geochemical characteristics of oils from taiwan

Pergamon Journal of Southeast Asian Earth Sciences, Vol. 9, No. 3, pp. 193-206, 1994

Elsevier Science Ltd Printed in Great Britain

Geochemical characteristics of oils from Taiwan

J.-N. OUNG*~" and R. P. PHILP* *School of Geology and Geophysics University of Oklahoma, Norman, OK 73019, U.S.A.; 1"Chinese Petroleum Corporation, Offshore and Overseas Petroleum Exploration Division, 4th Fl., No. 3, Peiping West Rd.,

Taipei, Talwan

(Received 29 October 1992; accepted for publication 9 April 1993)

Al~traet-----Geochemical studies on a number of oils from Taiwan have shown that they are sourced predominantly from terrigenous organic matter. Biomarker characteristics include the presence of high amounts of long-chain regular isoprenoids, bicyclic sesquiterpanes, diterpanes, low concentrations or absence of tricyclic terpanes, presence of several C24 tetracyclic terpanes and some source-specific pentacyclic terpanes and a predominance of C29 steranes.

For most oftbe oils, the n-alkanes extended to C30 or above and have CPI values of approximately 1, indicating the mature nature of these oils. The pristane/phytane ratios, generally in excess of 5, suggest the likely prevalence of a highly oxic, nearshore, marine-open peat swamp depositional environment which incorporated large amounts of terrigenous organic matter. The long-chain isoprenoids extending to C42, and possibly higher, are proposed to be regular head-to-tall isoprenoids possibly derived from polyprenols in higher plants. The sesquiterpanes present in the oils range from C~s and CI7 and are dominated by 8//(H)-drimane and 8fl (H)-homodrimane. The diterpanes include Ct742~0 tricyclic and Ci9 and C20 tetracyclic diterpanes. The presence of diterpanes in these oils suggests that resins derived from conifers contributed to the source of these oils. At least nine tetracyclic terpanes with carbon numbers ranging from 24 to 27 were detected in Taiwan oils. Two series of tetracyclic terpanes are apparent, one is the de-A-oleanane/ursane series and the other is the 17,21-secohopane series. The former is more source-specific than the latter and is probably related to the presence of oleanane. A number of pentacyclic terpanes, including oleanane, have been detected and many of these compounds may be of terrigenous origin and source-specific. Oleanane, de-A-oleanane/ursane and de-A-lupane can be used as indicators for angiosperms which have evolved since the late Cretaceous. Sterane distributions in the Taiwan oils are dominated by the C29 steranes and include regular and rearranged steranes, indicating the terrigenous nature of the organic matter in the source rocks. The significance of the variations observed in the biomarker distributions of the oils from Taiwan is discussed in some detail in this paper.

INTRODUCTION

TAIWAN IS an island situated 600km southwest of Okinawa, 350 km north of Philippines and 160 km off the coast of mainland China. Three major geologic provinces, i.e. the Central Range, the Coastal Range and the Western Foothills are apparent on this island (Ho 1986) as shown in the simplified geologic map of Taiwan in Fig. l a. From both the sedimentary and structural viewpoints, the more important prospect for oil and gas lies in western Taiwan and may be considered as comprising all the areas of the Tertiary, mainly Neogene, non-metamorphosed sediments. The western Neogene basin, bounded on the east by the Paleogene indurated rocks in the Central Range, extends offshore into the Taiwan Strait (Kuang and Wu 1986). Abundant gas and associated oil have been produced from the reservoirs distributed in the sedimentary rocks ranging from uppermost Oligocene through Neogene in the foothills and coastal plain belt of western Taiwan. The main oil- and gas-producing horizons in western Tai- wan, in descending stratigraphic order, are the sandstone reservoirs in the Kueichulin, Nanchuang, Talu, Peiliao, Shihti, Taliao/Piling, and Mushan Formations of Miocene age and the Wuchihshan Formation of Oligocene age. The main oil and gas fields include

onshore Chingtsaohu-Chiting, Paoshan, Chinshui, Paishatun, Tiehchenshan, and Chuhuangkeng fields, and Hsinchu offshore fields. All of these fields produce gas in commercial quantities, condensate and oil. An oil and gas map of Taiwan showing the prospective structures including the oil and gas producing fields is given in Fig. lb (Meng et al. 1972). The Oligocene Wuchihshan Formation and the Neogene formations are still considered to be the most favorable for discovery of additional reservoirs.

With regard to the source rocks for the hydrocarbons in Taiwan, geochemical evaluation has shown that the great thickness of dark shales in the Tertiary sequence would seem sufficient to provide a rich source for petroleum. The source rocks in the potential Oligocene and Neogene sediments are moderately mature or mature with fair to good richness and are represented chiefly by woody material with minor amounts of herbaceous or amorphous material which produces condensate and gas plus a small amount of crude oil (Chang 1987, Kuang and Wu 1986, Chou et al. 1987). To further understand the petroleum geochemistry of the Taiwan oils and to aid further petroleum exploration efforts, systematic geochemical studies on Taiwan oils have been undertaken and the results are described in this paper.

193

194 J.-N. OUNG and R. P. PHILP

OIL AND GAS MAP OF TAIWAN

SCALE (KM)

o 2O 4O 60KI~ =

~SiNCHU

• TAIPEI

. , .;_:!/.~i!i

PENGHU ~

'7 o...~ ,oo ~, 2 2

119 ) 12 121 ° 122 e I I I I

Fig. la. Simplified geologic map of Taiwan showing the major geologic provinces.

~ E N ~ U ~ S

II .1 ~'PCC

LUTAO ~7 FIELD/STRUCTURE ~ ~ CS - Chinlhui CT -CNt ing CTH-Chingt$oohu CHK - Chuhuongkeng PST - Poisltotun PS - P(IOlhon TCS -Tiehchem~. 9n I LANXSU ~ PCC - i-,ochongcm THS - T o i h l i

Fig. lb. Oil and gas map of Taiwan showing location of major oilfields.

