song hong basin, northern vietnam (oligocene dong ho ... · potential source rocks in the ne song...

19Journal of Petroleum Geology, Vol. 28 (1), January 2005, pp 19 - 38

SOURCE ROCK PROPERTIES OF LACUSTRINEMUDSTONES AND COALS(OLIGOCENE DONG HO FORMATION), ONSHORESONG HONG BASIN, NORTHERN VIETNAM

H. I. Petersen1*, Vu Tru2, L. H. Nielsen1, Nguyen A. Duc2 and H. P. Nytoft1

Oligocene lacustrine mudstones and coals of the Dong Ho Formation outcropping aroundDong Ho, at the northern margin of the mainly offshore Cenozoic Song Hong Basin (northernVietnam), include highly oil-prone potential source rocks. Mudstone and coal samples were collectedand analysed for their content of total organic carbon and total sulphur, and source rock screeningdata were obtained by Rock-Eval pyrolysis. The organic matter composition in a number of sampleswas analysed by reflected light microscopy. In addition, two coal samples were subjected to progressivehydrous pyrolysis in order to study their oil generation characteristics, including the compositionalevolution in the extracts from the pyrolysed samples. The organic material in the mudstones ismainly composed of fluorescing amorphous organic matter, liptodetrinite and alginite withBotryococcus-morphology (corresponding to Type I kerogen). The mudstones contain up to 19.6wt.% TOC and Hydrogen Index values range from 436–572 mg HC/g TOC. From a pyrolysis S2versus TOC plot it is estimated that about 55% of the mudstones’ TOC can be pyrolised intohydrocarbons; the plot also suggests that a minimum content of only 0.5 wt.% TOC is required tosaturate the source rock to the expulsion threshold.

Humic coals and coaly mudstones have Hydrogen Index values of 318–409 mg HC/g TOC.They are dominated by huminite (Type III kerogen) and generally contain a significant proportionof terrestrial-derived liptodetrinite. Upon artificial maturation by hydrous pyrolysis, the coals generatesignificant quantities of saturated hydrocarbons, which are probably expelled at or before a maturitycorresponding to a vitrinite reflectance of 0.97%Ro. This is earlier than previously indicated fromDong Ho Formation coals with a lower source potential.

The composition of a newly discovered oil (well B10-STB-1x ) at the NE margin of the SongHong Basin is consistent with contributions from both source rocks, and is encouraging for theprospectivity of offshore half-grabens in the Song Hong Basin.

1Geological Survey of Denmark and Greenland (GEUS),10 Øster Voldgade, DK-1350K Copenhagen, Denmark.

2Vietnam Petroleum Institute, Yen Hoa, Cau Giay, Hanoi,Vietnam.

*author for correspondence, email: [email protected]

INTRODUCTION

The northern offshore part of the Cenozoic Song HongBasin in northern Vietnam has been penetrated by onlya few exploration wells (Fig. 1). This appliesparticularly to that part of the basin north of the SongLo Fault Zone, where only two wells have been drilled.The region’s exploration history has been reviewed

by Nielsen et al. (1999) and Andersen et al. (in press).However, two petroleum systems have been provedto exist in the northern Song Hong Basin sourcedrespectively by Miocene coals and Paleogenelacustrine facies (mudstones and coals).

A Miocene coal-sourced petroleum system hasbeen identified in a narrow, structurally inverted zonebetween the Song Chay and Vinh Ninh/Song Lo faultzones, where a thick, predominantly deltaic Miocenesuccession has been tested in faulted anticlines formedby late Miocene inversion (Andersen et al., in press).The 103-TH-1x and 102-CQ-1x wells encounteredsub-commercial volumes of gas, condensate and oilin these Miocene deltaic sandstones (Fig. 1). Thehydrocarbons in well 103-TH-1x have a typical

20 Source rock properties, Oligocene Dong Ho Formation, Song Hong Basin (northern Vietnam)

terrestrial geochemical signature, including thepresence of oleanane (Total, 1990), and the waxy, earlymature oil in well 102-CQ-1x has been correlated tocoals in the basal Lower Miocene section in well 102-HD-1x located about 25 km further to the south(Geochem Group Limited, 1994).

The most important oil-generating source rocksin SE Asia are lacustrine mudstones (Katz, 1990;Sladen, 1997; Todd et al., 1997). Recently, forexample, lacustrine mudstones were reported to bethe source for two groups of oils in the western partof the Chinese Pearl River Mouth Basin offshoresouthern China (Fig. 1), and a third oil group wasrelated to a coaly source (Huang et al., 2003b). Gasaccumulations in the Chinese Qiongdongnan Basinhave been correlated to coal-bearing strata (Huang etal., 2003a). A lacustrine/coal sourced petroleumsystem has also been demonstrated to exist in the SongHong Basin. An oil show in well GK63 at the marginof the Hanoi Trough, the landward extension of theSong Hong Basin, has organic geochemical featurestypical of a predominantly lacustrine-sourced oil(Petersen et al., 2004a). The lacustrine/coal sourcedpetroleum system has recently also been proved toexist north of the Song Lo Fault Zone in the NE SongHong Basin, as the sub-commercial oil encounteredby well B10-STB-1x in Devonian carbonate rocks wasprincipally generated from lacustrine facies, but withsome contribution from a coaly source (Petersen etal., 2004a).

In spite of the B10-STB-1x oil discovery, sourcerock facies have not been encountered in the wellsdrilled in the NE Song Hong Basin. Seismic sectionsshow, however, that the Palaeogene Thuy Nguyen andKien An Grabens (locations on Fig. 1) contain syn-rift successions with distinct, continuous high-amplitude reflectors, a facies pattern commonlyinterpreted as lacustrine shale-prone units (Rangin etal., 1995; Andersen et al., 1998, in press). Mappingof this seismic facies in the NE Song Hong Basinindicates the regional occurrence of lacustrinemudstones in the syn-rift successions of the undrilledPalaeogene half-grabens, which suggests that thelacustrine/coal sourced petroleum system may bewidespread in the NE Song Hong Basin.

Previous evaluations of the quality of offshoresource rocks and their hydrocarbon generationproperties has relied on studies of potential sourcerock analogues exposed at a few localities in the DongHo area and on Bach Long Vi Island, a Palaeogeneinversion structure in the Gulf of Tonkin (locationson Fig. 1) (Petersen et al., 2001, 2004a). In the DongHo area, immature Oligocene lacustrine mudstonesand humic coals have for several years been knownto be exposed on a river bed location (loc. 1, Fig. 2).Geochemical data from artificially matured (by

hydrous pyrolysis) highly oil-prone mudstones fromDong Ho show considerable similarity to the B10-STB-1x oil (Petersen et al., 2004a). It has beensuggested that the oil was primarily sourced frommature Dong Ho-like mudstones in the Thuy NguyenGraben and/or Kien An Graben located to the SE ofthe B10-STB-1x well (Fig. 1) (Petersen et al., 2004a).Samples from Bach Long Vi Island may represent aslightly more saline lacustrine facies than the DongHo mudstones, and similar facies may have provideda minor contribution to the B10-STB-1x oil.

