Post-generative alteration e•ects on petroleum in the ... · Post-generative alteration e•ects...

Post-generative alteration e�ects on petroleum in the onshoreNorthwest Java Basin, Indonesia

Haposan Napitupulua,b, Leroy Ellisc,d,*, Richard M. Mittererb

aPertamina EP Jl. Merdeka Timur No.6, Jakarta 10110, IndonesiabUniversity of Texas at Dallas, Richardson, TX 75083-0688, USA

cARCO Exploration and Production Technology, 2300 West Plano Parkway, Plano, TX 75075-8499, USAdTerra Nova Technologies, PMB 409, 18352 Dallas Pkwy, Ste. 136, Dallas, TX 75287, USA

Received 15 March 1998; accepted 4 November 1999

(returned to author for revision 15 June 1998)

Abstract

Northwest Java Basin oils, largely derived from the ¯uvial-deltaic to nearshore marine Talangakar formation of Oli-gocene to Early Miocene, range from heavy oils to extremely light oils and retrograde condensates, with API gravities ofa suite of oils ranging from about 17� to 53�. Heavy oils, with API gravities less than 22�, all of which are in shallow

reservoirs, are biodegraded. Pristine oils concomitant with related derivative residual and retrograde condensate oiltypes indicate evaporative fractionation phenomena. Post-generative alteration processes are widespread in this highlyfaulted region. Pristane to phytane biomarker ratios of retrograde condensates and residual oils have been shown to be

severely a�ected by evaporative fractionation. Principal component analysis (PCA) of isotope and biomarker dataidenti®ed two oil families associated with source rocks of the Talangakar formation. One group is suggested to be derivedfrom more marine in¯uenced delta-front to prodelta depositional settings, while the second group is attributed to ahigher plant-rich delta-plain to delta-front depositional environment. Correlation of these oil families with the varied

depositional environments of the Talangakar formation has allowed a more re®ned approach to the identi®cation ofhydrocarbon migration pathways in the Northwest Java Basin. Multivariate statistical analysis is shown to be an e�ec-tive tool in correlating high gravity condensate oil types. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Oil; Condensate; Evaporative fractionation; Water washing; Biodegradation; Principal component analysis (PCA); NW

Java Basin

1. Introduction

Crude oil compositions, although initially controlledby the nature and maturity of organic matter in thesource rock, are subject to a complex series of sub-

sequent compositional modi®cations that may occurduring migration and within the reservoir (Lafargue andBarker, 1988). Gross changes in oil composition are

generally attributed to thermal maturation and bio-degradation e�ects. Thermal maturation, a consequenceof increasing burial depth and higher temperature, will

form increasingly lighter gravity oils until extreme tem-peratures result in cracking of the parent kerogen and/

or oil to gas. By contrast, biodegradation by subsurfacemicrobial communities at shallow depths leads to hea-vier (or low API gravity) oils. In addition, more com-

plex phenomena involving evaporative fractionation,water washing, deasphalting, mineral catalysis, gravitysegregation, subsurface PVT (pressure-volume-tempera-

ture) e�ects, and dewaxing may all contribute, to vary-ing extents, to alteration of crude oils either in thereservoir or along migration pathways.Light oils (usually from 30 to 50� API gravity) and/or

retrograde condensates (ranging up to 60� API gravity)may, in most cases, be de®ned as the lowmolecular weightportion of crude oils that becomes entrained/miscible with

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.

PI I : S0146-6380(99 )00154-0

Organic Geochemistry 31 (2000) 295±315

www.elsevier.nl/locate/orggeochem

* Corresponding author.

E-mail address: [email protected] (L. Ellis).

a gas phase, and transported away from the primarycrude oil reservoir. This re-migration tends to be direc-ted along permeable stratigraphic units or faults toshallower reservoir depths where lower geothermal

temperatures and pressures result in the entrained ¯uidscondensing to a separate liquid phase. The new reservoiris, therefore, partitioned into a two-phase gas±liquid

system, similar to the parent reservoir that sourced thehydrocarbons. Other occurrences of light oils may beattributed to overmature products derived directly from

the parent kerogen at late stages of catagenesis/earlymetamorphism (Hunt, 1996). Oils of this type are theresult of thermal cleavage reactions, which tend to pro-

duce a greater proportion of n-alkanes relative to bran-ched and cyclic alkanes (Philippi, 1975). Condensates(usually > 50�API gravity) may be de®ned as a naturalwet gas accumulation in which liquid hydrocarbons,

such as those comprising the gasoline range, exist toge-ther with gases in a single phase at some equilibriumde®ned by PVT conditions in the reservoir (McCain

1973; Hunt, 1996). At surface temperatures and pres-sures, liquids may condense from these `wet gases' andform extremely light API gravity ¯uids (i.e., con-

densates). Two of the most important processes respon-sible for the formation and alteration of light oils andretrograde condensates are evaporative fractionation

and water washing.

1.1. Evaporative fractionation

The term evaporative fractionation is suggested forthe complex PVT phenomena involved in the phase-separation of gas from reservoired oil (Thompson,

1983). In this process, the original reservoir becomesoversaturated as a consequence of reservoir pressuredepletion, or through additional hydrocarbon charging,

involving migration of methane and other migratedlight hydrocarbons (C2±C5) from the original sourcerock that has now reached the gas window. Over®llingof the reservoir capacity past spill point permits the

escape and re-migration of gases and ¯uids. Other sce-narios may involve loss of gases from the parent reser-voir, perhaps due to faulting, overburden removal,

seismic activity, or simply leakage (e.g. via micro-fracturing) through a gas-permeable seal rock thatallows only movement of gases, and other low mole-

cular weight components. In all cases, gases serve as themobile phase in which entrained hydrocarbon liquidsare distributed vertically and/or horizontally along

`paths of least resistance' in the sedimentary sequencebefore ®nally reaching a new reservoir or escaping to thesurface. As gases migrate upward, carrying entrainedoil, subsurface pressure is reduced, and heavier hydro-

carbon components in the gas phase liquefy and con-dense enroute. Hence, as pressure and temperaturedecrease at shallower depths, new gas±liquid phase PVT

equilibria are continuously established, ultimatelyresulting in the shallowest oils and condensates beingthe highest API gravity ¯uids (assuming no microbialdegradation). The process of evaporative fractionation

is likely to be continuous, with hydrocarbon ¯uidsredistributing and accumulating on multiple occasionsby the same process within the generative basin, until

eventually only dry or wet gas accumulations exist in areservoir (Thompson, 1987, 1988). The frequency ofoccurrence of gas-condensate accumulations depends

upon the frequency of occurrence of the separationprocess, which in turn, depends upon the frequency offault movement and ultimately upon the availability

and supply of hydrocarbon gases to the trap. Thetotal distance that oil and condensate may remigrate toother traps can vary from a few hundred meters to morethan one hundred kilometres (Vandenbroucke et al.,