EXPERIMENTAL

Forty-eight oil samples from several fields in Taiwan were investigated in this study (Table 1). Due to the very low concentration of asphaltenes in most of the oils, all of the oils were analyzed directly by gas chromatography to determine the n-alkane, pristane and phytane distributions. For analysis of the other series of biomarkers the oils were separated into saturates, aromatics, and polar fractions by thin-layer chromatography. The saturate fractions were further treated with molecular sieves (S-115, Union Carbide) and the branched/cyclic fractions analyzed for terpanes, steranes and long-chain isoprenoids using combined gas chromatography-triple stage quadrupole mass spectrometry (TSQ 70, Finnigan MAT) (Oung 1989).

maturation and partly due to migration effects. Oils from the deep reservoirs and of higher maturity levels generally have higher API values. However, vertical migration may make some oils in shallow reservoirs show higher API values than their associates in the deeper reservoirs. The woody type kerogen (Type III) is the major source for oils in the Miocene and upper Oligocene formations of western Taiwan (Chou et al. 1987). Oils produced from Taiwan, mainly from Tertiary basins, are rich in paraffin waxes, low in sulfur content and low in asphaltenes (Meng et al. 1972, Chou et al. 1987). These properties are a reflection of the high input of terrigenous organic material to their source rocks (Philp and Gilbert 1986). The biomarker distributions in Taiwan oils also show many characteristics which are commonly associated with oils derived from terrigenous sources.

RESULTS AND DISCUSSION

Most oils produced in Taiwan have API values ranging from 30 to 60 (Table 2). The variation in API values for the oils of different depths is proposed to be due to

n-ALKANES, PRISTANE AND PHYTANE

All the Taiwan oils studied, with the exception of two samples, namely oil THS-1 and an oil seep from Chiah- sien Creek in the south Taiwan area, have whole oil

Geochemical characteristics of oils from Taiwan 195

Table 1. List of Taiwan oil samples*

Sample No. Well name

1 A-1 2 A-I 3 K-I 4 K-2 5 K-2 6 K-2 7 K-3 8 K-3 9 K-3

I0 K-4 11 K-4 12 K-5 13 K-11 14 K-12 15 K-12 16 K-15 17 K-15 18 L-1 19 S-I 20 S-2 21 K-16 22 CS--45 23 CS-53 24 CS-54 25 CS-58 26 CS-59 27 CS~50 28 CS-67 29 CS--69 30 CS~i9 31 CS-70 32 CS-72 33 CS-74 34 T-2 35 T-8 36 TH-3 37 TH-5 38 TH-14 39 HK-110 40 W-I 41 ST-5 42 PC-3 43 PS-3 44 PS-9 45 TCS-23 46 CHI 47** Chiahsien Creek 48 THS

Field

Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinehu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinchu offshore Hsinehu offshore Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chinshui Chiting Chiting Chingtsaohu Chingtsaohu Chingtsaohu Chuhuangkeng Penghu offshore Paishatun Pachangchi Paoshan Paoshan Tiehchenshan Chiahsien

Taihsi

Age Depth (m)

Miocene 3533.5-3539 Oligocene 3641.5-3649 Miocene 2381-2404 Miocene 3020-3042 Miocene 3038-3042 Miocene 3085-3103 Miocene 2760.5-2763 Miocene 2920-2946 Miocene 3253.2-3259.7 Miocene 1911-1915.8 Miocene 1924-1935 Miocene 3119.5-3136 Miocene 2888-2976 Miocene 2551-2558 Miocene 3261-3308 Miocene 3442.5-3444 Miocene 3587.5-3588.5 Oligocene 3480-3485 Miocene 2254-2281 Miocene 2841-2849 Miocene 2689.5-2690.2 Miocene 2327-2385 Oligocene 4371-4462 Miocene 2156-2209 Miocene 3124-3283 Miocene 2292-2333 Miocene 2197-2247 Miocene 3640-3660, 3693-3699

Oligocene 4601--4782.1 Miocene 3218-3237 Miocene 3852-4211.5 Oligocene 4584-4851 Miocene 3259-3269 Miocene 3281-3288 Miocene 2853-2854 Miocene 2000-2003.5 Miocene 3860.5-3868 Miocene Eocene 2001-2023 Miocene Miocene 1888-1920 Miocene 3790.5-3807.5 Miocene 3568-3592.5 Miocene 2796-2834 Miocene

Miocene 1223-1234

*For location refer to Fig. la. **Oil seep sample.

chromatograms dominated by n-alkanes (Fig. 2). The chromatograms of the THS-1 oil and the Chiahsien oil seep are devoid of n-alkanes which is indicative of biodegradation. The asphaltene fraction of the THS-1 oil was sealed in a glass tube and pyrolyzed at 300°C for 72 hr and produced a pyrolysate dominated by n-alkanes. This indicates that the source material for the THS-I oil originally produced n-alkanes which were subsequently removed by biodegradation.

Because many of the oil samples are condensates, some with API values as high as 60 (Table 2), the n-alkane distributions do not show any apparent predominance in the C,,--C3o range, typically indicative of a contribution from higher plant source material. However, the n-alkane distributions in most of the oil samples do extend to C30 and above (see examples in Fig. 2). The carbon preference index (CPI) values for all samples is approximately one, indicating that they are all quite mature (see Table 2).

The high wax content, indicated by the abundance of n-alkanes in the chromatograms, together with pristane/ phytane ratios in excess of 5 (Table 2), and high pristane/ n-heptadecane (n-C17) ratios (generally higher than 1 in Hsinchu offshore field), collectively suggest that the depositional environment of the source rocks was a highly oxic nearshore marine--open peat swamp which incorporated large amounts of terrigenous organic matter (Didyk et al. 1978). The pristane/n-heptadecane and pristane/phytane ratios of oils from the Hsinchu offshore field are significantly higher than those of oils from Chinshui (CS) onshore field which may imply that the source rocks of the oils in the Hsinchu offshore field were deposited in a more oxic nearshore depositional environment than those of the CS field, which is consistent with the geological history of the area. Prior to the uplift of Taiwan island, i.e. the collision of the Philippine Sea plate and Eurasian plate during the Plio-Pleistocene, the Hsinchu offshore

196 J.-N. OUNO and R. P. PmLp

Table 2. Acyclic hydrocarbon composition of Taiwan oils

Sample No. Well name CPP Pr/17 b Ph/18 c Pr/Ph d API

1 A-1 1.09 1.10 0.13 8.32 34.1 2 A-1 1.10 1.26 0.14 9.33 3 K-I 1.12 0.83 0.12 7.93 37.1 4 K-2 1.13 0.80 0.12 7.14 31.5 5 K-2 1.04 1.51 0.19 8.97 50 6 K-2 1.09 1.45 0.19 9.47 37.1 7 I£-3 1.13 2.58 0.27 12.46 59.5 8 K-3 1.07 1.06 0.15 8.64 49.7 9 K-3 1.13 5.37 0.47 12.49 53