Coals from Dong Ho loc. 1 with Hydrogen Indexvalues of 200–242 mg HC/g TOC have been shownnot to expel oil before a maturity corresponding to avitrinite reflectance of approximately 1.03–1.15%Rohas been reached (Petersen, 2002). This means thatthese coals will to a large extent be immature in, forexample, the Kien An Graben (see discussion inAndersen et al., in press). New data presented in thepresent paper, however, document the presence ofconsiderably more oil-prone humic coals in otheroutcrops of the Dong Ho Formation (Fig. 3), whichmay be capable of expelling liquid hydrocarbons atlower maturity. Despite this, the highly oil-pronelacustrine mudstones constitute the most promisingpotential source rocks in the NE Song Hong Basin.

To date, information concerning the generationcapability and characteristics of potential lacustrineand coal source rocks in the offshore Song Hong Basinhas been restricted to the study of a limited numberof samples from the Dong Ho river bed locality (loc.1) and from Bach Long Vi Island (Fig. 1) (Traynorand Sladen, 1997; Petersen et al., 2001, 2004a).However, a recent study of the distribution of theOligocene Dong Ho Formation, based on numerousshallow wells and outcrops (Tru et al., 2002), hasprovided a series of new representative samples (Fig.2). In the present study, we report on a larger numberof potential source rock samples than in previousstudies. The results provide new insights into thecompositional variability and generation potential ofpotential mature source rock equivalents in theoffshore Song Hong Basin.

GEOLOGICAL SETTING ANDSTRATIGRAPHY

The Song Hong Basin forms part of the left-lateraltranstensional complex developed along the Red RiverFault (Rangin et al., 1995; Nielsen et al., 1999;Andersen et al., in press), and is one of a number ofbasins located along the western margin of the EastVietnam Sea (South China Sea) (Fig. 1). During theCenozoic, reactivation of pre-existing faults andPalaeogene extension resulted in the formation of aseries of grabens and half-grabens in the Song Hong

21H. I. Petersen et al.

Fig. 1. (a) Location map of sedimentary basins offshore Vietnam. The Beibuwan, Qiongdongnan and PearlRiver Mouth basins occur within Chinese teritory. The outlined area corresponds to Fig. 1b.

(b) The structural framework of the northern part of the Song Hong Basin and the Hanoi Trough. Note thatonly two wells have been drilled in the NE Song Hong Basin north of the Song Lo Fault Zone. The outlinedarea corresponds to Fig. 2 (Dong Ho area). TNG: Thuy Nguyen Graben.

Kien An Graben

TNG

Beibuwan Basin

HainanIsland

Bach LongVi Island

Dong Ho,Oligocene outcropsHanoi

Hanoi Trough

Song Chay Fault

Main depocentre

Song Lo Fault Zone

Vinh Ninh Fault Zone

102-CQ-1X

GK63

103-TH-1X107-PA-1X

20°N

50 km

Post-rift sediments restingdirectly on pre-CenozoicbasementDry wellOil/Gas shows

Gas discovery

B10-STB-1X

Hanoi

21°N

108°E107°E

102-HD-1X

103-TG-1X

Tien Hai gas fieldD14-STL-1X

200 km110°E

10°N

15°N

20°N

CHINA

LAOS

GULF OF TONKIN

THAILAND

CAMBODIA

VIETNAM

BORNEO

HAINAN

SOUTH CHINASEA

BEIBUWANBASIN

SONG HONG BASIN

RED RIVER FAULT

DA NANG SHEAR ZONE

PHU KHANH BASIN

PATTANIBASIN

TUY HOA SHEAR ZONE

NAM CON SON BASINCUU LONG BASIN

PEAL RIVER MOUTH BASIN

QIONGDONGNANBASIN

NAM CON SON BASIN

MALAY BASIN

105°E 115°E

(b)

(a)


Basin and the Hanoi Trough (Fig. 1), within whichsiliciclastic sediments accumulated including syn-riftlacustrine deposits and thin humic coal seams,presumably of Oligocene age. During the LateOligocene to Early Miocene post-rift phase, deltaic andparalic coal-bearing strata were deposited in the HanoiTrough and in the northern part of the Song HongBasin. In general, the syn-rift sediments are separatedfrom the post-rift deposits by an unconformity, datedat 30 Ma, which is probably related to the onset ofsea-floor spreading in the East Vietnam Sea and theformation of oceanic crust (Rangin et al., 1995; Nielsenet al., 1999).

Palaeogene extension led to the general NW-SEorientation of grabens and half-grabens in the northernSong Hong Basin (Fig. 1), whereas tectonic elementsin the neighbouring Beibuwan Basin are mainlyoriented NE-SW (Rangin et al., 1995; Packham, 1996).At the northern margin of the Song Hong Basin, a seriesof fault-related narrow and more or less east-westoriented depressions were formed. These troughs arefilled with Oligocene lacustrine sediments, referred toas the Dong Ho Formation, which are conformablyoverlain by Neogene fluvial and floodplain depositsof the Tieu Giao Formation (Fig. 3). In the study area,sediments of the Dong Ho Formation are present inthe Hoanh Bo Trough, which covers an area of about150 km2, and which can be divided into the minor DongHo and Cua Luc troughs (Fig. 2) (Tru et al., 2002).The smaller Dong Ho Trough to the NW consists of ahalf-graben dipping to the north. The larger Cua LucTrough is bounded by faults to the north, south andeast, and the fill dips 12–15o. Several shallow wells

drilled for water supply and exploration for coal andoil shales in the two troughs (locations on Fig. 2)have shown that the Dong Ho Formation consists ofpebble beds, sandstones, mudstones, coal beds andoil shales, while the Tieu Giao Formation consists ofclaystones, siltstones and sandstones in the lower partand sandstones and pebble beds in the upper part (Fig.4) (Tru et al., 2002). Cross-sections based onoutcrops, well data and geophysical data show thatthe Dong Ho Formation has a thickness of about 150m in the eastern part of the Hoanh Bo Trough, andincreases to a thickness of 300–350 m in the centralpart (Figs. 3, 5). The overlying Tieu Giao Formationis 20 m thick at the margins, and increases to athickness of 250 m in the central parts. The thicknessdistribution of the two formations reflects the reliefof the underlying Triassic rocks, which form the basinfloor.

After deposition of the two formations, the studyarea was subjected to regional compression withreactivation of pre-existing faults resulting in foldingand inversion. In most of the northern Song HongBasin nearby to the south and east of the study area,the compression culminated in latest Miocene timeand resulted in deep truncation of the inversionstructures (Rangin et al., 1995; Nielsen et al., 1999;Andersen et al., in press); the uplift and folding ofthe Hoanh Bo Trough was probably related to thisevent. Outcrops of the Dong Ho Formation occuralong the southern margin of the Dong Ho Troughand the western and southern margin of the Cua LucTrough. Following the compressional phase,subsidence resumed and a virtually undisturbed and

Fig. 2. Detailed map of the Dong Ho area showing areas where the Dong Ho Formation is exposed at thesurface. The “classic” Dong Ho river bed locality (loc. 1) and the three new outcrops (loc. 2–4) are indicatedon the map together with drilled wells, and the profiles of the cross-sections AA’, BB’, and CC’ shown in Fig. 5.(Modified from Tru et al. , 2002).

Lk.K5

Lk.K5

Lk.K9

Dong Ho Trough

Lk162Lk162b

Lk.K10

Lk158Lk.K11

Lk.K5

Lk610Lk208

Lk.K7

Lk.K8

Lk4

Lk6

Lk.K2

Lk301Lk304

Lk.14Lk.K4

Lk.K3

Lk29

Lk.K12

A

A`A

B

C`

C

B`

A`

Loc. 2: New irrigation trench

Loc. 1: Classic river bed locality

Vinh Cua Luc

Song Dien

Vong

C u a L u c T r o u g h

Loc. 3: Hào-2 Khu Dong Dang

Dong Ho Fm. at surface

Well

Rivers and streams

Quaternary &Mesozoic rocks

Sea

Cross section

Oil shale; Dong Ho Fm.

