. The coals havereached the condensate zone in the deepest part inthe Kien An Graben. The maturity map in Fig.9bshows that particularly in the Kien An Graben, thecoals will be in the early or main oil generating zonesat the present day. However, as noted above, the onsetof hydrocarbon generation is anticipated to occur at avitrinite refletance of 0.5%Ro; this may be optimisticfor coals. Petersen (2002) showed that the start of theoil window (“effective oil window”) for humic coals(Type III kerogen) with an HI of 200 mg HC/g TOCfrom Dong Ho occurs at ~1.03–1.15%Ro. By applyingthe kinetic Easy%Ro model (Burnham and Sweeney,1989; Sweeney and Burnham, 1990), it was shown

that these coals have not reached the oil expulsionthreshold in the modelled Thuy Nguyen and Kien AnGrabens. However, using the Yükler modelled vitrinitereflectance gradient, these coals would have reachedthe start of the oil window in the Thuy Nguyen Grabendespite a threshold for oil expulsion at ~1.03–1.15%Ro. Coals with much higher HI values (>300mg HC/g TOC) recently sampled from outcrops inthe Dong Ho area seem to expel oil at lower maturity(Petersen et al., 2005). Similar coals will probablybe oil-mature in the Thuy Nguyen Graben even whenapplying the Easy%Ro model. This would also beconsistent with indications of some contribution fromcoals/Type III kerogen to the principally lacustrine-sourced oil in well B10-STB-1X (Petersen et al., 2004).

Miocene source rocks (model events 11 and 13)Modelling results of Type I kerogen show thatlacustrine mudstones in the Miocene succession inthe Ca260–Ca230 interval (model event 11) arecurrently in the early oil generating zone, and theinterval is in the main oil and condensate generatingzones (Fig.8a) only in the central part of the invertedarea between the Song Chay and Song Lo fault zones.The interval is immature on the undeformed platformto the east of the fault zone. In the Miocene intervalbetween Ca205 and Ca207 (model event 13),lacustrine mudstones have reached the early oil

0.00 1.00 0.00 1.00 0.00 0.40 0.80 1.20 1.60 2.00 20 56 92 128 164 2000

1200

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th (m

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Optimisation plots, 102-CQ-1X well

Vitrinite Ref. [%Ro] Temperature [C]

Fig.7. Optimisation plot from well 102-CQ-1X using the standard heat flow scenario. Calculated and measuredvalues of sterane and hopane isomerisation ratios, vitrinite reflectances, and temperature are plotted versusdepth.


Fig.8. Modelled present-day hydrocarbon generation zones of Type I (A) and Type III (B) kerogens of modelevents 4, 11, and 13 along a depth-converted seismic profile (composite of seismic lines 93-29, 89-1-36, and89-1-36A). Locations of pseudo-wells are indicated ( see Fig.1b). The dashed lines in (A) show modelledvitrinite reflectance values obtained by using the Yükler model, whereas the Ro value in italics (0.75) wasobtained using the EASY%Ro model. 0.75%Ro indicates the start of the oil window for lacustrine mudstonesfrom the Dong Ho riverbed locality determined by artificial maturation (hydrous pyrolysis) (Petersen et al.,2004). KAG: Kien An Graben; TNG: Thuy Nguyen Graben.

10 km5 times verticalexaggeration

102-CQ-1X, 2km offset NSW NE

2000 1500 500 400 200 1500 1000 500PW98-8

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Fig.9. Yükler-modelled present-day hydrocarbon generation zones for Type I (A) and Type III (B) kerogens atseismic marker Ca380 (base model event 4) in the mapped syn-rift prisms in the NE Song Hong Basin. Thelocation of the map is indicated in Fig.1b.

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generating zone if the Yükler model is used (Fig.8a).The study by Petersen et al. (2004), using a vitrinitereflectance of 0.75%Ro as the start of the oil windowand the Easy%Ro model, showed that lacustrinemudstones will only be oil mature in the Ca260–Ca230 interval in the deep central syncline. Theonshore well GK63 NW of well 102-CQ-1X (Fig.1b),has an oil show displaying several geochemicalfeatures that suggests a lacustrine source withcontributions from Type III kerogen (Petersen et al.,2004). This confirms the presence of active lacustrinesource rocks to the south of the Song Lo Fault Zone.

The modelling results show that Miocene coals ininterval Ca260–Ca230 (model event 11) are currentlyin the oil and condensate generating zone in the centralpart of the inverted area between the Song Chay andSong Lo fault zones (Fig.8b). However, the basal partof the interval is in the dry gas generating zone in thedeep synclines between the inversion ridges. Theinterval is immature on the undeformed platform tothe east of the fault zone. In the study by Petersen(2002), coals with a start of the effective oil windowbetween 1.03 and 1.15%Ro (HI ~200) were modelledto be immature, even in the deep synclines. However,

Fig.10. Yükler-modelled timing of entry to main oil window of Type III kerogen (coals) for midpoint of modelevent 11. Major inversion structures in Blocks 102 and 103 are indicated. The location of the map is indicatedin Fig.1b (dashed area).