I0 K-4 ND ~ 1.20 0.13 10.94 57.9 I1 K-4 1.07 1.10 0.15 8.22 45.8 12 K-5 0.97 1.16 0.16 9.08 37.7 13 K-I1 1.04 1.12 0.15 8.51 14 K-12 1.11 1.37 0.17 9.27 55.3 15 K-12 1.04 1.16 0.15 8.55 47.7 16 K-15 1.02 1.50 0.20 7.93 48 17 K-15 1.06 1.69 0.20 9.34 45.3 18 L-1 0.98 1.10 0.18 8.43 36.2 19 S-I 1.01 1.28 0.18 6.97 41.4 20 S-2 1.13 1.42 0.17 10.51 50 21 K-16 1.19 1.32 0.17 8.36 22 CS-45 ND 0.55 0.08 12.31 23 CS--53 0.98 0.34 0.07 6.26 24 CS-54 1.16 0.92 0.12 8.35 25 CS-58 1.12 0.45 0.07 7.04 26 CS-59 1.10 0.44 0.06 6.30 27 CS-60 ND 0.54 0.09 9.21 28 CS-67 0.99 0.31 0.05 7.52 29 CS--69 1.16 0.26 0.05 9.01 30 CS-69 1.08 0.60 0.08 8.57 31 CS-70 1.16 0.33 0.07 5.36 32 CS-72 1.13 0.28 0.05 5.62 33 CS--74 1.08 0.25 0.05 5.09 34 T-2 1.02 0.88 0.11 8.56 54 35 T-8 1.02 0.95 0.13 8.02 36 TH-3 1.03 0.82 0.12 7.38 39 37 TH-5 ND 1.12 0.14 14.96 54 38 TH-14 1.10 1.05 0.14 10.19 50 39 HK-110 0.91 0.36 0.06 7.11 40 W-I 1.15 0.45 0.28 1.75 33 41 ST-5 0.93 0.81 0.12 5.99 42 PC-3 1.00 0.86 0.14 13.49 43 PS-3 1.06 0.87 0. I 1 7.48 36 44 PS-9 ! .06 0.77 0.11 7.73 47 45 TCS-23 ND 0.90 0.12 9.92 46 CHI 1.03 1.63 0.24 9.17 47 Chiahsien Creek ND ND ND ND 48 THS ND ND ND 3.95

sCPI = [C23 + C25 + C27 -~- C29) "~- (C25 "~- C27 bPr/17:Pdstane/n-Ct7 alkane. cph/18: Phytane/n-Cis alkane. dpr/Ph: Pristane/Phytane. eNot available.

"~ C29 "~- C3t)]/[2"(C~ + C26 + C2s + C30)].

a rea was closer to the sediment source in the nor th- west than the CS area and hence the depos i t iona l env i ronment was nearshore and subsequent ly more oxic. Based on a geological s tudy o f the s t ra t ig raphy and sed imento logy o f the Miocene in western Ta iwan (Chou 1980), the Miocene sediments are a b o u t 3000m in thickness a long the nor thwes te rn coas t o f Ta iwan and become gradua l ly thicker t oward the southeas t and south o f western Taiwan. This general t rend o f thicken- ing o f sediments is a ccompan ied by a general decrease o f the gra in size o f the sands tone and the sands tone percentage, indica t ing tha t the sediments were t rans- po r t ed chiefly in these di rect ions (Chou 1980). The pa leocur ren ts resulted in the depos i t ion o f sediments in a cont inenta l -para l ic -ner i t ic bas in in nor thwes tern Taiwan.

L O N G - C H A I N I S O P R E N O I D S

Long-chain isoprenoids are c o m m o n const i tuents in most Taiwan oils, and the m/z 183 mass f ragmentogram from one o f the Miocene oils f rom the Hsinchu offshore field (in Fig. 3). shows the typical dis t r ibut ion o f long- chain isoprenoids. I t appears that this series o f isoprenoids extends to at least C42 and possibly higher. I t is proposed that these long-chain isoprenoids are members o f the regular head-to-tai l series, possibly derived f rom poly- isoprenols to known to occur in var ious species o f higher plants such as conifers (Han and Calvin 1969). Long- chain ol igoterpenyl alcohols have also been previously identified in extracts from certain higher plants ( Ibata et al. 1984). The relatively high concentra t ion o f the long- chain head-to- ta i l i soprcnoids and the predominance of

$ I I

w

j - <~ _J t~ r~

Prl

2O

._ . J I

Pri

pn

20

II

25

I

J J, J,,l,~

3 0

I N C R E A S I N G R E T E N T I O N T I M E - - ~ '

Fig. 2. Gas chromatograms showing the typical distribution of n-alkanes observed in Taiwan condensates and oils. (Numbers represent carbon numbers of n-alkanes. Pr = Pristane and Phy = Phytane.)

5' I I

LIJ I--- Z

LU > I-- <I . J LU ne

% I O O "

50

M / Z : 183.0 34

5

24 26

29 28

30 31 35

36

40

. . . . ] . . . . i , , , , I ' ' ' ' i , , , , I ' ' ' ' i . . . . I ' ' ' ' i , , , , I ' i , , i , , , ,

i500 2000 2500 3000 3~X:X) 4000

E t 0 5 2 . 8 8 3

Fig. 3. Partial m/z 183 fragrnentogram showing the typical distribution of long-chain isoprenoids observed in most Taiwan oils. (Numbers represent carbon number of particular components.)

197

198 J.-N. OUNG and R. P. PmLP

% I00"

IJJ I-- 5 0 _z

I.U > I.- < ..I bJ IZ

M / Z , IS l . 2

Pent ocy¢U¢ terpones

Direr

B icyc l lc sesquiterpones

)ones

Te t rocycllc terpones

. . . . . . . . . . . . . . . . . . 2o:oo 3o,oo' . . . . . . . . . . . . . . . . . ' 4o,'oo sol oo . . . . ' . . . . eo,oo' . . . . . . . . . 7o:oo' . . . . . . . . . eo,oo' . . . . . . . . . so,oo' . . . . . . . . . ,oo : oo . . . . . . . . . , ,o, oo' . . . . . . . .

m, E~ OS 4.1 '32

I N C R E A S I N G R E T E N T I O N T I M E - - - : ,

Fig. 4. An m/z 191 fragmentogram showing the typical terpane distribution of terpanes observed in most Taiwan oils.

pristane over phytane support the proposal that the oils of Tertiary age were derived from higher plant source material deposited in a non-reducing environment.