Faults

1 km

Loc. 4: Huu Nghi Bridge


Fig. 3. Generalised stratigraphic column for the Dong Ho area (modified from Tru et al. , 2002).

Per

iod

Form

atio

n

Div

isio

n

Lith

olog

y

Description

Thic

knes

s (m

)

5-12

500

-100

050

0-1

000

500

-100

050

0-1

000

500

-100

050

-350

20-2

50

Quaternary

Mio

cene

-Plio

cene

Tiêu

Gia

o

Olig

ocen

e

Don

g H

oH

òn G

ai

Car

boni

fero

us–P

erm

ian

Qua

ng H

anh

Tria

ssic

Car

boni

fero

us

Dà

Tràn

g

Low

erM

iddl

eU

pper

Pebble beds,sandstones and siltstones,brown, grey to dark grey in colour.

Pebble beds, conglomerates, coarse- tomedium-grained sandstones; grey,light grey and dark grey siltstones andsandstones with interbedded thin layersof coal and carbonaceous mudstones;oil shale; asphalt-bearing sandstones;plant fossils.

Pebble beds, breccia, sandstones, siltstones, and mudstones with inter-bedded thin coal lenses; plant fossils.

Pebble beds, sandstones, siltstones,mudstones and 10–25 commercial coalseams; plant fossils.

Pebble beds, breccia, sandstones, silt-stones, mudstones with interbedded thin coal lenses; plant fossils.

Dark grey platy dolomite, light grey andblocky in upper part; foraminifera.

Dark grey to light grey dolomite withinterbedding of platy silicate and thinlayers of mudstone; foraminifera.


thick uppermost Miocene–Pliocene and Quaternarysediment package was deposited in the Song HongBasin, where the Tieu Giao Formation isunconformably overlain by 5–12 m of Quaternarysediments.

SAMPLE MATERIAL AND METHODS

Fifteen outcrop samples of the Oligocene Dong HoFormation were collected from three localities in theDong Ho and Cua Luc troughs (locs. 2, 3, 4) (Fig. 2;Table 1). Nine of the samples were collected from anew trench excavated for irrigation (loc. 2), exposingabout 1.5–2.0 m of dark grey to brownish and blackmudstones (Fig. 2). After removing as much surfacematerial as possible to minimize the effects of surfaceweathering, a 1.60m thick profile was sampled here(Fig. 6a). Locality 2 is located approximately 320 m

west of locality 1, where mudstones and humic coalsof the Dong Ho Formation are exposed in the riverbed (Traynor and Sladen, 1997) (Fig. 6b).

The second new locality (Hào 2; loc. 3) is locatedat Khu Dong Dang, Hoanh Bo (Fig. 2), where anapproximately 3 m deep trench exposes part of theDong Ho Formation. From the base, the sectionconsists of about 1.3 m of grey mudstones and sandymudstones, overlain by about 0.30 m of white-greymudstones, and then 0.45–0.60 m of brownish–blackmudstones (oil shales) and humic coals, from whichfour highly organic-rich mudstone and humic coalsamples were collected (Table 1). The section iscapped by less than 1 m of Quaternary sediments.Two samples, an organic-rich mudstone and a humiccoal, were collected at Huu Nghi Bridge, Viet Hung(loc. 4), approximately 200 m south of loc. 3 (Fig.2). The samples were taken from the bed of a small

Fig. 4. Stratigraphic section drilledby the LK14 well (location in Fig. 2;see also cross-section CC’ in Fig. 5).About 21 m of the Dong HoFormation was penetrated.Soil

Sandstone

Grey mudstone

Mainly sandstone

Mudstone

Mainly sandstone

Mainly sandstone

Light mudstone

Mainly sandstone

Sandy mudstone

Sandstone

Description

Age

/Fm

.

Dep

th b

elow

surfa

ce (m

)

Thic

knes

s (m

)

Q 0.60 0.60

3.80

1.40

4.40

5.80

8.502.70

2.10

4.10

4.10

10.60

14.70

15.90 1.20

20.00

2.1022.10

17.40

39.50

42.20 2.70

11.70

53.90

4.90

58.80

3.00

61.80

63.00 1.20

6.20

1.80

4.50

Lith

olog

y

75.50

69.20

71.00

Neo

gene

Tiê

u G

iao

Fm.

Olig

ocen

e D

ong

Ho

Fm.

LK14 well

Light grey and yellowish grey mudstones

Clay- and sandstone

Clay and sandstone

Light mudstone

Dark grey mudstone, highly organic-rich (oilshale)Sandstone with gravels

1m of mudstone; sandstone;0.5m at base: gravel and breccia


Fig.

5. C

ross

-sec

tion

s (s

ee F

ig. 2

) sh

owin

g th

e pr

edic

ted

geom

etry

and

thi

ckne

ss o

f the

Don

g H

o Fo

rmat

ion

in t

he D

ong

Ho

area

. Obs

erva

tion

s at

the

out

crop

loca

tion

s sh

ow t

hat

the

Don

g H

o Fo

rmat

ion

is t

herm

ally

imm

atur

e in

ter

ms

of h

ydro

carb

on g

ener

atio

n in

thi

s ar

ea (

mod

ified

from

Tru

et

al. ,

200

2).

-100

-200

-1000

-200

0

A 00

Cua

Luc

Tro

ugh

Cua

Luc

Tro

ugh

Cua

Luc

Tro

ugh

Don

g H

o Tr

ough

WE

WE

SN

A´

020

0040

0060

0080

0010

000

(m)

C`

C

-1000

-200

-300(m

)

-100

0 -200

-300

(m)

020

0040

0060

0080

0010

000

1200

0

1400

016

000

1800

020

000

(m)

Lk K

10

-300

-300

-300

-200

-100

-300

-200

-100

Lk K

7LK

304

Lk.K

4Lk

.K3

LK.2

08LK

610

Lk30

1

020

0040

0060

0080

0010

000

1200

014

000

1600

0 (m

)

BB

`

Lk14

Lk30

1Lk

K11

Sea

/rive

rQ

uate

rnar

yTi

êu G

iao

Fm.

Don

g H

o Fm

.Tr

iass

icO

il sh

ale

Faul

tW

ell

Lk K

5(D

H)

Lk K

11Lk

K12

(m)

(m)

(m)

(m)


river where a coal seam and black carbonaceousmudstones are exposed; the mudstone sample wascollected about 50 m downstream from the coal seam.In addition to new data from these localities,previously reported data from loc. 1 (Petersen et al.,2001, 2004a) are included in the graphs presented inthis paper and in the discussion which follows.

The samples were analysed for total organic carbon(TOC) and for total sulphur (TS) by combustion in aLECO CS-200 induction furnace, and additionallypyrolysed on a Vinci Rock-Eval 5 or Rock-Eval 6instrument (Table 1). Carbonate-bonded carbon wasremoved by HCl treatment before combustion.