1983).A consequence of the evaporative fractionation pro-

cess is the formation of ``derivative'' light oils (retro-

grade condensates), condensates and residual oils fromthe ``pristine'' parent oils. In each case, gross composi-tional changes in the hydrocarbon components of the

gases and liquids take place (Thompson, 1987). In theevaporative fractionation process, low molecular weightsaturated hydrocarbons are preferentially incorporated

into the vapour phase, relative to aromatic components,and tend to remigrate from the original reservoir. Non-biodegraded and remigrated light oils or condensates,are generally reservoired at shallower depths, and will

typically exhibit an enrichment in light ends and n-par-a�ns concomitant with higher API gravities relative tothe parent oil. By comparison, residual oils, which are

generally found in deeper reservoirs, usually show a lossof light ends and an increase in aromaticity concomitantwith lower API gravities relative to the parent oil

(Thompson, 1987; Dzou and Hughes, 1993; Curiale andBromley, 1996; Hunt, 1996).Dzou and Hughes (1993) investigated the K ®eld oils,

o�shore Taiwan, which produce predominantly gas

along with small amounts of liquid hydrocarbons.Gases and liquids from these ®elds could be correlatedto the same deltaic source rock and maturity, thus

invoking a suspected process of evaporative fractiona-tion of deeper, initially remigrated oil. Hunt (1996) usedbulk properties, GC patterns, and fractionation index

plus the B value (Toluene/n-C7) to identify oils thathave undergone evaporative fractionation. Holba et al.(1996) used o�shore Louisiana oils to show that residual

oils from evaporative fractionation processes are usuallyfound in deeper reservoirs and are characterized by lossof light ends and elevated aromaticity. Reservoiredresidual oils may also receive later recharging by

hydrocarbon contributions. Oils observed to haveundergone numerous cycles of evaporative fractionationwith subsequent recharge were found to exhibit a ``V''

296 H. Napitupulu et al. / Organic Geochemistry 31 (2000) 295±315

pattern in C5±C7 n-alkane distributions on the gaschromatogram (Holba et al., 1996).

1.2. Water washing and biodegradation

Crude oils may be substantially altered during migra-tion, or in the reservoir, by water washing concomitant

with biodegradation (Seifert and Moldowan, 1979;Lafargue and Barker, 1988). Biodegradation involvesmicrobial alteration of crude oil in the reservoir and

may be possible whenever the oil pool is in contact withshallow meteoric waters. Among the saturated hydro-carbon classes, normal alkanes are the ®rst compound

type consumed by bacteria. Hence, biodegraded oils canusually be characterized by the absence, or presence ofvery low concentrations, of n-alkanes, simple branchedalkanes and alkylcyclohexanes relative to isoprenoid or

bicyclic alkanes that are more resistant to microbialalteration (Evans et al., 1971; Volkman et al., 1983).Water washing is particularly e�ective within the low-

boiling range of hydrocarbons and results in a decreasedAPI gravity, followed by sequestering of low molecularweight aromatics (i.e., benzene, toluene) before light

alkanes and naphthenes are removed (Connan, 1984;Palmer, 1984). Conditions favorable for water washingare known to exist during oil migration, especially when

oil is migrating through a hydrologically active water-wet carrier bed and reservoir system. Water washingwithout concomitant biodegradation is indicated by: (1)a decrease in the amount of aromatic and low molecular

weight n-alkanes while naphthenes are unaltered, (2)partial removal of C15+ aromatics while C15+ alkanesare una�ected, and (3) a decrease in sulphur-bearing

aromatics (especially dibenzothiophene) (Palmer, 1984;Lafargue and Barker, 1988). In most cases, the loss ofbenzene and toluene is a good indicator that water

washing has occurred. However, in cases where there isa complete absence of gasoline range components orlight ends, loss of higher molecular weight species suchas dibenzothiophene relative to phenanthrene may also

prove to be a good indicator (Palmer, 1984).Biodegradation and water washing processes often

occur together. Assessing the extent of water washing on

migrating and reservoired crude oils is di�cult becauseof the more dominant e�ect of microbial alteration ofthe oil. Biodegradation typically occurs in the shallowest

reservoirs in a basin where viable bacterial communitiesin meteoric waters commonly exist and are transported,in some cases, along with migrating waters to reservoirs.

With time, an active hydrologic system, and favorablestatic subsurface conditions, microbial alteration ofcrude oil may be extremely pronounced and lead tosigni®cant changes in the gross compositional matrix

of the oil (Lafargue and Barker, 1988; Peters andMoldowan, 1993). In cases such as these, the e�ects ofbiodegradation far exceed those of water washing. In

deeper reservoirs and associated migration conduits,bacteria communities are less likely to be viable mainlydue to the e�ects of increasing temperature. In thesescenarios, water washing e�ects can be more pro-

nounced and, therefore, easier to observe.In this study, detailed geochemical analyses of

Northwest Java Basin oils are examined in order to

understand the origin of light oils and condensates.Signature compositional variations exhibited by sam-pled petroleum ¯uids derived from two distinct NW

Java oil families reveal the varying extent of theseaforementioned processes.

2. Regional geological setting

The NW Java Basin is located between the Bogor

Trough to the south, the continental Sunda Plate to thenorth, the Tanggerang High to the west, and the Arja-winangun High to the east (Fig. 1). This basin is part of

a series of basins (e.g., Palembang, Sunda, Asri) thatoriginated on the southern edge of the Sunda cratonduring a major Eocene-Oligocene orogenic period of

dextral wrenching (Daly et al., 1987; Gresko et al.,1995).In the NW Java Basin, an extensive accumulation of

Tertiary sediments up to 5000 meters thick covers acratonic basement of pre-Tertiary age (Nayoan, 1972),with the stratigraphic succession ranging in age fromLate Paleocene (?) - Mid-Eocene to Holocene (Fig. 2).

The Oligocene to Middle Miocene sediments weredeposited in a general transgressive sequence. The rockunits of this sequence comprise Talangakar, Baturaja

and Cibulakan formations. This transgressive sequenceconsists mainly of shale interbedded with sandstone,siltstone and coal which covers the northern half of the

basin, grading southward into deep water shale andcarbonate facies. Talangakar strata, which were depos-ited in deltaic to shore environments range from 150 mto greater than 900 m thick in the shelf area and basin

axis depocenters. The coals and carbonaceous shales ofthis depositional system exhibit excellent hydrocarbonsource rock characteristics (Fletcher and Bay, 1975;

Roe and Polito, 1977; Gordon, 1985; Robinson, 1987;Pramono et al., 1990; Noble et al., 1991, 1997; Wu,1991).

At least ten active petroleum systems, with 150 sepa-rate oil and gas ®elds, have been recognized in this basinby Noble et al. (1997). Oil and gas in the onshore NW

Java Basin have migrated northward through onshorestructural highs to o�shore basins (Noble et al., 1997).Hydrocarbons are distributed in the o�shore andonshore NW Java Basin in Upper Oligocene (Talangakar

formation) to Upper Miocene (Parigi formation) reser-voirs, and the oils are predominantly derived from ¯uvio-deltaic source rocks (Table 1).