TERPANES

The predominant features of the terpane distributions in the Taiwan oils include the presence of bicyclic sesquiterpanes and diterpanes, the absence, or low concentration, of tricyclic terpanes, a relatively high concentration of several tetracyclic terpanes and the presence of

a series of pentacyclic hopanoid triterpanes plus a number of non-hopanoid triterpanes, e.g. 18a (H)-oleanane (Fig. 4). The m/z 191 chromatogram in Fig. 4 is representative for these oils and although there are some variations the major features are very similar for all of the oils analyzed in this study•

1. Sesquiterpanes and diterpanes

A representative example of a m/z 123 mass fragmentogram showing the sesquiterpane and diterpane distributions of an oil from Hsinchu offshore field is

I !

) - I--

Z bJ I'-- _z

tel

/ UJ w

% I 0 0 -

50'

M / Z : 123.0

• z~o

Ct6 BicyclJc sesquiter pones

C rs Bicyclic sesquiterpones

Czo Tetrecyti¢ dtterpones

)5

Tricyclic diterpones

C)7 Oe C,s

It N

• , , ~ . ~ . . . .

4 ( X ) ' ' ' ' ' ' ' tO00 i200 i400

t E t ' 0 6 5,693

I N C R E A S I N G R E T E N T I O N T I M E - - - >

Fig. 5. The m/z 123 fragmentogram showing distributions of sesquiterpanes and diterpanes from one of the Taiwan oils. (For peak auimnments refer to Table 3 and presentative structures in Appendix I.)


Table 3. Summary of structural assignments for the bicyclic sesquiterpanes and diterpanes in Taiwan oils

Peak no.* Assignment Structure** Remark

1 Rearranged CIs Bicyclane I Alexander et al. 1984 2 Rearranged Cts Bicyclane II Alexander et al. 1984 3 8~ (I-I)-Drimane Ilia Alexander et al. 1984 4 Cis Bicyclane -- Oung 1989 5 C~s Bicyclane -- Oung 1989 6 Ci6 Bieyclane -- Oung 1989 7 818 (H)-Homodrimane IIIb Alexander et aL 1984 8 CI7 Bicyclane llIc Dimmler et aL 1984 9 4~ (H)-19-Norisopimarane Via Noble et al. 1986

10 Ct7 Tricyclane -- Oung 1989 11 C~9 Tricyclane -- Oung 1989 12 17-Nortetracyclic diterpane IV Noble et al. 1986 13 ent-Beyerane V Noble et aL 1985a, b 14 Isopimarane Vlb Noble et al. 1986 15 16~ (H)-PhyUocladane VIIb Noble et aL 1985a, b 16 ent-16~ (H)-Kaurane IIXb Noble et aL 1985a, b 17 16a (H)-Phyllocladane VIIa Noble et al. 1985a, b 18 ent-16a (H)-Kaurane IIXa Noble et al. 1985a, b *For peak no. refer to Fig. 5. **For structure refer to Appendix I.

given in Fig. 5 and major peak assignments are listed in Table 3. Structures I-VIII are representative of the basic structural types for these compounds.

Sesquiterpanes. T h e series of C~-CI7 bicyclic sesquiterpanes in these oils is dominated by 8//(H)-drimane and 8//(H)-homodrimane. The distribution of these compounds is similar to that previously observed in Australian and New Zealand oils (Philp et al. 1981, Philp 1985), which are known to be derived from terrigenous source material and the Athabasca tar sand bitumen (Dimmler et al. 1984). However, the Athabasca tar sand bitumen contains bicyclic terpanes with carbon numbers extending to 24. A recent report on a Chinese Boghead coal has also demonstrated the presence of extended bicyclic terpanes to at least C20 and probably higher (Wang et al. 1990). The widespread occurrence of bicyclic sesquiterpanes containing the drimane skeleton in crude oils suggests a ubiquitous source(s) for these compounds. Alexander et al. (1984) proposed that bicyclic alkanes of the drimane type are probably formed by biological alteration of hopanoid precursors during diagenesis. The similarity of the bicyclic sesquiterpane distributions found in Taiwan, Australian, and New Zealand oils and Athabasca oil sand suggests similarities in the distribution of the precursor compounds in a wide range of geological environments. Such an observation supports a microbial origin for these compounds.

Diterpanes. The diterpanes present in most of the Taiwan oils include C~7-C20 tricyclic and CI9 and C20 tetracyclic diterpanes (Fig. 5). The tricyclic and tetracyclic diterpenoids are abundant in higher plants and are major constituents of conifer resins (Hanson 1972) and occur as analogous diterpanes in petroleum and rock extracts derived from higher plant sources (Simoneit 1977, Philp et al. 1983, Snowdon and Powell 1982, Livesey et al. 1984).

Trieyclic diterpenoids are widespread in conifer resins, and are particularly well represented in the southern conifer families of Podocaepaceae and Araucariaceae

(Gough 1964, Thomas, 1969). More specifically, the

Dacrydium genus (Podocarpaceae family) is rich in diterpenoids of this type, and contains compounds such as isopimara-7,15-diene (Grant et al. 1967) and rimuene (Salasoo 1984, and references therein), likely precursors of isopimarane and rimuane, respectively. The C20 tetracyclic diterpenes occur widely in the leaf resins of conifers, and compounds from these sources are the most likely precursors of phyllocladane, kaurane and beyerane (Noble et al. 1985a). The Ci9 tetracyclic diterpanes with either the phyllocladane, kaurane or beyerane skeletons have not been isolated from conifers, which indicates that the 17-nortetracyclic diterpane identified in petroleum is in all probability derived from a C20 compound by subsurface chemical reactions. These transformations may involve decarboxylation, or oxidation-reduction of an olefinic moiety, depending on the nature of the precursor compound(s) (Noble et al.

1986). The sources for the Ci7"-Ci9 tricyclic diterpanes found in the Taiwan oils are not readily apparent but may be derived from the thermal degradation of higher molecular weight tetracyclic diterpanes.

The presence of isopimarane, beyerane, phyllocladane and kaurane diterpenoid type skeletons in these Taiwan oils strongly suggests that conifer resins contributed to the source materials of these oils. The presence of phyllocladane, a marker for the Podocarpaceae family of conifers, is particularly significant since the resins derived from conifers are often rich in lipids. If deposited under suitable geological conditions, resins have been proposed as good source materials for crude oils (Snowdon 1980, Snowdon and Powell 1982, Shanmugam 1985).