Based on the screening data, 12 samples wereselected for microscopic analyses (Table 2). Thesamples were crushed to the grain size range 63 mm– 1 mm and embedded in epoxy. Particulate blockswere polished, and studied microscopically inreflected light and oil immersion. The organic matterwas characterised in white and fluorescence-inducingblue light using a Zeiss incident light microscope anda Swift point counter. A total of 500 macerals andminerals were counted in each sample. Huminitereflectance measurements (%Ro, random) were performedon the maceral eu-ulminite by means of a Leitz MPV-

SP system (Table 2). None of the samples showedsigns of weathering, such as reduced fluorescenceintensity.

Two coal samples (8246, loc. 3; 7573, loc. 4) withhigh HI values (Table 1) were solvent-extracted bymeans of CH2Cl2/CH3OH (93 vol/7 vol), and theasphaltenes were precipitated by addition of 40-foldn-pentane. The remaining fraction was separated intosaturated and aromatic hydrocarbons and NSOcompounds (Table 3). The saturated hydrocarbonswere analysed by gas chromatography (GC) on aHewlett Packard 5890 gas chromatograph equippedwith a 25 m HP-1 WCOT column and a flameionisation detector (FID). The two coal samples (8246and 7573) were artificially matured by stepwisehydrous pyrolysis at five temperatures for 72hr at eachtemperature (240°C/72hr, 270°C/72hr, 300°C/72hr,330°C/72hr, 345°C/72hr). Hydrous pyrolysis wascarried out in stainless steel HPLC columns filled withabout 0.3 g of ground sample mixed with water(thoroughly stirred). The cooled samples were washedwith water, and the samples were recovered on 0.45mm polycarbonate filters and dried at roomtemperature. The hydrous-pyrolysed samples wereanalysed for TOC content and by Rock-Eval pyrolysis

Fig. 6. (a) Dark grey to brownish and black highly oil-prone lacustrine mudstones of the Oligocene Dong HoFormation exposed in a new irrigation trench (loc. 2). About 1.60 m was sampled. Lens cap (at arrow) forscale.(b) The “classic” Dong Ho Formation locality (loc. 1) where lacustrine mudstones and coals are exposed in ariver bed .

a b


(Table 3). Equivalent reflectance values (%Reqv) ofthe measured Tmax values were derived by thecorrelation Tmax=51.96%R+398.39 of Petersen (2002).The samples hydrous-pyrolysed at 240°C/72hr,330°C/72hr and 345°C/72hr were solvent-extractedand the saturated fractions were analysed by GC asdescribed above (Table 3).

RESULTSOrganic geochemistry

The source rock screening data of the samples arelisted in Table 1. The nine carbonaceous mudstonesamples from loc. 2 have considerable TOC contents,whereas the TS contents are low. Rock-Eval pyrolysisS1 and S2 yields and Hydrogen Index (HI) values arehigh. The two humic coals (8243 and 8246) and twocoaly mudstones (8244 and 8245) from loc. 3, andthe coal (7573) and carbonaceous mudstone (7574)from loc. 4 have high TOC contents. TS contents arelow for all six samples. Both S2 yields and HI valuesare high for these coaly source rocks.

Organic petrographyThe detailed petrographic and kerogen compositionsof the samples are shown in Table 2.

(i) location 2 samplesThe organic matter of the six carbonaceous mudstones

analysed from loc. 2 are dominated by liptiniticmaterial of which fluorescing amorphous organicmatter (AOM), intimately associated with mineralmatter, constitutes the major component (Fig. 7a).Alginite with a morphology similar to the extantBotryococcus alga is present in all samples. Thecommon association of alginite with the fluorescingAOM and liptodetrinite, and the morphology andyellowish fluorescence property of the latter twocomponents, suggest that both were derived to a largeextent from the degradation of algal material. Thehuminite content is low, whereas inertinite is absent.Framboidal pyrite was observed in minor proportionsin all samples. The maceral composition implies a highproportion of Type I kerogen (alginite+liptodetrinite+AOM) followed by much smalleramounts of terrestrial Type II kerogen (remainingliptinite) and Type III kerogen (huminite).

(ii) location 3 samplesThe samples from loc. 3 are much richer in huminite.Samples 8243 and 8246 are humic coals, whereassamples 8244 and 8245 are coaly mudstones (Fig. 7b–d). Coal sample 8246 yields a huminite reflectance of0.45%Ro. Early generated heavy bitumen orhydrocarbons (exsudatinite) have been observed (Fig.7b). The total liptinite content in the four samples maybe considerable. It is dominated by liptodetrinite,which in the two coal samples and mudstone sample

Table 1. Screening data, Dong Ho Formation.

7575 LM 0.05–0.10 4.09 0.13 433 0.88 22.36 5467576 LM 0.20–0.25 5.20 0.13 434 1.31 22.92 4417577 LM 0.35–0.40 12.92 0.04 428 2.72 72.10 5587578 LM 0.55–0.60 10.39 0.37 430 2.44 50.62 4877579 LM 0.75–0.80 8.29 0.45 427 2.23 44.72 5397580 LM 1.00 6.83 0.21 430 1.53 33.80 4957581 LM 1.20 11.43 0.56 429 2.27 57.46 5037582 LM 1.40 12.71 0.53 435 2.56 65.14 5137583 LM 1.60 10.25 0.58 430 2.43 58.56 572 8243 HC - 54.07 0.40 421 6.19 193.16 3578244 CM - 25.94 0.33 426 3.87 106.02 4098245 CM - 35.75 0.38 419 3.87 127.11 3568246 HC - 50.85 0.49 420 3.98 161.67 318 7573 HC - 53.85 0.68 426 4.38 180.30 3357574 LM - 19.61 0.22 428 4.05 85.58 436 LM: lacustrine mudstone; CM: coaly mudstone; HC: humic coal; HI: hydrogen index; TOC: total organic carbon; TS: total sulphur

2: Dong Ho,outcrop intrench.Sampledprofile 1.60 mthick.

3: Hao 2,Khu DongDang, HoanhBo

4: Huu NghiBridge, VietHung

S1 S2(mg HC/g rock) Sample

no.Locality Lithology

Sampleposition

(m)

TOC TS

Tmax

HI(mg HC/g TOC)(wt.%) (wt.%) (°C)


Com

posi

tion

Don

g H

o, lo

c. 2

K

hu D

ong

Dan

g, lo

c. 3

H

uu N

ghi B

ridge

,

75

75

7577

75

78

7579

75

81

7583

82

43

8244

82

45

8246

75

73

7574

Spo

rinite

1.

4 3.

6 2.

6 2.

0 3.

4 3.

2 1.

4 1.

4 1.

4 0.

6 2.

0 1.

2C

utin

ite

0 0.

2 0.

6 0.

4 0.

6 1.

0 3.

4 1.

0 0.

6 0

2.0

3.0

Res

inite

0

0 0

0 0

0 0.

4 0.

6 0

0 0.

4 0

Alg

inite

4.

6 0.

8 0.

6 1.

4 1.

6 4.

0 0.

4 0.

4 1.

0 0.

6 0.

4 0.

8Li

ptod

etrin

ite

8.4a

22

.6a

17.0

a 11

.8a

9.8a

10

.0a

13.4

b 11

.4a

9.8b

10

.2b

7.6b

13

.8a

Fluo

resc

ing

AO

Mc

13.4

42

.4

38.2

46

.8

57.6

58

.6

1.0

8.8

5.2

2.6

1.4

25.2

Hum

inite

4.

2 5.

2 6.

0 4.

2 5.

8 5.