H. Napitupulu et al. / Organic Geochemistry 31 (2000) 295±315 297

3. Analytical

Oils [22, see Table 1] sampled from NW Javaexploration and production wells (well-head separa-tors), were analyzed as summarized in Tables 2±4. Oils

were sampled directly from the well-head and collectedin sealed glass vials. The biodegraded oil (JTB-194) wasobtained from Pertamina Lab in Cirebon. Samples were

kept at ambient conditions for a short time period (3±4weeks) until arrival at ARCO Exploration Research andTechnical Services Laboratories (Plano, TX), where allsamples were subsequently stored in a refrigerator. All

sample preparation and GC, GC±MS analyses wereperformed within a few months of their arrival. Wholeoils were analyzed by gas chromatography on a HP

5890 GC equipped with a 60 m�0.25 mm i.d. capillarycolumn coated with a 0.25 mm dimethylploysiloxane

(DB-1) phase. Sample preparation of the oils involvedtopping under a stream of nitrogen at 40�C for 1 h, beforetreatment with an excess of pentane to precipitate

asphaltenes. The pentane soluble fraction was subse-quently concentrated, with the polar NSO fraction thenremoved using a Waters Sep-Pak Plus CN cartridge

using pentane as eluent. The apolar saturated, andmoderately polar aromatic hydrocarbon, fractions wereisolated by medium-pressure liquid chromatography(MPLC) using deactivated silica and activated silica

columns.Gas chromatography±mass spectrometry (GC±MS)

was performed on saturate and aromatic fractions using a

Fig. 1. Northwest Java Basin location map.


HP 5890 GC equipped with a 60 m�0.25 mm i.d capil-lary column coated with a 0.1 mm phenyl methylpoly-siloxane (DB-5) phase. The concentrations of selectedbiomarkers were determined by adding standards (100

ppm), 5b-cholane for saturates and d10-anthracene foraromatics. Response factors for the components of interestrelative to the internal standards were taken as a nominal

value of 1.0. While this is not strictly true, it is e�ective forthe purpose of comparing samples with one another.Carbon isotopic analyses were performed at the stable

isotope laboratory at U. T. Dallas and at Coastal Sci-

ence Laboratory (Austin, TX). Stable carbon isotoperatios are reported in parts per thousand (per mil) rela-tive to the PDB standard with a precision of 0.2%.

Fig. 2. Generalized stratigraphic column for Northwest Java Basin (adapted from Arpandi and Patmosukismo, 1975; Arianto, 1993;

Gresko et al., 1995; Noble et al., 1997).


Table 1

Sample name, location, reservoir age and inferred depositional environment of source rocks for NW Java Basin

Well Depth (m) Sub-basin Reservoir age Depositional environment of source rock

CCH-P5 1027±1031 Rengasdengklok High Upper Miocene Near shore

SIN-5 1456±1468 Jatibarang Upper Miocene Near shore

RDL-2 1071±1075 Rengasdengklok High Middle Miocene Near shore

JTB-194 468±473 Jatibarang Middle Miocene Near shore

CLT-1 1800±1803 Pasirputih Middle Miocene Near shore

SDS-1 1840±1846 Pasirputih Middle Miocene Near shore

PGD-3 1693±1695 Pasirputih Middle Miocene Near shore

RDG-45 1411±1418 Jatibarang Middle Miocene Near shore

PCT-1 1538±1548 Jatibarang Middle Miocene Near shore

CMS-21A 1807±1811 Pamanukan High Middle Miocene Fluvio - Deltaic

SNT-1 1328±1331 Jatibarang Lower Miocene Near shore

TBN-1 1821±1827 Ciputat Lower Miocene Fluvio - Deltaic

PMK-2 2195±2199 Pasirputih Lower Miocene Near shore

CLU-5 1811±1816 Pasirputih Lower Miocene Near shore

JNG-1 832±838 Ciputat Lower Miocene Near shore

BJR-2 1578±1580 Pasirputih Lower Miocene Fluvio - Deltaic

WLU-2 1575±1579 Pamanukan High Lower Miocene Fluvio - Deltaic

TGB-24 1621±1667 Pamanukan High Lower Miocene Near shore

JTB-128 1845±1849 Jatibarang Lower Miocene Fluvio - Deltaic

CMB-7 2367±2372 Pamanukan High Upper Oligocene Fluvio - Deltaic

SBD-1 2111±2114 Pamanukan High Upper Oligocene Fluvio - Deltaic

KPT-1 1645±1647 Jatibarang Upper Oligocene Fluvio - Deltaic

Table 2

Bulk oil characteristics of NW Java oils

13C (PDB, per mil) Oil composition (%)

Well API %S Oil Sat Aro NSO Asp Sat Aro NSO Asp Sat/Aro

CCH-PS 20.7 1.19 ÿ23.0 ÿ22.0 ÿ23.2 ÿ24.5 44 45 11 ± 1.0

SIN-5 36.1 0.09 ÿ27.2 ÿ28.9 ÿ26.8 ÿ27.6 ÿ27.0 64 21 11 3 3.1

RDL-2 32.1 0.13 ÿ27.1 ÿ27.9 ÿ26.6 ÿ27.0 ÿ27.4 63 22 11 4 2.8

JTB-194 17.8 0.42 ÿ27.2 ÿ28.2 ÿ26.8 ÿ27.0 ÿ26.2 39 39 18 3 1.0

CLT-1 32.9 0.08 ÿ28.3 ÿ29.6 ÿ27.1 ÿ28.2 ÿ28.1 64 29 6 1 2.2

SDS-1 43.6 0.07 ÿ27.0 ÿ28.7 ÿ26.2 ÿ28.7 ± 72 21 7 ± 3.4

PGD-3 50.1 0.15 ÿ27.0 ÿ27.9 ÿ25.8 ÿ29.2 ± 72 14 13 1 5.2

RDG-45 30.2 0.24 ÿ27.5 ÿ29.0 ÿ16.4 ÿ27.0 ÿ25.3 62 25 12 1 2.4

PCT-1 40.3 0.08 ÿ27.2 ÿ28.3 ÿ26.2 ± ± 76 20 4 ± 3.7

CMS-21A 40.6 0.08 ÿ27.1 ÿ27.8 ÿ26.4 ÿ27.6 ÿ26.3 58 19 15 8 3.1

SNT-1 53.4 0.02 ÿ26.6 ± ± ± ± ± ± ± ± ±

TBN-1 32.0 0.11 ÿ28.6 ÿ30.2 ÿ27.7 ÿ27.5 ÿ26.6 71 16 9 5 4.5

PMK-2 35.2 0.05 ÿ28.3 ÿ29.2 ÿ26.8 ÿ28.5 ± 58 36 6 ± 1.6

CLU-5 33.7 0.12 ÿ28.2 ÿ28.9 ÿ26.4 ÿ28.2 ± 68 25 7 ± 2.8

JNG-1 28.1 0.25 ÿ26.4 ÿ27.7 ÿ25.6 ÿ26.1 ÿ24.4 62 24 11 3 2.5

BJR-2 43.3 0.02 ÿ27.3 ÿ26.5 ÿ25.7 ± ± 51 44 4 ± 1.2

WLU-2 33.1 0.07 ÿ27.9 ÿ28.4 ÿ27.9 ÿ28.8 ÿ27.0 38 9 13 40 4.3

TGB-24 35.6 0.20 ÿ27.6 ÿ29.1 ÿ26.4 ÿ27.2 ÿ25.9 64 27 8 1 2.4

JTB-128 22.7 0.09 ÿ28.4 ÿ29.5 ÿ27.2 ÿ27.4 ÿ27.9 52 21 13 14 2.4

CMB-7 23.8 0.14 ÿ28.6 ÿ29.6 ÿ27.6 ÿ27.6 ÿ27.9 44 31 17 8 1.4

SBD-1 25.8 0.16 ÿ28.2 ÿ29.1 ÿ27.1 ÿ27.3 ÿ27.0 53 22 13 12 2.5

KPT-1 45.1 0.02 ÿ27.6 ÿ28.0 ÿ26.8 ÿ28.4 ± 68 27 5 ± 2.6

Notes: ÿ, No data acquired or available.