2. Tricyclic, tetracyclic and pentacycl ic terpanes

A m/z 191 mass fragmentogram showing the distributions of tricyclic, tetracyclic and pentacyclic terpanes of an oil from Hsinchu offshore field is shown in Fig. 6 and peak assignments are given in Table 4.

Tricyclic terpanes. The presence of the tricyclic terpanes is now generally considered to be an indicator of

200

e z so

J.-N. OUNG and R. P. PHILP

M/Z.~ 191.2 30

25

19 32

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IPC÷06 4 .?32

INCREASING RETENTION TIME---> Fig. 6. The partial m/z 191 mass fragraentogram for an oil from Taiwan showing the distributions of tricyclic, tetracyclic

and pentacyclic terpanes. Peak assignments are given in Table 4.

Table 4. Identifications of terpanes present in the m/z 191 fragmentogram

Peak No.* Assignment Structure**

1 C23 Tricyclane (cheilanthane type) 2 C24 Tricyclane (cheilanthane type) 3 C25 Tricyclane (degraded tetracyclane) 4 C24 Tetracyclane (de-A oleanane/ursane) 5 C~ Tetracyclane (de-A lupane) 6 C25 Tetracyclane (cheilanthane type) 7 C25 Tetracyclane 8 C24 Tetracyclane 9 C24 Tetracyclane (17,21-secohopane)

10 C26 Tricyclane (cheilanthane type) 11 C~ Tetracyclane 12 C2s Tetracyclane 13 C27 Tetracyclane 14 C26 Tetracyclane 15 C27 Pentacyclane 16 C27 Pentacyclane 17 C27 Pentacyclane (Ts) 18 C30 Pentacyclane 19 C27 Pentacyclane (Tm) 20 C29 Pentacyclane 21 C30 Pentacyclane 22 C2s Pentacyclane (bisnorhopane) 23 C3o Pentacyclane 24 C29 Pentacyclane 25 C29 Pentacyelane (C29-norhopane) 26 C~9 Pentacyclane 27 C30 Pentacyclane (X-C30), 17~ (H)-diahopane 28 C29 Pentacyclane (C29-moretane) 29 C30 Pentacyclane (oleanane) 30 C~ Pentacyclane (C~-hopane) 31 C30 Pentacyclane (C~-moretane) 32 C3~ Pentacyclane (C31-homohopane, 20S) 33 C3~ Pentacyclane (C3~-homohopane, 20R) 34 C3j Pentacyclane (C3rmoretane) 35 C32 Pentacyclane (C32-bishomohopane, 20S) 36 C32 Pentaeyelane (C32-bishomohopane, 20R) 37 C33 Pentacyclane (C33-trishomohopane, 20S) 38 C33 Pentacyclane (C33-trishomohopane, 20R) 39 C~ Pentacyclane (C~4-tetrakishomohopane, 20S) 40 C~ Pentacyclane (C~-tetrakishomohopane, 20R) 41 C35 Pentacyclane (C35-pentakishomohopane, 20R) 42 C35 Pentacyclane (C3~-pentakishomohopane, 20R)

I II

III

VI

IV VII V

VIII

*For peak No. refer to Fig. 6. **For structure refer to Appendix II.

a marine or algal source material (Aquino Neto et al. 1983, Oung 1987). The absence, or low concentration, of tricyclic terpanes in most Taiwan oils suggests that the majority of these oils are derived mainly from terrigenous organic source material. This is also a characteristic feature of oils from many other parts of the world thought to be derived from terrigenous source materials (Philp and Gilbert 1986).

Although the concentration of tricyclic terpanes is generally low, there are some variations in their concentrations in oils from various fields. The varying concentrations of tricyclic terpanes in oils from different reservoirs implies that some of the oils are derived from source rocks containing varying proportions of marine or algal organic matter. The concentrations of the tricyclic terpanes relative to the hopanes are higher in oils from the CS onshore field compared to those from Hsinchu offshore field. This observation agrees with the arguments given above for the fact that the source region of the terrigenous source material for the hydrocarbons was situated northwest of the island of Taiwan.

Tetracyclic terpanes. The diversity of tetracyclic terpanes present in the Taiwan oils is remarkable. At least nine tetracyclic terpanes with carbon numbers ranging from 24 to 27 could be assigned in these oils (Fig. 6 and Table 4). Proposed structures for these tetracyclic terpanes are shown in Appendix II. Use of GC-MS/MS in the parent mode indicated that there were at least two series of tetracyclic terpanes present in the majority of these samples (Oung 1989, Woolhouse et ai. 1992). One series of tetracyclic terpanes consisted of the components represented by peaks 4 (C24), 7 (C25), 11 (C26) and 13 (C27), and the other consisted of peaks 9 (C24), 12 (C25) and 14 (C26) in Fig. 6.

The components giving rise to peaks 4, 5, 8 and 9 (Fig. 6) are C~ tetracyclic terpanes previously observed in oils from the Taranaki Basin, New Zealand (Czochan- ska et al. 1988). Mass spectra of the compounds


corresponding to peaks 4 and 5 have been acquired and lead to the proposal that they are ring-A degraded triterpanes (Czochanska et al. 1988). The component responsible for peak 4 is similar to an unidentified tetracyclic terpane previously reported in Nigerian crude oils and assigned a structure based on a ring-A degraded oleanane or ursane structure (de-A oleanane/ursane, Ekweozor et al. 1981, Czochanska et al. 1988). It would appear from a purely quantitative point of view that oils with high oleanane concentrations have a correspondingly high concentration of the degraded oleanane-peak 4. The component producing peak 5 has been assigned as a ring-A degraded lupane (de-A lupane, Corbet et al. 1980) and the compound producing peak 8 is an unidentified C2a tetracyclic terpane previously reported only in oils from Taranaki Basin, New Zealand (Czochanska et al.

1988). In addition to the Taiwan oils this compound has now been detected in several oils and rock extracts from the Mahakam Delta, Indonesia and from Nigeria, which were examined in this study for comparative purposes (Fig. 7). The samples in which this component is present also contain significant amounts of oleanane suggesting that this unknown tetracyclic compound may be derived from a component possibly oleanane, indicative of terrigenous source material. The compound producing peak 9 is C24-17,21-secohopane (Aquino Neto et al. 1983) previously reported in relatively high abundance in coals and oils derived from terrigenous source materials (Philp and Gilbert 1986). The occurrence of a high abundance of this compound relative to the tricyclic terpanes has been proposed as an indicator of a significant input of higher plant material (Trendel et al. 1982) although in reality it is probably related to elevated levels of microbial activity in the original depositional environment.