0 61

.4

40.8

56

.6

75.8

66

.0

25.8

Iner

tinite

0.

2 0

0 0

0 0

7.4

1.2

1.8

3.2

9.4

3.4

Pyr

ite

0.2

0.2

0.4

0.4

0.2

0.8

0 0

0 0.

6 0.

4 0.

4O

ther

min

eral

s 67

.6

24.8

34

.6

33.0

21

.0

17.4

11

.2

34.4

23

.6

6.4

10.4

26

.4

Ty

pe I

kero

gen

26

.4

65.8

55

.8

60.0

69

.0

72.6

1.

4 20

.6

6.2

3.2

1.8

39.8

Type

II k

erog

en

1.4

3.8

3.2

2.4

4.0

4.2

18.6

3.

0 11

.8

10.8

12

.0

4.2

Type

III k

erog

en

4.2

5.2

6 4.

2 5.

8 5.

0 61

.4

40.8

56

.6

75.8

66

.0

25.8

Type

IV k

erog

en

0.2

0 0

0 0

0 7.

4 1.

2 1.

8 3.

2 9.

4 3.

4

Vi

trini

te re

flect

ance

, %R

od - -

- -

- -

- -

- 0.

45

0.45

-

a Lip

tode

trini

te, p

roba

bly

mai

nly

deriv

ed fr

om a

lgin

ite (i

nclu

ded

in ty

pe I

kero

gen)

b Lip

tode

trini

te, p

roba

bly

mai

nly

deriv

ed fr

om te

rres

trial

lipt

inite

(inc

lude

d in

type

II k

erog

en)

c Flu

ores

cing

am

orph

ous

orga

nic

mat

ter i

ntim

atel

y as

soci

ated

with

min

eral

mat

ter;

orga

nic

mat

ter a

ntic

ipat

ed m

ainl

y to

be

deriv

ed fr

omal

gal m

ater

ial (

incl

uded

in ty

pe I

kero

gen)

d Ref

lect

ance

mea

sure

men

ts c

arrie

d ou

t on

eu-u

lmin

ite, t

he lo

w-r

ank

equi

vale

nt to

the

vitri

nite

mac

eral

col

lote

linitelo

c. 4

Tabl

e 2.

Org

anic

pet

rogr

aphi

c an

d ke

roge

n ty

pe c

ompo

siti

ons

(vol

. %)

and

mea

n vi

trin

ite

refle

ctan

ce (

rand

om)

valu

es.


8245 is considered to have been derived principallyfrom terrestrial liptinite (Fig. 7c,d). In the other coalymudstone sample (8244), the liptodetrinite isconsidered mainly to have been derived from algalmaterial. In all four samples, some alginite withBotryococcus morphology is present. The inertinitecontent is generally low in all four samples, and pyritehas only been detected in one of them. In terms of kerogentypes, the four samples are dominantly Type III.

(iii) location 4 samplesSample 7573 from loc. 4 is a huminite-dominated coalwith a reflectance of 0.45%Ro. Sample 7574 isdominated by liptinite of which fluorescing AOM isthe primary component. Alginite with Botryococcus-morphology has been detected. The huminite contentis also high. In terms of kerogen types, the coal sampleis dominated by Type III, whereas Type I kerogenconstitutes the primary kerogen in the mudstonefollowed by a considerable proportion of Type IIIkerogen.

Hydrous pyrolysis (artificial maturation)During hydrous pyrolysis, the HI of coal sample 7573from loc. 4 reaches a maximum of 410 mg HC/g TOC

Fig. 7. (a) Lacustrine mudstone, loc. 2 (sample 7583). The organic matter is characterised by alginite, sporinite,and liptodetrinite in a groundmass of fluorescing AOM (reflected blue light, oil immersion). (b) Humic coal,loc. 3 (sample 8243). Orange–yellowish fluorescing early generated exsudatinite (heavy bitumen/oil) intrudedinto cleats in the huminite (reflected blue light, oil immersion). (c) Humic coal, loc. 3 (sample 8243). Abundantsporinite, cutinite, and liptodetrinite in huminitic groundmass (reflected blue light, oil immersion). (d) Samearea as shown in (c), but in reflected white light (oil immersion).

a b

c d

30 m

30 m

30 m

30 m

at 0.63%Reqv after which it decreases, in particularabove 0.80%Reqv (Fig. 8; Table 3). Tmax does notchange (0.97%Reqv) from a hydrous pyrolysistemperature of 330°C/72hr to 345°C/72hr, whereasthe HI further decreases. The S1 yields (normalised toTOC) increase gradually up to 0.80%Reqv from whichpoint S1 increases more rapidly to a maximum at thefinal hydrous pyrolysis temperature (Table 3). Despitethe pronounced drop in HI from 330°C/72hr to 345°C/72hr, the accompanying increase in S1 yields is limited(Fig. 8; Table 3).

The HI of coal sample 8246 from loc. 3 increasesfrom the immature sample to a well-defined maximumof 365 mg HC/g TOC at 0.69%Reqv, which is followedby a pronounced decrease at 1.07%Reqv (Fig. 8; Table3). The S1 yields show a gradual build-up to amaximum at 1.07%Reqv.

The n-alkane distribution of the two immaturecoals are heavy-end skewed with a clear dominanceof odd-numbered alkanes (Fig. 9). The n-alkanesmaximise at nC27 and nC28. At a hydrous-pyrolysistemperature of 330°C/72hr (Tmax = 450°C; 0.99%Reqv),coal 8246 from loc. 3 still shows a heavy-end skewedn-alkane distribution, while coal 7573 (Tmax = 449°C;0.97%Reqv) from loc. 4 shows a unimodal distribution

30 Source rock properties, Oligocene Dong Ho Formation, Song Hong Basin (northern Vietnam)S

ampl

eTe

mp.

(°C

/72

h)E

xtra

ct y

ield

(mg

SO

M/g

TO

C)

Asp

hal.

(%)

Sat

.(%

)A

ro(%

). N

SO

(%)

TOC

(wt.%

)T m

ax(°

C)

%R

eqv*

S1

S2

S1/

TOC

HI

7573

raw

240

270

300

330

345

raw

240

270

300

330

345

83 151 - -

345

315

82 114 - -

197

234

47.1

762

.81

- -32

.74

24.9

6

65.9

568

.70

- -26

.51

28.0

0

7.01

9.34 - -

35.4

056

.52

6.84

7.45 - -

41.2

641

.75

3.27

1.65 - -

8.07

6.96

1.58

4.26 - -

9.79

3.88

89.7

289

.01

- -56

.52

36.5

2

91.5

888

.30

- -48

.95

54.3

7

53.8

552

.17

53.8

055

.32

54.8

254

.21

50.8

557

.98

57.8

160

.07

61.6

957

.52

426

423

431

440

449

449

420

424

434

439

450

454

0.53

0.47

0.63

0.80

0.97

0.97

0.42

0.49

0.69

0.78

0.99

1.07

4.38

12.1

620

.61

40.7

379

.10

92.1

7

3.98

8.21

18.4

839

.77

62.8

281

.71

180.

3020

9.69

220.

3820

7.59

150.

8499

.88

161.

6719

2.45

211.

0519

5.04

137.

6399

.96

8.13

23.3

138

.31

73.6

314

4.29

170.

02

7.83

14.1

631

.96

66.2

110

1.83

142.