4. Results

4.1. Bulk oil characteristics

The locations and ages of crude oils and rock samplesused in this study are given in Table 1, with some asso-

ciated geochemical data presented in Tables 2±4. Bulkoil characteristics for 22 onshore NW Java Basin crudeoils are listed in Table 2. The oils vary in appearance at

room temperature from highly viscous black tarry sub-stances to colourless light oils. API oil gravities rangefrom 17.8 to 53.4�, re¯ecting oil classi®cations from

heavy to light crude oils and retrograde condensates(Hunt, 1996). As shown in Fig. 3, a plot of API gravityagainst depth for the oils, three out of ®ve of the med-

ium gravity oils occur at depths deeper than 1800 mwhile 12 out of 15 light oils and retrograde condensatesoccur at depths between 1300 and 1850 m. The twoheavy oils identi®ed in this study occur at depths shal-

lower than 1300 m. These data show that most of thelighter API gravity crude oils and retrograde con-densates are found in shallower reservoirs, whereas the

medium API gravity crude oils are generally found indeeper reservoirs (exceptions are JNG-1 and RDG-45).

Observations such as these, in which the shallowerreservoirs in a basin contain the lighter gravity ¯uids,are consistent with data suggesting that evaporativefractionation processes may be responsible (Dzou,

1990; Dzou and Hughes, 1993; Holba et al., 1996). Theheavy API gravity oils (CCH-P5 and JTB-194) analyzedin this study are located in the shallowest sections of the

basin and display obvious signs of biodegradation,including depletion of n-alkanes and isoprenoids (Fig.4). Table 2 shows that CCH-P5 and JTB-194 also pos-

sess the highest sulphur contents of the oils analyzedwith values of 1.19 and 0.42% S, respectively (averageof NW Java oils in this study is 0.18% S), a character-

istic often associated with biodegraded petroleums(Hunt, 1996). CCH-P5 oil is unusual in that it is reportedto be derived from a carbonate source rock (Napitupuluet al., 1997).

Table 2 lists the stable carbon isotope measurements(expressed in per mil PDB) for the 22 NW Java oils,including data for the respective saturate, aromatic,

Table 3

Geochemical data for NW Java Basin oilsb

Well Pr Pr CPI C27R 20S 20S Oleanane DBT % Rc

Ph n-C17 CPI C29R 20S+20R 20S+28R C30 Hopane Phen

CCH-P5 4.9 3.3 1.2 1.10 0.24 ±a 0.04 0.20 0.54

SIN-5 7.9 1.8 1.2 1.54 0.52 0.53 0.04 0.11 0.93

RDL-2 6.1 1.6 1.2 1.43 0.57 0.71 0.39 0.10 0.88

JTB-194 ± ± ± 1.61 0.58 0.67 0.46 0.02 0.69

CLT-1 9.8 5.5 1.2 2.63 0.54 0.59 0.36 0.06 0.75

SDS-1 9.7 3.2 1.2 1.43 0.45 0.56 0.43 0.06 0.74

PGD-3 13.8 2.1 ± 1.49 0.41 ± 0.30 0.03 0.78

RDG-45 5.3 1.3 1.2 2.04 0.56 0.67 0.40 0.30 0.98

PCT-1 11.6 1.7 1.3 1.85 0.47 ± 0.29 0.09 0.88

CMS-21A 8.2 2.0 1.2 1.10 0.54 0.51 0.31 0.12 0.88

SNT-1 5.6 1.8 ± ± ± ± ± ±

TBN-1 7.7 2.2 1.2 0.60 0.50 0.64 0.39 0.09 0.82

PMK-2 7.9 1.6 1.1 4.00 0.63 0.71 0.45 0.16 0.85

CLU-5 9.0 2.0 1.2 2.56 0.59 0.78 0.37 0.13 0.92

JNG-1 5.1 1.3 1.2 2.27 0.55 0.75 0.76 0.56 0.89

BJR-2 14.4 2.0 ± 1.04 0.47 ± 0.26 0.13 0.86

WLU-2 2.4 1.4 1.1 0.55 0.50 0.52 0.15 0.06 0.75

TGB-24 6.5 1.6 1.2 2.27 0.52 0.78 0.46 0.19 0.97

JTB-128 8.1 4.6 1.3 0.72 0.53 0.54 0.24 0.12 0.77

CMB-7 9.6 11.3 1.4 0.85 0.54 0.58 0.43 0.06 0.76

SBD-1 7.6 2.7 1.2 0.93 0.52 0.53 0.30 0.10 0.85

KPT-1 16.7 4.1 1.2 0.89 0.50 0.54 0.37 0.04 0.74


De®nitions and methods of measurement: Pr/Ph=pristane/phytane (GC); Pr/n-C17=pristane/n-heptadecane (GC); CPI=carbon

preference index (GC); C27R/C29R=C27 aaa 20R-cholestane/C29 aaa 20R-ethylcholestane (m/z 217); 20S/(20S+20R), 20S and

20R diastereomers of 5a(H),14a(H),17a(H)-ethylcholestane (m/z 217); 20S/20S+28R triaromatics, C20 pregnane/(C20 pregnane

+C28 20R stigmastane) (m/z 231); Ol/C30H=18a(H)-oleanane/C30 17a(H)-hopane (m/z 191); DBT/PHEN=dibenzothiophene (m/z

184)/phenanthrene (m/z 178); %Rc=calculated equivalent vitrinite re¯ectance, 0.60 (MPI-1)+0.40 (for Ro<1.35%) (m/z 192).


NSO and asphaltene fractions for each oil. Fig. 5(a) and(b) present `Galimov' or `Stahl' -type isotope plots

(Galimov 1973; Stahl 1978) for nine oils that are classi-®ed as `pristine' or `pristine-like' based on whole oilchromatograms, in which, each oil displays relativelylimited signs of post-generative alteration e�ects such as

evaporative fractionation, water washing or biode-gradation. Hence, while the de®ned `pristine-like' oilswe have used in this study may well re¯ect recharged

residual oils, they approximate as best as is possiblewhat the original pristine oil may have actually lookedlike (i.e. `pristine-like', see Figs. 13 and 14). From Fig. 5,

two distinct isotope trends can be discerned among thenine oils plotted. Group 1 samples (Fig. 5a: PMK-2,SIN-5, TGB-24, RDG-45, SDS-1 and RDL-2) have a

heavier saturate fraction carbon isotopic composition(d13Csat>ÿ29.2%) than Group 2 samples (Fig. 5b:TBN-1, JTB-128 and CMB-7) (d13Csat <ÿ29.5%).Similarly, for the aromatic fraction, Group 1 samples

have heavier carbon isotopic values (d13Caro>ÿ26.8%)relative to Group 2 samples (d13Caro>ÿ27.2%). Theindividual line pro®les these types of curves depict can

provide important geochemical correlation information.Typically, oils and bitumens show a general enrichmentin 13C for fractions of increasing polarity and boilingpoint. However, variations in source rock facies con-

comitant with secondary processes such as migration,deasphalting or thermal maturation may in¯uence theisotopic composition of each fraction resulting in ìrre-

gular' line pro®les (Chung et al., 1981). For the desig-nated `pristine-like' oils shown in Fig. 5, no signi®cante�ect of secondary processes are inferred, hence the

variations in the isotopic line pro®les between theGroup 1 and Group 2 oils may, therefore, be attributedto source rock heterogeneity. For Group 1 oils, these

line variations are re¯ected as heavier isotopic char-acteristics of the whole oil, saturate and aromatic frac-tions, relative to Group 2 oils, which are characterizedby a lighter isotopic line pro®le. Interestingly, the iso-

topic values recorded for the NSO fractions in both oilgroups are quite similar.