The tetracyclic terpane series containing peaks 4 (C24), 7 (C25), 11 (C26) and 13 (C27) in the Taiwan oils corresponds to a series of C 2 4 ~ 2 7 tetracyclic terpanes also detected in crude oils and source rock extracts from various fields of the Tertiary Niger delta (Ekweozor et

al. 1981). It is proposed that these compounds were formed by thermal degradation, namely sequential cleavage of the terminal ring, of pentacyclic precursors of the oil in the reservoir. An unusual C25 tricyclic terpane (peak 3 in Fig. 6) was detected in some of the Taiwan oils and has also been reported in Nigerian oils (peak Q2, Ekweozor et al. 1981). The C25 tricyclic terpane was thought to be formed by thermal cleavage of the tetracyclic terpanes.

Another tetracyclic terpane series containing the components giving peaks 9 (C24), 12 (C25) and 14 (C26) in Fig. 6 corresponds to a tetracyclic series (17,21-secohopanes), extending from C24 to C27, detected by Aquino Neto et al. (1983) in a biodegraded petroleum from Aquitaine Basin, France. Aquino Neto et al. (1983) proposed that this tetracyclic terpane series is closely related to the pentacyclic hopanoids of microbial origin by cleavage of the 17(21) bond during early diagenesis or maturation.

The presence of these two series of tetracyclic terpanes in various oil samples may provide some information

I ! >-

Z I~1 I - z_ LIJ >

/ IJJ n-

M / Z 191.2

TAIWAN

T~racyclic I 4 terpanel r . I

'5 119

oleonone

,I,,Iol~e

NEW ZEALAND

NIGERIA

,.L, JL,.J~ ; ~

INDONESIA

Tm

45 I I I I TS

3OO0 35O0 4OO0 450O 5OOO

INCREASING RETENTION TIME--->

Fig. 7. Partial m/z 19! fragmentograms of oils f rom various basins worldwide showing the presence of tetracyclic terpanes and some unidentified C30 pentacyclic terpanes. (Peak assignments refer to Table

4 and structures are shown in Appendix II.)

concerning the origin of these compounds. It is interest- ing to note that all the oils containing de-A oleanane/ursane generally have high concentrations of oleanane. However, this is not the same for 17,21-secohopanes. The 17,21-secohopanes have been found to be present in many oils which do not contain detectable amounts of oleanane, i.e. oils from Australia (Philp and Gilbert 1986). It is, therefore, proposed that the de-A oleanane/ursane series (peaks 4, 7, 11 and 13) is more source-specific than the 17,21-secohopane series (peaks, 9, 12 and 14) and probably, like oleanane, is derived from precursors originating in angiosperms. The relative concentration of the 17,21-secohopanes may reflect the extent of microbial activity. The relative concentrations of de-A oleanane/ursane and de-A lupane probably reflect the types of higher plants contributing to the source materials for these oil samples, more specifically angiosperms.

It is proposed that similar mechanisms are operative for the formation of both the de-A-oleananes and de-A-lupanes from their pentacyclic precursors. The specific

202 J.-N. OUNG and R. P. PHILP

mechanism may involve thermal degradation, or diagenesis (Corbet et al. 1980). Richardson and Miller (1983) studied a terrigenous oil, whose precise origin was not given although it was probably from somewhere in Indonesia, and suggested that the presence of oleananes indicated a contribution of terfigenous source material to the oil. The possible presence of lupanes led to the further conclusion by Richardson and Miller (1983) that the oil was of Tertiary age. Lupanes are proposed to have an origin from terrigenous higher plants (Rullkotter et al. 1982) and their precursors, lupenols, are thought to be produced exclusively by angiosperms, particularly Indonesian sapotaceous trees which have evolved since the late Cretaceous and flourished during the Tertiary. The use of oleananes and lupanes to indicate samples of Tertiary age has been supported by an investigation of these compounds in terrigenous rock extracts of various ages (Oung, unpublished data).

The concentrations of the tetracyclic terpanes in oil samples may be related to the concentration of precursors and the extent of their degradation, e.g. thermal evolution of oils in the reservoir as proposed by Ekweozor et al. (1981) and Aquino Neto et al. (1983). The presence of the tetracyclic terpanes in the Taiwan oils suggests that these oils are mainly sourced from terrigenous source material which has been subjected to extensive thermal alteration.

Pentacyclic terpanes. A. Hopane series

In all the oils examined in this study, the relative concentration of the extended hopane series (>C3:) decreases exponentially and, unlike oils sourced under highly reducing conditions does not contain high concentrations of the C35 hopanes relative to the C34 compounds. The extent of isomerization at various chiral centers within the hopanes and steranes have been widely used to assess maturity levels of oils. Values for the 22S/(22S + 22R) epimer ratios measured for C3~ homohopanes in Taiwan oils are given in Table 5 and for most of the oils this value is close to the equilibrium value of about 0.6%, indicating that these oils are derived from source rocks at an advanced stage of thermal maturity. One exception was an oil from the Hsinchu offshore field (Sample 14 in Table 1) which has a value of 0.23 for this ratio (Table 5) which may be related to the presence of coal seams near the reservoir. A similar value was previously reported for an Australian oil and was proposed to result from the migration of an oil through an immature brown coal seam containing a high concentration of the C31 22R homohopane (Philp and Gilbert, 1982).

The ratio of 17/~ (H),21 ~ (H)-moretanes to 17~ (H),21//(H)-hopanes has also been used a maturity parameter but, compared to the 22S and 22R ratio, this value shows incomplete isomerization for most of the Taiwan oils. The extent of 17//(H),21a (H)-moretane to 17~(H),21//(H)-hopane isomerisation (M30/H30 in Table 5) is generally in the range of 0.1-0.2 for most of these oils which is within the range observed for oils

from the Tertiary basins as described by Grantham (1986). The M30/H30 ratio range typical for mature crude oils which have reached complete isomerisation is generally in the range 0.03-0.06 (Seifert and Moldowan, 1980). An explanation for this discrepancy offered by Grantham (1986) is the importance of time in these isomerization reactions. In young Tertiary basins, such as the Neogene basin in Taiwan, sediments have been buried rapidly and have reached a high temperature rapidly, but insufficient time has passed for the isomerization to reach equilibrium.