05

335

402

410

375

275

184

318

332

365

325

223

174

8246

*%R

eqv

calc

ulat

ed fr

om T

max

acc

ordi

ng to

the

rela

tion

esta

blis

hed

by P

eter

sen

(200

2)

(mg

HC

/g ro

ck)

(mg

HC

/g T

OC

) which, apart from two pronounced peaks at nC27 andnC29, has a maximum at nC21 (Fig. 9). At the highesthydrous-pyrolysis temperature of 345°C/72hr, the n-alkane distribution of coal 8246 (Tmax = 454°C;1.07%Reqv) has attained a more or less unimodal shapewith a maximum at nC21, whereas coal 7573 (Tmax =449°C; 0.97%Reqv) displays a light-end skewed n-alkane distribution that maximises at nC20 and extendsto at least nC38 (Fig. 9).

The asphaltene-free extracts from the twoimmature samples are completely dominated by polarcompounds, and the composition does not changeupon hydrous pyrolysis at 240°C/72 hr (Fig. 10; Table3). Hydrous pyrolysis at 330°C/72 hr significantlyincreases the generation of saturated hydrocarbons,which corresponds to a maturity of 0.97–0.99%Reqv.Upon further artificial maturation, the proportion ofsaturated hydrocarbons in the extract from coal 8246does not change, whereas the proportion of saturatedhydrocarbons in the extract from coal 7573 increasesto 57% (Fig. 10; Table 3). For comparison with thenew results, the extract composition of the hydrous-pyrolysed Dong Ho 3 coal (HI = 200 mg HC/g TOC)from loc. 1 is also shown (Petersen et al., 2001;Petersen, 2002).

DISCUSSIONDepositional environment

The Dong Ho Formation mudstones from loc. 1 havebeen interpreted to have been deposited in aprincipally freshwater lake environment under low-energy and oxygen-deficient conditions (Traynor andSladen, 1997; Petersen et al., 2001, 2004a). A similarlacustrine origin is also indicated for the mudstonesfrom loc. 2 and the carbonaceous mudstone (sample7574) from loc. 4. TS contents <0.58 wt.% are withinthe range observed in recent freshwater mires (e.g.Cameron et al., 1989; Phillips and Bustin, 1996) (Fig.11), and the presence of alginite with a morphologysimilar to the recent alga Botryococcus in several ofthe samples further implies a freshwater origin.However, small amounts of pyrite, commonly inframboidal form, suggest a saline influence on thelakes, as pyrite formation is related to the reductionof sulphate in saline water by anaerobic bacteria (e.g.Postma, 1982; Cohen et al., 1984; Brown and Cohen,1995). The fine grain size of the inorganic matter (clayand silt) implies a low-energy depositionalenvironment. The fine-grained mineral matrix andlow-oxygen conditions favoured the preservation oforganic matter, as indicated by elevated TOC contentsand the high proportion of fluorescing AOM. The highTOC content, the dominance of Type I kerogen andonly minor amounts of Types II and III kerogen, alongwith HI values generally above 500 mg HC/g TOC,

Tabl

e 3.

Ext

ract

ion

and

scre

enin

g da

ta o

f the

hyd

rous

pyr

olys

ed c

oals

.


Fig. 8. The evolution in the Hydrogen Index and S1/TOC with increasing maturity (hydrous pyrolysistemperature) for two coal samples (loc. 3, 8246;and loc. 4, 7573). The relationship between Tmax and vitrinitereflectance is based on the correlation in Petersen (2002). OR: original sample; 240, 270, 300, 330, 345: hydrouspyrolysis temperature/72 hours.

correspond to the so-called lacustrine “algal organicfacies” of Carroll and Bohacs (2001). This organicfacies typifies the “fluctuating-profundal faciesassociation”, characteristic of progradational andaggradational basin fill successions. This faciesassociation is a significant component of balanced-fill lake basins, where the creation of availableaccommodation space roughly balances the sedimentand water input (Bohacs et al., 2000). The balanced-fill lake type provides optimal conditions for organicmatter accumulation, and the “fluctuating-profundalfacies association” includes some of the worlds richestsource rocks (Carroll and Bohacs, 2001). For example,Lake Victoria, East Africa, is presently classified as ashallow balanced-fill lake, and analysis of an earlyHolocene and a Recent sample from a core drilled at32 m water depth a few kilometres offshore (Talbot,1988) yielded TOC contents of 18.31–19.05 wt.%, anorganic matter composition including amorphousmaterial together with Botryococcus algae, spores/pollen and plant cuticles, and HI values of 510–570mg HC/g TOC.

The remaining samples comprise a coaly mudstone(sample 8244, loc. 3) and humic coals, which wereformed in wet, oxygen-deficient peat mires. The zeroto very low content of pyrite observed in the coals

and the low TS contents (0.33–0.68 wt.%) areconsistent with a freshwater mire origin (Fig. 11;Tables 1 and 2). The composition of the coalymudstone and humic coals corresponds to the so-called “algal-terrestrial organic facies”, whichcontains a mixture of Type I, terrestrial Type II, andType III kerogen (Bohacs et al., 2000; Carroll andBohacs, 2001). This organic facies is characteristicof the “fluvial-lacustrine facies association”, whichis typical of overfilled lake basins in which thesediment supply exceeds the availableaccommodation space. In the coals, the content ofType III kerogen (huminite) is clearly dominant. As aconsequence, the HI values may vary considerably,and the source potential is mixed oil and gas. The oilgenerative potential of the investigated coals is,however, significant (see below).

Hydrocarbon generative potential: lacustrinemudstonesThe majority of the analysed samples are lacustrinemudstones with a considerable content of organicmatter which, apart from the Type III kerogen-dominated coaly mudstone (sample 8244) from loc.3, can generally be classified as Type I kerogen(Tables 1 and 2). The remaining samples are humic

0420

430

440

450

460

HI

S1/TOC

HI

S1/TOC

0.42

0.61

0.80

0.99

1.19%Reqv Tmax(C)

100

OR

240 240

270 270

300 300

330345

330

OR

OROR

200

mg HC/g TOC

300 400

Sample 8246, loc. 3: Sample 7573, loc. 4:


Fig. 9. Gas chromatograms (saturate fraction) of three immature coals and when subjected to hydrouspyrolysis at 240°C/72 h, 330°C/72 h and 345°C/72 h. The n-alkanes show an evolution from a heavy-end skeweddistribution to a more or less unimodal or light-end skewed distribution with maximum at nC20–21 andextending to at least nC38.

Hào

-2, H

oanh

Bo

(824

6), l

oc. 3

Huu

Ngh

i Brid

ge (7

573)

, loc

. 4D

ong

Ho

river

bed

(Don

g H

o 3)

, loc

. 1

290.

45 %

Ro

Original samples 240C/72 hr 330C/72 hr 345C/72 hr

0.45

%R

o0.

41 %

Ro

0.49

%R

eqv

0.47

%R

eqv

0.47

%R

eqv

0.99

%R

eqv

0.97

%R

eqv

1.03

%R

eqv

1.07

%R

eqv

0.97

%R

eqv

1.15

%R

eqv

29

33 33

25 25

21 21

17

29

3325

2117

29

3325

21

17

29

33

2521

17

29

33

25

21

17

29

33

25

21

17

29

33

25

2117

29

33

2521

17

29

33

25

21

17

29 29

33 33

25 25

21 21

17 1717


coals dominated by higher land-plant derived organicmatter, corresponding to Type III kerogen.