4.2. Principal component analysis

In order to examine the two oil groupings in greater

detail, principal component analysis (PCA) was per-formed using bulk oil, carbon isotope and biomarkerdata (Tables 2 and 3). Due to the fact that many integral

geochemical variables will be compromised as a con-sequence of evaporative fractionation, biodegradationand water washing phenomena, an initial determinationof the most important or signi®cant oil parameters was

®rst established using 17 geochemical variables for thenine inferred pristine or pristine-like oils. Fig. 6(a)shows a scores plot of all 17 biomarker and bulk oil

variables for the samples. From this plot, the `weighting'of the variables comprising the ®rst and most signi®cantprincipal component (accounting for 36% of the var-

iance) clearly indicates two collective sets of dependentvariables, with one set of variables representing samplesexhibiting à more terrestrial in¯uence' and the secondset of variables representing samples exhibiting à more

marine in¯uence.' Variables indicative of terrestrial orhigher plant contributions include compounds andparameters such as oleanane, high pristane/phytane, C29

sterane and C19 and C20 tricyclic terpanes (Powell andMcKirdy, 1973; Reed, 1977; Hunt, 1996). Variablessuggestive of a marine in¯uence may include high

abundance of dibenzothiophene, C23 tricyclic terpane,C27 sterane and C29 hopane compounds (Huang andMeinschein, 1979; Hughes, 1984; Peters and Moldowan,

1993). Taking into account the ¯uvial deltaic source ofthe NW Java oils, the oils can be distinguished intofamilies containing a more signi®cant terrestrial orhigher plant character and families exhibiting more of a

marine in¯uence.Fig. 6(b) is a factor plot of the main principal com-

ponents for 21 NW Java oils (SNT-1 was omitted due to

Table 4

Compositional ratios for gasoline fraction of NW Java Basin

oils

Well Heptane

value (H)

Isoheptane

value (I)

Toluene/n-C7

(B)

n-Heptane/MCH

(F)

CCH-P5 5.0 0.1 17.9 0.1

SIN-5 17.3 1.0 1.2 0.4

RDL-2 10.3 0.8 1.0 0.2

JTB-194 0.4 ± ± ±

CLT-1 ± ± ± ±

SDS-1 7.6 1.0 0.1 0.2

PGD-3 19.6 1.3 1.0 0.5

RDG-45 14.7 0.9 3.1 0.3

PCT-1 3.1 0.9 0.2 0.1

CMS-21A 20.2 1.0 10.6 0.4

SNT-1 22.5 1.3 0.8 0.5

TBN-1 7.8 0.8 0.5 0.2

PMK-2 19.4 1.0 5.8 0.4

CLU-5 11.5 1.1 2.6 0.5

JNG-1 3.9 0.7 1.0 0.1

BJR-2 18.3 1.1 4.6 0.3

WLU-2 12.0 0.5 1.1 0.3

TGB-24 16.9 1.2 1.5 0.4

JTB-128 14.4 0.7 1.8 0.3

CMB-7 10.7 0.5 2.3 0.2

SBD-1 3.5 0.4 0.5 0.1

KPT-1 17.3 0.8 1.4 0.3


De®nitions and methods of measurements (see Thompson,

1979): Heptane value=100*n-C7/(CH+2MH+23DMP+11DMCP

+3MH+1C3DMCP+1T3DMCP+1T2DMCP+NC7+MCH).

Isoheptane value=(2MH+3MH)/(1C3DMCP+1T3DMCP+

1T2DMCP).


lack of biomarker data). Due to the gross compositionalvariations associated with the oils (i.e. light, medium,and heavy/biodegraded oils), only those `more robust'variables less likely to be a�ected by post-generative

processes (e.g., evaporative fractionation, biodegrada-tion, migration fractionation) were included. Conse-quently, a�ected variables such as CPI, Pr/Ph, Pr/nC-17

and carbon isotope data were omitted in order to mini-mize potential weighting of the principal componentswith data likely to skew the results and subsequent

interpretations. The two oil family groupings, initiallybased on carbon isotope data of the pristine-like oils(Fig. 5), are also clearly seen in Fig. 6(b). In addition to

the pristine-like oils previously classi®ed, samples WLU-2, SBD-1, KPT-1 and CMS-21A are correlated withGroup 2 oils, suggested to exhibit a relatively greaterterrestrial in¯uence. Furthermore, samples PGD-3,

PCT-1, JTB-194, CLT-1, CLU-5 and JNG-1 can beassociated with Group 1 oils, suggested to exhibit arelatively greater marine in¯uence. Sample CCH-P5,

proposed to be derived from a carbonate source rock, isthe only oil that could not be correlated to either group.The PCA methodology employed here involves ®rst

determining oil groupings/families based on pristine or

pristine-like oils, before subsequently incorporating allother altered oils/condensates, (i.e. those a�ected by theaforementioned secondary phenomena) using a more

restricted group of `robust' PCA variables. This metho-dology serves to constrain and guide the statistical ana-lysis, such that provided the initial groupings are

maintained, the oils/condensates a�ected by secondaryprocesses can then also be correlated with some con-®dence. While this procedure can not claim to be 100%

foolproof, it does provide a level of statistical con®denceand e�cacy otherwise unobtainable, and allows for oil/condensate correlations on seemingly `impossible sam-ples' that may be devoid of important geochemical

parameters. This type of `guarded' statistical treatmentmay well represent the safest approach to determining oil/condensate correlations and family groupings involving

Fig. 3. API gravity versus depth for NW Java oils. Twelve of 15 light oils and retrograde condensates occur at depths between 1300

and 1850 m. GOR=Gas to oil ratio (McCain, 1973; Hunt, 1996).


hydrocarbon ¯uids that are severely a�ected by second-ary processes.