The ratio of the two C~7 trisnorhopanes, 18,, (H)-tris- norhopane (Ts) to 17~ (H)-trisnorneohopane (Tin) is generally regarded as a sensitive maturity indicator for source rocks and oils derived from similar types of organic matter (Seifert and Moldowan 1978, 1980, 1981). The oils from the Hsinchu offshore field generally have Ts/Tm values lower than that of the oils from the CS onshore field (Table 5). Since both fields have similar sources of terrigenous organic matter, the difference in Ts/Tm ratio suggests that the Hsinchu offshore oils are less mature than the CS oils, consistent with the geological and other geochemical data. The CS field is located closer to the Taihsi-Taichung Neogene basin center than Hsinchu offshore field (Fig. 2) and the vitrinite reflectance values (%Ro) for the corresponding source formations are higher in the CS field than in the Hsinchu offshore field (Chou et al. 1987). However, the small differences in organic source material between these two fields may also be a possible reason for the differences in the Ts/Tm values.

B. Non-hopanoid terpanes

The m/z 191 chromatogram in Fig. 6 indicates a number of non-hopanoid terpanes to be present in the Taiwan oils, many of which are probably source-specific. Peak 18 is a C30 compound which elutes between Ts and Tm. Although this C30 pentacyclic terpane has not been unequivocally identified, its presence in these oils from Taiwan as well as other oils from terrigenous sources (e.g. the Indonesia oil shown in Fig. 7) but not marine- derived oils suggests that its origin is related to terrigenous source material. A number of pentacyclic terpanes (e.g. peaks 20-24 in Fig. 6) including the C~8-bisnorhopane elute in the region between Tm and C~9 norhopane. Two Taiwan oil samples from Hsinchu offshore field (Samples 7 and 9 in Table 1) contain a number of C30 pentacyclic terpanes in exceptionally high concentrations in this region of the ehromatogram. Interestingly a similar distribution of these C30 compounds has also been found in oils from the Taranaki Basin, New Zealand and from Nigeria (Fig. 7). Although the identities of these compounds have not been unambiguously established, their presence permits them to be used as additional correlation or source parameters. The relative concentration of these C30 terpanes appears to be significantly higher in samples containing higher amounts of oleanane (Fig. 7). This may imply a relationship between the source of oleanane


Table 5. Biomarker parameters for maturity and source determination in Taiwan oils

Sample no. Well name 22S" M30/H30 b Ts/Tm Ole/H3(F C~R/S + R d Czg//[/e

1 A- I 0.59 0.23 0.25 0.17 0.52 0.50 2 A-I 0.60 0.25 0.13 0.34 0.52 0.46 3 K-1 0.59 0.13 0.50 0.17 0.50 0.57 4 K-2 0.61 0.16 0.31 0.15 0.49 0.53 5 K-2 0.57 0.20 0.32 0.26 0.65 0.47 6 K-2 0.53 0.24 0.27 0.33 0.66 0.45 7 K-3 0.57 0.28 0.I0 1.40 0.58 0.44 8 K-3 0.60 0.14 0.34 0.23 0.60 0.48 9 K-3 0.60 0.30 0.08 1.57 0.60 0.43

10 K-4 0.55 0.18 0.50 0.45 0.51 0.49 11 K-4 0.59 0.15 0.34 0.21 0.50 0.54 12 K-5 0.60 0.23 0.29 0.36 0.52 0.53 13 K-11 0.58 0.15 0.65 0.16 0.53 0.51 14 K-12 0.23 0.20 0.41 0.25 0.77 0.48 15 K-12 0.56 0.24 0.32 0.20 0.53 0.51 16 K-15 0.58 0.15 0.34 0.31 0.48 0.52 17 K- 15 0.63 0.20 0.31 0.13 0.54 0.48 18 L-1 0.61 0.20 0.23 0.17 0.49 0.53 19 S-1 0.57 0.17 0.27 0.19 0.55 0.52 20 S-2 0.60 0.22 0.22 0.22 0.60 0.48 21 K-16 0.58 0.21 0.18 0.45 0.60 0.47 22 CS-45 0.53 0.13 0.50 0.15 0.58 0.42 23 CS-53 0.56 0.19 0.47 0.11 0.60 0.48 24 CS-54 0.54 0.25 0.28 0.14 0.54 0.49 25 CS-58 0.60 0.13 0.67 0.08 0.50 0.55 26 CS--59 0.54 0.12 0.44 0.12 0.46 0.58 27 CS~0 0.58 0.16 0.58 0. I 1 0.52 0.53 28 CS-67 0.56 0.10 1.14 0.19 0.51 0.64 29 CS~9 0.48 0.19 0.42 0.18 0.49 0.57 30 CS~9 0.56 0.13 1.00 0.18 0.46 0.61 31 CS-70 0.56 0.10 0.87 0.29 0.48 0.62 32 CS-72 0.52 0.14 0.62 0.12 0.53 0.61 33 CS-74 0.50 0.11 0.89 0.05 0.51 0.63 34 T-2 0.58 0.14 0.44 0.17 0.50 0.54 35 T-8 0.55 0.12 0.78 0.48 0.48 0.60 36 TH-3 0.56 0.17 0.28 0.I0 0.51 0.54 37 TH-5 0.53 0.15 0.67 0.38 0.63 0.50 38 TH- 14 0.58 0.13 0.37 0.11 0.47 0.57 39 HK-110 0.53 0.16 1.15 0.18 0.49 0.57 40 W-1 0.55 0.18 1.35 0.I0 0.68 0.35 41 ST-5 0.60 0.13 0.39 0.10 0.42 0.57 42 PC-3 0.56 0.13 0.73 0.08 0.55 0.56 43 PS-3 0.59 0.11 0.91 0.20 0.46 0.59 44 PS-9 0.61 0.12 0.69 0.17 0.46 0.59 45 TCS-23 0.51 0.18 0.63 0.20 0.55 0.47 46 CHI 0.62 0.11 1.09 0.24 0.47 0.53 47 Chiahsien Creek 0.57 0.49 1.21 0.38 0.35 0.48 48 THS 0.58 0.24 0.20 0.19 0.52 0.52

"22S: ratio of 22S/(22S + 22R) for C3~-homohopanes. bM30/H30: Ratio of C30 moretane to C3o hopane. cOle/H30: Ratio of oleanane to C30 hopane. dC29R/S + R: Ratio of 20R C29 steranes in all C29 steranes. cC29~: Ratio of C29 tiff steranes in all C29 steranes.

and the C30 compounds. Peak 27 previously assigned as the C30-triterpane X (X-C30) which has been observed in many oils from Australia and New Zealand (Philp and Gilbert 1986, Czochanska et al. 1988) has now been identified as a 17~(H)-diahopane (Moldowan et al. 1991).