The high content of algal-derived Type I kerogenin the lacustrine mudstones has a profound influenceon the source potential as shown by the high HI values(Table 1). HI values up to 690 mg HC/g TOC havebeen obtained from mudstones sampled from loc. 1(Fig. 2) (Petersen et al., 2001). The highly oil-pronecomposition of the mudstones is emphasised by theTOC versus S2 plot, on which the mudstones areclassified as excellent petroleum source rocks with agenerative capacity generally between 20 and 100 kghydrocarbons/ton rock (Fig. 12). The generationpotential is to a large extent linearly correlated to the

total content of Type I kerogen (AOM+alginite+liptodetrinite; liptodetrinite is mainlyconsidered to represent algal remnants) as shown bythe correlation value of r2=0.88 (Fig. 13). The HI valueis not zero at a zero content of Type I kerogen, showingthat other kinds of organic material also influence thegenerative potential, although a strong correlationexists between the HI and the content of Type Ikerogen for the lacustrine mudstones. The other typesof organic matter contributing to the generativepotential may be recognisable terrestrial Type IIkerogen (i.e. sporinite and cutinite) and/orsubmicroscopic material, including bacterial residues.The latter may be illustrated by the two Type II

Fig. 10. The evolution in extract composition of the artificially matured coal samples. Data from a coal fromloc. 1 with an initial HI of about 200 mg HC/g TOC is also included (see Petersen et al., 2001). The two newcoals with significantly higher HI generate considerably larger amounts of saturated hydrocarbons.

Fig. 11. TS plotted against TOC (Berner and Raiswell, 1983) suggesting that the depositional environmentswere dominantly freshwater.

0 20

0.5

Khu Dong Dang (8246), loc. 3

Huu Nghi Bridge (7573), loc. 4

Dong Ho (Petersen et al., 2001; Petersen, 2002), loc. 1

Evolution in extract composition from 330C/72 hr to 345C/72h

Vitrinite reflectance values calculated from Tmax (see Petersen, 2002)

40

0.970.990.97

0.97

1.071.15

1.03

60 80 100

1000

8020

6040

4060

2080

0100

Saturates

Pola

rsArom

atics

• • • •• •• • •

•1.5

1.0

0.5TS (w

t.%)

Mar

ineFr

esh

water

Dong Ho, loc. 2Khu Dong Dang, loc. 3Huu Nghi Bridge, loc. 4

TOC (wt. %)

2 4 6 8 10 12 14 20 22 24 26 360

0


kerogen-lean samples (0.2–0.4 vol.% Type II kerogen;Petersen et al., 2004a) from Bach Long Vi Island,which despite a relatively low content of Type Ikerogen still display significant source potential (HI= 195–278 mg HC/g TOC) (Fig. 13). Similarly, therelatively high HI values of the two samples from BachLong Vi Island and loc. 2 indicated as “outliers” onFig. 13 may be caused by a high proportion ofsubmicroscopic lipid material. Also, a significantlyincreased proportion of Type III kerogen (i.e.huminite) will influence the source potential, and theelevated HI for the two samples from loc. 3 (8244,8245) may partly be attributed to the high proportionof such organic matter (Fig. 13; Table 2). In sourcerocks dominated by Type III kerogen, such as humiccoals, a much more complex relationship between thesource potential and the organic composition exists(Petersen and Rosenberg, 2000).

Langford and Blanc-Valleron (1990) used a TOCversus S2 plot to determine the true average HI valueand the adsorption of generated hydrocarbons insource rocks. TOC is plotted versus S2 for the samplesfrom loc. 2 in Fig. 14. By restricting samples to loc.2, only source rocks from the same stratigraphic levelare included. The samples plot in the upper part ofthe Type II kerogen area and the slope of the regressionline (r2=0.96) yields an average HI of 550 mg HC/g

TOC, emphasising the oil-prone nature of themudstones. The interception of the regression line withthe x-axis is interpreted to represent the matrixadsorption of generated hydrocarbons, i.e. the amountof organic matter necessary before hydrocarbons canbe expelled from the source rock (Langford and Blanc-Valleron, 1990). In this way a minimum of 0.5 wt.%TOC is suggested to be required before liquidhydrocarbons can be expelled from the Dong Hosamples. Hence, with measured TOC contentsbetween 4.09 and 12.92 wt.%, matrix adsorption ofgenerated hydrocarbons is considered to be ofsubordinate importance (Fig. 14; Table 1). In addition,the slope of the regression line indicates that 55% ofthe TOC is pyrolisable to hydrocarbons (S2/TOC x10; Langford and Blanc-Valleron, 1990). This valueis comparable to the 50–55% of the TOC in mudstonesfrom loc. 1 which were determined to be involved inpetroleum formation by Petersen et al. (2001).

The highly oil-prone nature of the lacustrine sourcerocks is consistent with the very pronounced liquidhydrocarbon generation over a relatively narrowtemperature range from these rocks during artificialmaturation (hydrous pyrolysis) (Petersen et al., 2001,2004a). Such generation characteristics are typical ofType I source rocks, due to their rather narrowactivation energy distribution and pronounced

Fig. 12. S2 yields plotted against the TOC content for the lacustrine and coaly mudstones demonstrating theirhighly oil-prone nature. Data for loc. 1 are from Petersen et al. (2001, 2004). S2 and TOC cut-off values basedon Bordenave et al. (1993).

•

•

•

• • • •••••

TOC (wt. %)

S2

(kg

hydr

ocar

bons

/ton

rock

)

2

86

100

10

1

0.1

0.1 1 10

4

2

86

4

2

86

4

2

2 3 4 5

Poor

Poo

r

Good

Goo

d

Excellent

Exc

elle

ntFa

ir

6 7 8 9 2 3 4 5 6 7 8 9 2 3 4 5

•••

•

Dong Ho, loc. 1Dong Ho, loc. 2Khu Dong Dang, loc. 3Huu Nghi Bridge, loc. 4


Fig. 13. Correlationbetween the HydrogenIndex and the content ofcalculated Type I kerogenfor the lacustrine and coalymudstones. The correlationis strong for the lacustrinemudstones (r2 = 0.88; n =14). The two coalymudstones from loc. 3 maybe influenced by thecontent of Type III kerogen(see discussion in text).Data for loc. 1 and BachLong Vi Island are fromPetersen et al. (2001,2004a).

Fig. 14. S2 yields plottedagainst TOC content forthe lacustrine mudstonesfrom loc. 2. The averageHydrogen Index is 550 mgHC/g TOC. The regressionline (r2 = 96) cuts the x-axis at 0.5 wt.% TOC,suggesting that only 0.5wt.% TOC is required foroil expulsion to occur (seeLangford and Blanc-Valleron, 1990).

••• •••

700

600

500

400

300

200

100

0

HI (

mg

HC

/g T

OC

)

Type I kerogen (vol%)

0 2010 30 40 50 60 70 80 90 100

Outliers

Type IIIkerogen-rich

•Dong Ho, loc. 1Dong Ho, loc. 2Khu Dong Dang, loc. 3Huu Nghi Bridge, loc. 4Bach Long Vi Island

Stippled line: n = 14Slope: 5.22Corr. coef.: 0.94 (r2=0.88)

40

30

20

10

S2

(mg

HC

/g ro

ck)

TOC (wt. %)

0 2 4 6 8 10 12 14 16

• ••

••

•••

•

80

70

60

50

0

Type I

Type II

Type III

HI=550


activation energy peak (Tissot et al., 1987; Jarvie,1991; Tegelaar and Noble, 1994; Ruble et al., 2001;Petersen et al., 2001, 2002, 2004a).