4.3. Source parameters

The main source intervals attributed to the NW JavaBasin oils are the ¯uvial-deltaic sub-units of the Upper

Oligocene Talangakar formation (Fig. 2) (Fletcher andBay, 1975; Roe and Polito, 1977; Gordon, 1985;Robinson, 1987; Pramono et al., 1990; Wu, 1991; Noble

et al., 1991, 1997). Pristane-to-phytane ratios for thepristine-like oils used in this study are very high, aver-aging 7.6 (� � 1:4), suggesting a relatively oxic deposi-

tional setting. This ratio must be used with somecaution as an indicator of redox condition during sedi-mentation, as it may also re¯ect the relationship

between the chemistry of the environment and the pre-cursor organisms (Didyk et al., 1978; ten Haven et al.,1987; Dzou, 1990).Oleanane, a land plant biomarker derived from angio-

sperms is present in all NW Java oils used in this study,with the exception of SNT-1. Sample SNT-1 represents anextreme example of a retrograde condensate and is devoid

of any high molecular weight species greater than 15carbons (Fig. 7). Many of the oils used in this study arewaxy and exhibit high abundances of para�ns between

C20 and C35. Carbon preference indices for these oilsaverage 1.2, re¯ecting strong contributions of higherplant-derived C25 to C35 odd-carbon-numbered paraf-®ns. Based on these para�n distributions, all of the

Group 2 oils, characterized as containing a relativelygreater higher plant contribution, are waxy, while asigni®cant number of Group 1 oils (9 of 12), character-

ized as containing a relatively greater marine in¯uence,are non-waxy.Fig. 8 is a ternary diagram of the relative abundance

of C27, C28 and C29 steranes for the oils used in thisstudy. Data from this plot show that Group 1 samplesare clearly weighted in favour of the C27 steranes while

Group 2 samples are clearly weighted towards a highercontent of C29 steranes. These data further support theinterpretation that Group 1 samples exhibit a greatermarine character relative to the more terrestrially-

dominated Group 2 samples. It is important to be awareof possible bicadinane interferences on sterane m/z217 ion chromatograms, especially when dealing with

Fig. 4. Gas chromatograms of two biodegraded oils (JTB-194 and CCH-P5).


south-east Asian petroleum systems. Our analysesshowed only the C28 20S sterane stereoisomer (not used)was compromised by bicadinane interference.Fig. 9, a plot of the saturate versus aromatic carbon

isotope values for the nine pristine-like oils, shows veryclearly the distinction between the assigned pristine-likeoils ofGroups 1 and2. Isotopic di�erences can generally be

attributed to proportional di�erences in the amount ofhigher plant and algal derived organic matter in the sourcerock that generated the crude oils.Modern terrestrial lipids

from C3 plants have light isotopic signatures, usuallyd13Csat=ÿ30% or lighter, which in distal deltaic systemsmaybecomeprogressively dilutedwith isotopically heavieralgal material in more distal marine environments (Collis-

ter et al., 1994; Collister andWavreck, 1997). TheGroup 2oils are isotopically lighter than Group 1 oils, in goodagreement with the aforementioned proposal that the

Group 2 oils are more terrestrially dominated with iso-topically lighter, plant organic material. These data sup-port a proximal depositional setting for the Group 2 oils.

The Group 1 oils, while still terrestrially dominated, pos-sess a greater proportion of isotopically heavier, marinealgal organic matter consistent with an inferred distal

depositional setting for these oils.

4.4. Maturity parameters

Sterane and triterpane hydrocarbon biomarkers areuseful maturity indicators and geochemical correlationtools (cf. Peters and Moldowan, 1993). Evaluation of

condensate and light crude oil maturities may be di�-cult, however, due to the lean concentration of relativelyhigher molecular weight sterane and triterpane hydro-carbons (C27+ compounds) in these oils.

Fig. 10(a) is a plot of API Gravity against 20S/(20S+20R) C29 sterane isomerization. With the excep-tion of biodegraded sample CCH-P5, all the oils show

sterane isomerization values ranging between 0.41±0.63.Higher sterane values than can be derived from normalsterane isomerization equilibration (equilibrium range

0.52±0.55, cf. Peters and Moldowan, 1993) were recor-ded for ®ve of the NW Java Basin oils. While biode-gradation can result in arti®cially high sterane ratiosabove 0.55, this e�ect would provide a possible expla-

nation for biodegraded sample JTB-194 only (cf. Petersand Moldowan, 1993). Coelution problems with afore-mentioned species such as bicadinanes on m/z 217

selected ion chromatograms, used to calculate steraneisomerization ratios, are common with Tertiary-age oilsfrom Indonesia and may arti®cially elevate sterane ratios.

Only bicadinane T1 was found to coelute with the C28

20S sterane (not used) in this study. Interestingly, nineof the 14 higher gravity oils, such as PGD-3, KPT-1,

BJR-2 and PCT-1 (but not SNT-1), have lower steraneisomerization values compared to the values recordedfor the heavier medium-gravity oils. These data indicatethat the high gravity oils used in this study are probably

not high maturity oils, rather their origin may beattributed to evaporative fractionation processes leadingto retrograde condensates formation. Further support for

Fig. 5. `Galimov' or `Stahl'-type isotope plots for nine pristine-like NW Java Basin oils showing two distinct isotope trends.


this interpretation is obtained from calculated equiva-lent vitrinite re¯ectance values based on the MethylPhenanthrene Index-1 (MPI-1) (Radke et al. 1986).

Calculated equivalent vitrinite re¯ectance (% Rc) valuesfor the NW Java Basin oils range from 0.69 to 0.98%[Fig. 10(b)], suggesting early to middle maturity levels

for the parent source rock. These values are in agree-ment with published estimates of Talangakar sourcerock maturity levels (VRo of 0.7±0.75 %) in the NW

Java Basin (Pramono et al., 1990). Therefore, geologicfactors rather than source maturity appear to be gov-erning and controlling the evaporative fractionationphenomena observed in this study.

Light hydrocarbon maturity data obtained fromgasoline range hydrocarbon components are also asses-sed. Fig. 11, a plot of isoheptane values (I) against

heptane values (H), indicates that all the light oils/ret-rograde condensates (with the exception of biodegradedsamples CCH-P5, JTB-194, CLT-1) are of normal

maturity. These data further support the proposal thatthe high gravity oils in this study, are most likely theresult of evaporative fractionation processes and not

high thermal maturity.

5. Discussion

5.1. Evaporative fractionation

The distributions and concentrations of gas (C1±C4)and gasoline (C5±C14) range components in oils are a�ec-ted by a complex combination of subsurface processes

Fig. 6. (a) Scores plot of 17 geochemical variables for nine pristine-like NW Java Basin oils. One set of variables re¯ects a more

terrestrial in¯uence with the second set re¯ecting a more marine in¯uence (see Appendix for parameter de®nitions). (b) Factor plot of

the main principal components for 21 NW Java Basin oils illustrating two oil families. A subset of 10 `robust' selectively screened

geochemical variables was used for the bulk sample set.


related to source type, migration, evaporative fractio-nation, water washing, and thermal maturation (Ley-thaeuser et al., 1979; Welte et al., 1982). Physical

partitioning of oils may result in compositional changesduring remigration, greatly a�ecting parameters com-monly used for geochemical characterization of migration

¯uids (Thompson andKennicutt, 1990). Thompson (1988)noted that migration e�ects in deltaic environments, whereoil tend to undergo multiple migration processes, are very

pronounced and require special consideration.The 22 onshore NW Java Basin oils were analyzed for

gas and gasoline range hydrocarbons, with associated

Fig. 7. Gas chromatogram of retrograde condensate SNT-1.

These high gravity ¯uid types are usually found in the shal-

lowest reservoirs.

Fig. 8. Ternary diagram of relative abundance of C27±C28±C29 aaa 20R steranes in NW Java Basin oils.