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of the present study suggests that that the upper coal-bearing interval is of limited areal extent, and maturitymodelling based on a detailed and consistent break-down of the geological record indicates that coals inthis interval are generally thermally immature. Thelower coal-bearing interval is found in all four wellsclose to the presumed Oligocene–Miocene boundary.However, the number and thickness of individual coalbeds decrease from north to south, with few coals (allof which are less than 1 m thick) being present in thewells drilled in block 103.

Maturity modelling suggests that oil-prone coalswith HI>300 mg HC/g TOC in this lower interval arecurrently in the oil and gas condensate window, atleast in the deep synclines where lacustrine mudstonesalso are oil mature. Yükler modelling suggests thatthis interval entered the main oil window in the centralpart of the zone prior to 5 Ma ago, i.e. before theformation of the Late Miocene inversion structures.The Yükler model is more “optimistic” than theEasy%Ro kinetic model with regard to the onset ofoil generation. The Easy%Ro model suggests later oilgeneration which, together with the higher maturitythresholds for oil expulsion derived from artificialmaturation experiments, is more favourable in relationto the time of trap formation. These results demonstratethat the maturity modelling approach has implicationsfor the evaluation of the area’s prospectivity.

Lacustrine mudstones, similar to the highly oil-prone Oligocene mudstones and coals exposed in theDong Ho area and on Bach Long Vi Island, areinterpreted to be preserved in a system of undrilledNE–SW oriented Paleogene half-grabens in theoffshore part of the basin NE of the Song Lo FaultZone based on distinct, continuous, high reflectionseismic amplitudes. Considerable overlap existsbetween the shale-prone seismic facies and themodelled extent of present-day oil and condensategenerating zones, suggesting that active Paleogenesource kitchens exist in the NE part of the Song HongBasin. Compared to earlier results by Nielsen et al.(1999), the modelled hydrocarbon generation tookplace at a considerably later point in time. Thisdifference is important, as it allows a longer time forpost-rift structures to have been formed and for buriedhills to have been sealed prior to hydrocarbonexpulsion and migration, and therefore increases theplay area. The oil recorded in well B10-STB-1X hasbeen sourced mainly from “Dong Ho type” lacustrinemudstones with some contributions from coals andlends support to the presence of an active lacustrine-coal sourced petroleum system.

The documentation from wells of at least twoactive petroleum systems and the delineation of likelyPalaeogene kitchen areas are encouraging for futureexploration in the area.

higher quality coals, such as those recorded in well102-CQ-1X with HI >300 mg HC/g TOC, may havereached the start of the oil window. This would be inaccordance with the typical terrestrial geochemicalsignatures of early mature waxy oil recovered in RFTsin well 102-CQ-1X and the condensate and gas testsrecovered in DSTs from well 103-TH-1X.

The Ca205–Ca207 interval comprising the uppercoal-bearing interval drilled in well 102-CQ-1X ismodelled to be thermally immature, apart from thesyncline located west of the 102-CQ-1X well location.Here, only the basal part has entered the main oilwindow (Fig.8b). This area overlaps the area wherethe interval is characterised by high reflectionamplitudes inferred to represent successions rich incoals. However, using a higher maturity threshold forthe start of the oil window suggests that these coalswill be immature with respect to hydrocarbongeneration and expulsion.

The timing of hydrocarbon generation versus trapgeneration in the structurally inverted zone may becritical. The timing of modelled entry to the main oilwindow of kerogen Type III (coals) at the midpointof model event 11 is illustrated in Fig.10, togetherwith the location of major inversion structures mappedat the Ca260 level. The midpoint of the lower coal-bearing interval entered the main oil window in thecentral part of the of the inverted zone prior to 5 Maago, i.e. prior to the formation of the Late Miocenecompressional structures and the unconformity(CaU100) which deeply truncates them. The IESPetromod model suggests later oil generation which,together with the higher maturity thresholds for oilexpulsion derived from artificial maturationexperiments, is more favourable in relation to trapformation.

CONCLUSIONS

The northern offshore part of the Cenozoic Song HongBasin is at an early stage of exploration with onlyfive wells so far drilled. A proven petroleum systemexists in a narrow, structurally inverted zonecharacterised by a thick, Miocene, predominantlydeltaic succession between the Song Chay and VinhNinh/Song Lo fault zones.

Oil to source rock correlation indicates thatMiocene coals are responsible for the sub-commercialoil-gas accumulations and shows in sandstones in twoof the four wells drilled on faulted anticlines andflower structures in this area. Coals are confined totwo separate intervals in the northernmost well, 102-CQ-1X. They exhibit high HI values and S2 yields onpyrolysis, classifying them as a potential rich sourcerock for both oil and gas if present in sufficientquantities. Seismic facies analysis carried out as part


ACKNOWLEDGEMENTS

The results presented herein were obtained from abasin modelling study of the Cenozoic Song HongBasin carried out as a joint project between theGeological Survey of Denmark and Greenland(GEUS) and the Vietnam Petroleum Institute (VPI, asubsidiary of PetroVietnam). The Danish contributionwas funded through a grant from the Danish EnergyResearch Programme (1313/97-0034) and theDANIDA sponsored ENRECA-project “Integratedanalysis and modelling of geological basins inVietnam and an assessment of their hydrocarbonpotential”.

The authors acknowledge valuable commentsduring journal review by B. Horsfield (GFZ, Potsdam)and an anonymous referee.

The paper is published with the permission ofPetroVietnam, Vietnam Petroleum Institute, andGEUS.

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