Hydrocarbon generative potential: humic coalsThe humic coal samples from loc. 3 and loc. 4 displayconsiderable source potential (HI = 318–357 mg HC/g TOC), and thus constitute a significant potentialsource rock in addition to the oil-prone lacustrinemudstones in the NE Song Hong Basin. The initialincrease in HI shown by the two hydrous-pyrolysedcoals (Fig. 8) is a well-known feature from naturallymatured coals (e.g. Teichmüller and Durand, 1983;Sykes, 2001; Petersen, 2002), and may be related tothe formation of an additional source potential duringstructural re-organisation of the organic matter andincorporation of water-derived hydrogen (e.g. Lewan,1997; Schenk and Horsfield, 1998). Petersen (2002)determined that the start of the oil window for humiccoals from loc. 1, with an HI of about 200 mg HC/gTOC, occurs at a vitrinite reflectance of 1.03–1.15%Ro. Previous maturity modelling of theanticipated source rock interval in the offshore KienAn and Thuy Nguyen Grabens in the NE Song HongBasin showed that it is likely that the coals are, atbest, only marginally mature in both grabens(Petersen, 2002; Petersen et al., 2004b; Andersen etal., in press). The considerably higher generativepotential of the coals analysed in the current studyindicates a different organic matter composition andtherefore better generation characteristics. Potentiallysubmicroscopic oil-prone entities may havecontributed to the high HI values (318–335 mg HC/gTOC) recorded.

A decrease in S1 yields (normalised to TOC) froma series of naturally matured coals, and from a seriesof samples artificially matured by progressive hydrouspyrolysis, has been used to determine the start of theoil window (effective oil window or expulsionwindow) (Price, 1989, 1993; Petersen, 2002). The twocoal samples subjected to hydrous pyrolysis in thepresent study do not show the expected decrease inS1 at high maturities (Fig.8). This may be explainedby the extensive hydrocarbon generation from bothcoals, in particular demonstrated by the lack of changein maturity of coal sample 7573 from a hydrouspyrolysis temperature of 330°C/72 hr to 345°C/72 hr(Fig. 8). This generation characteristic is observed inlacustrine source rocks (e.g. Petersen et al., 2004a),and suggests that most of the energy put into thesystem is used to convert the labile part of the organicmatter to hydrocarbons. A consequence may be thatthe limited volume of the column used for artificialmaturation of the samples is totally saturated withhydrocarbons during pyrolysis, thereby hindering thedetection of decreasing S1 yields. Such a situation was

likewise observed during hydrous pyrolysis oflacustrine mudstones from loc. 1 (Petersen et al.,2004a). However, several observations relating to coalsample 7573 may indicate when expulsion starts fromthis coal. Although the coal shows a considerable dropin HI from 330°C/72 hr to 345°C/72 hr, the S1 yieldsonly display a limited increase (Fig. 8). At the sametime, the proportion of saturated hydrocarbonsincreases from 35% to 57%, and the n-alkanes changefrom a unimodal to a light-end skewed distribution(Figs. 9, 10). These geochemical data suggest that anearly mature oil has at least been generated andpresumably expelled at 330°C/72, corresponding to aTmax of 449°C or 0.97%Ro. Initial generation of heavybitumen/hydrocarbons (exsudatinite; Fig. 7b) mayfacilitate saturation of the coals to the expulsionthreshold, as these low-maturity products are“converted” to true hydrocarbons at higher maturityby partial decomposition (Lewan, 1994). Thebeginning of the oil window at a VR of 0.97%Ro (oreven at lower maturity) is earlier than the estimated1.03–1.15%Ro for the coals from loc. 1 previouslyinvestigated (HI = 200 mg HC/g TOC) by Petersen(2002). Similar oil-prone coals may therefore havereached the oil window in deeply buried kitchens inhalf-grabens in the offshore Song Hong Basin(Andersen et al., in press). This is an importantobservation, because an earlier hydrocarbon expulsionfrom such coals may explain the terrestrialgeochemical signature in the principally lacustrine-sourced oil encountered in the B10-STB-1x well atthe margin of the NE Song Hong Basin (Fig. 1)(Petersen et al., 2004a). Based on the n-alkanedistributions and evolution in S1 yields, coal sample8246 seems to expel hydrocarbons at a higher maturitydespite a high HI, although the data are difficult tointerpret, as discussed above. During artificialmaturation, the coal behaves in a fashion intermediatebetween coal sample 7573 and the Dong Ho 3 coalfrom loc. 1 (Figs. 9, 10).

CONCLUSIONS

One of the main exploration risks in the Song HongBasin is the occurrence of high quality source rocksand the timing of hydrocarbon generation relative tothe formation of traps. This paper further documentspotential source rocks in the basin and adds to theprevious assessment of their generative potential. Ourevaluation of the distribution and organic facies ofnew outcrops of lacustrine mudstones, coalymudstones and humic coals in the Oligocene DongHo Formation in the Dong Ho area at the margin ofthe offshore Cenozoic Song Hong Basin permits thefollowing major conclusions to be drawn:

1. The highly oil-prone lacustrine mudstones


contain from 4.09–19.61 wt% TOC, with 60% of thesamples containing over 10 wt% TOC. The organicmaterial is composed principally of fluorescing AOM,liptodetrinite, and alginite with Botryococcus-morphology (corresponding to Type I kerogen). HIvalues range from 436–572 mg HC/g TOC, but 60%of the samples have an HI value exceeding 500 mgHC/g TOC. About 55% of the TOC is estimated to bepyrolisable into hydrocarbons, and only 0.5 wt% TOCmay be required for oil expulsion to occur.

2. Humic coals and coaly mudstones have HIvalues of 318–409 mg HC/g TOC. Huminite (TypeIII kerogen) is the dominant organic material, whileterrestrially-derived liptodetrinite is subordinary. Uponartificial maturation by hydrous pyrolysis, the coalsgenerate significant quantities of saturatedhydrocarbons. Expulsion of liquid hydrocarbonsoccurs probably not later than at a maturitycorresponding to a vitrinite reflectance of 0.97%Ro.This is earlier than previous estimates for Dong HoFormation coals, which had an HI around 200 mg HC/g TOC.

3. The present study shows that these two oil-pronepotential source rocks are thermally mature in buriedsyn-rift successions in the under-explored NE SongHong Basin, offshore northern Vietnam. This issupported by oil recovered from well B10-STB-1x atthe margin of the NE Song Hong Basin, as the oilwas principally generated from a lacustrine sourcewith an additional component from a coaly source.

ACKNOWLEDGEMENTS

This paper is a contribution to the establishment of aregional source rock database in Vietnam as describedin the DANIDA-sponsored ENRECA project“Integrated analysis and modelling of geologicalbasins in Vietnam and an assessment of theirhydrocarbon potential”. E. Melskens and J. Halskov(GEUS) drafted the figures. J. Ineson (GEUS) isthanked for improving the English. The authors alsoacknowledge the valuable reviews by J.A. Curiale(Unocal) and B.M. Thomas (Origin Energy ResourcesLtd). The paper is published with the permission ofPetroVietnam, Vietnam Petroleum Institute and theGeological Survey of Denmark and Greenland.

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