Fig. 9. d13C of saturate fraction versus d13C of aromatic fraction

for nine NW Java Basin pristine-like oils of Group 1 and Group

2. (Sofer line, d13C ARO=1.14 d13C SAT+5.46, Sofer, 1984).


gasoline compositional ratios shown in Table 4.Fig. 12 is a plot of two light hydrocarbon parameters

(n-heptane/methylcyclohexane against toluene/n-hep-tane) used to identify those samples a�ected by biode-gradation-water washing and evaporative fractionation

processes. High toluene/n-heptane ratios for the major-ity of the 22 oils suggest that evaporative fractionationprocesses have occurred to varying extents. Formationof retrograde condensates, such as those described pre-

viously, from pristine oils by post-generative processeslike evaporative fractionation also results in the forma-tion of `residual oils.' Residual oils are characterized by

loss of low molecular weight hydrocarbons, enhancedconcentration of aromatic compounds, and lower API

gravities than the original oils (Thompson, 1987; Dzou,1990; Dzou and Hughes, 1993). Figs. 13 and 14 showwhole oil gas chromatograms of Groups 1 and 2 pris-tine-like oils, respectively, together with associated ret-

rograde condensate and residual oil examples. Fig. 13(b)illustrates typical features associated with a majority ofGroup 1 oils (9 of 12), including a non-waxy para�n

distribution showing n-alkanes eluting between nC5 andapproximately nC30. Other features include high pris-tane/phytane ratios and a strong abundance of methyl-

cyclohexane (MCH). Fig. 13(a) is a Group 1 retrogradecondensate exhibiting prominent concentrations ofhydrocarbons eluting between nC5 and nC18, but devoidof higher molecular weight species above nC18, indicat-

ing that this oil is a product of evaporative fractionationprocesses. Figs. 13(c) depicts a Group 1 residual oil withan absence of low molecular weight hydrocarbons

below nC12, demonstrating that the light hydrocarbonsoriginally associated with this oil have been removedand transported away.

Fig. 14(b) shows typical features associated with all ofthe Group 2 oils (with the exception of extremely lightoils) from the NW Java Basin, including a waxy para�n

distribution indicated by the abundance of odd carbon-numbered n-alkanes eluting between nC20 and nC35.Other features, similar to those of Group 1 oils, includehigh pristane/phytane ratios and relatively high con-

centrations of methylcyclohexane (MCH). Fig. 14(a)and (c), as with Fig. 13(a) and (c), illustrates the forma-tion of derivative retrograde condensates and residual

Fig. 10. (a) 20S/(20S+20R) aaa C29 sterane versus API grav-

ity. Most NW Java Basin oils have sterane isomerization values

ranging from 0.41 to 0.63 (except CCH-P5). (b) Calculated

vitrinite re¯ectance (% Rc) versus API gravity for NW Java

Basin oils, suggesting early to middle maturity levels for the

parent source rock.

Fig. 11. Heptane value (H) versus isoheptane value (I) for NW

Java Basin oils. None of the light oils/retrograde condensate

are overmature (after Thompson, 1987; Holba et al., 1996).


oils from pristine unaltered oils via evaporative fractio-nation processes. From these examples, it is clear that

evaporative fractionation processes are important geo-logical phenomena a�ecting oils in the NW Java Basin.Vertical migration is very prevalent in this region due to

an extensive and complex fault system that has providedthe main conduits for hydrocarbon migration, and nodoubt, also is responsible for the evaporative fractiona-

tion processes a�ecting many of the oils.Evaporative fractionation e�ects, in addition to

altering the gross n-alkane distributions of the NW Java

oils, appear to have also induced changes in the pristane/phytane ratios of the retrograde condensates and resi-dual oils. Fig. 15 is a plot of Pr/Ph against Pr/n-C17 foreach of the Group 1 and Group 2 oils shown in Figs. 13

and 14. Retrograde condensates in each group havehigher pristane/phytane values relative to the parentpristine oil; conversely, residual oils in each group have

lower pristane/phytane values relative to the parentpristine oil. Evaporative fractionation processes appear

to increase the abundance of pristane relative to phytanein retrograde condensates, while reducing the abun-dance of pristane relative to phytane in the residual oils.

These observations, therefore, have obvious and impor-tant implications in oil correlation studies that comparepristane and phytane distributions in retrograde con-

densate, pristine and residual oil types.

5.2. Biodegradation and water washing

Biodegraded oils are represented by JTB-194 andCCH-P5; these oils have low API gravities (17.8±20.7�),low saturate/aromatic ratios, low para�nic contents,

and a de®ciency in low molecular weight aromatichydrocarbons (Fig. 4 and Table 2). These oils are loca-ted in the shallowest reservoirs in the basin, where

Fig. 12. Light hydrocarbon parameters [n-heptane/methylcyclohexane (F) versus toluene/n-heptane (B)] illustrating changes in oil

composition brought about biodegradation-water washing and evaporative fractionation processes (after Thompson, 1987).


favorable conditions for biodegradation are likely toexist.

Water washed oils are commonly characterized by theloss of low molecular weight aromatic hydrocarbonssuch as benzene and toluene (Palmer, 1984; Lafargue

and Barker, 1988). Fig. 16 depicts three whole oil chro-matograms of Group 1 oils that illustrate the e�ects ofwater washing on a retrograde condensate formed from

a related pristine-like oil. Samples PCT-1 and PGD-3[Fig. 16(a) and (b)] represent examples of Group 1 ret-rograde condensates formed by evaporative fractiona-

tion processes on a parent Group 1 pristine-like oil suchas TGB-24 [Fig. 16(c)]. All three oils have similarabundances of cyclohexane (CH) and methylcyclohex-ane (MCH), supporting previous assertions that satu-

rated hydrocarbons are less likely than aromatichydrocarbons to be a�ected by evaporative fractiona-tion and water washing processes. In addition, within

Fig. 13. Gas chromatograms of representative Group 1 oils

illustrating compositional changes associated with evaporative

fractionation: (a) a retrograde condensate with high abundance

of hydrocarbons eluting between nC5 and nC18, suggesting

formation by evaporative fractionation; (b) typical pristine-like

Group 1 oil; and (c) 1 residual oil has lost its light hydro-

carbons.

Fig. 14. Gas chromatograms of representative Group 2 oils

illustrating compositional changes associated with evaporative

fractionation processes: (b) a typical pristine-like Group 2 oil,

(a and c) a retrograde condensate and a residual oil derived

from a pristine-like oils as a result of evaporative fractionation.

Fig. 15. E�ect of evaporative fractionation on pristane/phy-

tane versus pristane/n-C17 parameters for Group 1 and Group

2 pristine-like oils. Pristane/phytane values are higher in retro-

grade condensates, and lower in residual oils relative to the

parent pristine-like oil in each group.


saturated hydrocarbon classes, naphthenic compoundtypes such as CH and MCH are more resistant thanlinear n-alkanes to biodegradation processes. SamplePCT-1 clearly shows the e�ects of water washing as

observed by the loss of benzene, toluene and xylene.

Concomitant with water washing, this oil may have alsoundergone limited microbial degradation resulting inremoval of the C5±C7 linear alkanes. With respect to thelight aromatic components in retrograde condensate

PGD-3 and pristine-like oil TGB-24, no loss of benzene,

Fig. 16. Whole oil gas chromatograms of Group 1 oils illustrating the e�ects of water washing on a retrograde condensate example (a)

formed from a related pristine-like oil (c). (a) and (b) are examples of Group 1 retrograde condensates formed by evaporative frac-

tionation processes, and (c) an inferred pristine-like Group 1 oil.


toluene or xylene appear to have occurred. These datasuggest that water washing processes have a�ected, andto a signi®cantly larger extent, retrograde condensatePCT-1 compared to retrograde condensate PGD-3 and

pristine-like oil TGB-24. As all three oils are located atsimilar depths, between 1500 and 1700 m, it may beenvisaged that evaporative fractionation processes

responsible for the formation of retrograde condensatePCT-1 also exposed this oil, in particular, to activecharged meteoric waters along the remigration pathway.

The e�ect of water washing in these instances canclearly be discerned against additional post-generativeprocesses such as evaporative fractionation and biode-

gradation. Fig. 16 shows that oils experiencing eva-porative fractionation may also exhibit the e�ects ofwater washing during re-migration from the originalreservoir into a shallower reservoir, or in the new reser-

voir itself.

5.3. Migration pathways of NW Java oils

Previous studies in the NW Java Basin concluded thatmigration pathways occurred laterally and/or vertically

out of the source area (Wu, 1991; Noble et al., 1997). Inthese reports, extended lateral migration pathways inthis basin were suggested to be oriented in a pre-

dominantly north±south direction along faults withvertical and cross-stratal migration conduits attributedto in-situ faulting concomitant with rapid transport of¯uids during periods of tectonic activity. Oil migration

along these complex fault systems is dependent on thelocation of nearby source rocks that have reached thermalmaturity. As oil migration distances in the NW Java Basin

are anticipated to be relatively short, the identi®cation ofdi�erent geochemical characteristics of reservoired oilsmay actually provide unique insights into source rock

variability in this region. Results from this study indi-cate that all the terrestrially-dominatedGroup 2 oils, withthe exception of TBN-1, are located in close proximityto the eastern ¯ank of the onshore NW Java Basin in

reservoirs on, or near, the present-day coastline. Oilswith Group 1 characteristics are exclusively located inonshore reservoirs in the central, western and southern

interior regions of the NW Java Basin. Fig. 17 shows aninferred sequence of depositional settings of the Talan-gakar formation from ¯uvial deltaic to nearshore mar-

ine environments based on biostratigraphic andpaleoenvironmental analyses of cores, drill cuttings andwell logs from the Late Oligocene to the Early Miocene

(Wu, 1991). As Fig. 17 illustrates, the Talangakar for-mation represents a marine transgressional sequenceduring this time interval, with ¯uvial deltaic depositsoverlain by nearshore marine sediments. The main

paleo-delta complex is located in the central portion ofthe present-day o�shore Arjuna Basin and in the eastern¯ank region near the present coastline. More marine

in¯uenced Group 1 oils appear to be concentrated in

present day central onshore regions (Pasirputih sub-basin) that correspond to nearshore marine and pro-delta Talangakar deposits. In addition, more terrestrial

in¯uenced Group 2 oils appear to be concentrated inpresent-day central o�shore regions (Wu, 1991; Nobleet al., 1997) and nearshore eastern coastal regions of theNW Java Basin that correspond to delta plain Talangakar

transgressive deposits. It is proposed, therefore, that themain delta plain to ¯uvial-deltaic complex of theTalangakar Formation is the source region for Group 2

Fig. 17. Paleogeographic maps of Talangakar formation

depositional environments in the NW Java Basin. (a) maximum

marine transgression and retreat of shoreline during NN1/N4-

Early Miocene time; (b) retreat of shoreline due to marine

transgression during NP25-Late Oligocene; and (c) initial

topographic depression ®lled with non-marine to marginal

marine sediment during NP24-Late Oligocene (from Wu, 1991).


oils, while prodelta to nearshore Talangakar depositsare the likely source rocks for the Group 1 oils.Proposed migration pathways of Group 1 and Group

2 oils from identi®ed mature source pods to identi®ed

NW Java Basin reservoirs are indicated (Fig. 18).Hydrocarbon migration pathways in the NW JavaBasin were previously largely proposed on the basis of

nearby reservoir locations and known geological faultconduits (Wu, 1991; Noble et al., 1997). The identi®ca-tion in this work of two distinct oil families in the NW

Java Basin assists in the identi®cation of hydrocarbonmigration pathways based on geochemical correlation.These data help to constrain and better re®ne likelymigration scenarios.

6. Conclusions

The main source rock of onshore NW Java Basin oilsis the ¯uvio-deltaic to nearshore Talangakar Formation.

The oils consist of light oils/retrograde condensates,residual oils, and pristine oils that have been a�ected byevaporative fractionation, biodegradation, and water

washing.NW Java oil samples are less mature than expected

for light oils and condensates generated by extremethermal processes. The compositional patterns and

maturity estimates infer that the origin of the light oilsand retrograde condensates in this basin are, in fact, dueto migration and evaporative fractionation e�ects.

Principal Component Analysis was e�ectively used tocorrelate retrograde condensate, pristine and residual oiltypes after screening of labile biomarker variables. Ingeneral, these oils can be classi®ed into a more marine

in¯uenced group and a more terrestrial dominatedgroup.Pristane/phytane ratios are signi®cantly a�ected by

evaporative fractionation processes, with light oils orretrograde condensates having higher Pr/Ph ratios andresidual oils having lower ratios than original parent

pristine oils.The e�ects of water washing, concomitant with eva-

porative fractionation processes, were observed tosequester low molecular weight aromatics such as ben-

zene and toluene.

Acknowledgements

The authors thank Pertamina EP and ARCO AEPTfor support of this project and permission to present theresults of our Northwest Java Basin study. We are

grateful to Erika Shoemaker-Ellis for graphical assis-tance with manuscript ®gures. The authors would alsolike to acknowledge Dr. Ron Noble and Dr. Leon Dzoufor reviewing the manuscript and providing many help-

ful suggestions.

Associate EditorÐR. Lin

Fig. 18. Petroleum systems of the NW Java Basin showing inferred migration pathways of Group 1 and Group 2 oils from identi®ed

mature source pods to identi®ed reservoirs (modi®ed from Noble et al., 1997).


Appendix

% C27=% C27 aaa 20R-cholestane (m/z 217).% C28=% C28 aaa 20R-methylcholestane (m/z 217).

% C29=% C29 aaa 20R-ethylcholestane (m/z 217).C27R/C29R=C27 aaa 20R-cholestane/C29 aaa 20R-

ethylcholestane (m/z 217).

C29H/C30H=C29 17a(H)-hopane/C30 17a(H)-hopane(m/z 191).Ol/C30H=18a(H)-oleanane/C30 17a(H)-hopane (m/z

191).TT(19+20)/TT23=C19+C20 tricyclic terpanes/C23

tricyclic terpane (m/z 191).

TT23/C30H, C23 tricyclic terpane/C30 17a(H)-hopane(m/z 191).DBT/PHEN = dibenzothiophene (m/z 184)/phenan-

threne (m/z 178).

Pr/Ph = pristane/phytane (GC).Pr/n-C17=pristane/n-C17 (GC).CPI2=Carbon preference index (GC).�API=API gravity (whole oil).% S=Sulfur content (%).d13CAro=d13C Oil aromatic fraction.

d13COil=d13C Whole oil.d13CSat=d13C Oil saturate fraction.

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