The presence of 18~ (H)-oleanane (peak 29 in Fig. 6) in all of the Taiwan oils is indicative of a terrigenous contribution to the source rocks. This compound has been reported in many terrigenous oils from Indonesia, Brunei and Sabah (Grantham et al. 1983), Nigeria (Whitehead 1974; Ekweozor et aL 1979) and New Zealand (Czochanska et aL 1988). However, Philp and Gilbert (1986) showed 18~ (H)-oleanane is not present in any of the Triassic/Jurassic oils from the Surat/Bowen Basin, Australia despite the abundance of higher plant

debris in their source rocks. It has now been fairly well established that the occurrence of oleanane parallels the evolution of the angiosperms which appeared in the late Cretaceous/early Tertiary. The presen~ of oleanane in Taiwan oils of Tertiary age but not in the rock extracts of terrigenous source from the early/middle Cretaceous in southwestern Taiwan offshore area is consistent with this inference (Oung, unpublished data).

The oils from Hsinchu offshore field generally have relatively higher concentrations of oleanane than oils from CS onshore field (Table 5). This again suggests that the source material for the Hsinchu offshore field contained more terrigenous source matter than the CS field, as suggested by the data of the tricyclic terpanes.

The presence of abundant diterpanes (e.g. phyllocladane and isopimarane) indicative of conifer

204 J.-N. Otmo and R. P, PHILP

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C 2 9 reorronged M/Z~ 217.2 s te l ' o r te$ t t r t -o5

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C 2 9 regulor steranes

70".00 ?5:00 8000 8~OO 90:00

INCREASING RETENTION T I M E - - - >

Fig, 8. The m/z 217 fragmentogram of GC-MS showing the representative sterane distribution in Taiwan oils.

(gymnosperm) debris, and oleanane, an indicator of angiosperms, in these oils suggests that both gymnosperms and angiosperms were present in the original source material.

STERANES

The sterane distributions for all of the oils examined in this study were similar suggesting similarities in the original source material. An m/z 217 sterane chromatogram illustrating a typical sterane distribution is shown in Fig. 8. This distribution shows a predominance of the C29 regular and rearranged steranes and relatively low concentrations of the C2s- and C27-steranes. The original proposal by Huang and Meinschein (1978) to use sterol distributions alone for predicting source material type has been criticized by some geochemists to be over simplified (Volkman, 1986). However, in the situation where there is a predominance of C~9 steranes and virtual absence of C27 and CES steranes, this is clearly indicative of terrigenous organic matter being the major source material for most of the oils in this study.

Within the C29 sterane distributions, the 13/~(H),17ct(H) rearranged steranes are the most abundant, suggesting that the oils have been generated from source rocks containing clay minerals (i.e. shales, marls) or from a relatively oxic environment. Rubinstein et al. (1975) suggested that rearranged steranes are formed by acid-clay-catalyzed backbone rearrangement of their sterene intermediates, consistent with the lithol- ogy of the source rocks in Taiwan. Exploratory drillings have shown that a great thickness of dark shales is present in the Tertiary formations.

For maturity assessment, the extent of side-chain epimerisation at C20 (20R/20R + 20S), as measured for the 50t (H),I4~ (H),17~t (H) C29-steranes (C29R/S + R; Table 5) are generally in the range of 0.5-0.6. These values are close to the equilibrium value of about 0.55

and are indicative of the maturity level for maximum hydrocarbon generation. Nuclear isomerisation [14~ (H),17~ (H)-14fl (n),17fl (n)] for both C29 20R and 20S epimers at this point is supposed to approach an equilibrium ratio which comprises approximately 0.75 14fl(H),17fl(H) C29-steranes (Seifert and Moldowan 1981, Mackenzie 1984). However the relative concentrations of 14fl (H), 17fl (H) C29-steranes for most Taiwan oils (shown as C:9flfl in Table 5) are around 0.50, considerably lower than what would be expected when compared to the C29 20R/20S + 20R ratios. This finding was also observed in the New Zealand oils (Czochanska et al. 1988) and is consistent with Grantham's (1986) observations of relatively low C29flfl concentrations in oils derived from Tertiary source rocks. The explanation being that insufficient time had elapsed in these Tertiary basins for the isomerisation to reach equilibrium despite the basins having attained high temperatures. Incomplete isomerisation was also observed in the ratio of C30-moretane to C30-hopane in these oils and discussed above.

CONCLUSIONS

The Taiwan oils examined in this study are generally highly paraffinic and low in their asphaltene and sulfur content. Geochemical studies have shown that Taiwan oils are derived from source rocks containing a significant proportion of higher plant source material including angiosperms and gymnosperms. A number of characteristic biomarker distributions have been observed in the Taiwan oils. These features include the presence of high quantities of long-chain regular isoprenoids, bicyclic sesquiterpanes and diterpanes, absence, or low concentrations, of tricyclic terpanes, presence of several C24 tetracyclic terpanes, some source- specific pentacyclic terpanes and a predominance of C29 steranes.

Geochemica l character is t ics o f oils f rom Ta iwan 205

The n -a lkane d i s t r ibu t ions and the relat ively high ra t ios o f pr i s tane over phy tane (general ly in excess o f 5) for mos t o f the Ta iwan oils suggest the l ikely prevalence o f highly oxic nearshore marine--open pea t swamp depos i t iona l env i ronments which inco rpora t ed large am oun t s o f ter r igenous organic mat ter . The long-chain i soprenoids extending to C42 and poss ib ly higher are p roposed to be regular head- to- ta i l i soprenoids which are poss ib ly der ived f rom poly isoprenols known to be present in higher p lan ts such as conifers. The presence o f d i te rpanes based on the i sopimarane , beyerane, phy l loc ladane and kau rane skeleton in Ta iwan oils suggests that resins der ived f rom conifers con t r ibu ted to the source o f these oils. This f inding is o f par t i cu la r significance to pe t ro l eum exp lora t ion since the resins which are rich in l ipids, i f preserved af ter deposi t ion , could be a good source for hydroca rbons .

The presence o f a number o f tetracycl ic terpanes in Ta iwan oils is a no teab le feature o f the b i o m a r k e r dis t r ibut ions . Wi th the a id o f M S / M S , a to ta l o f a t least nine tetracycl ic te rpanes with ca rbon numbers ranging f rom 24 to 27 could be assigned. Two series o f tetracycl ic te rpanes are apparen t , one is the de-A o leanane /ursane series and the o ther is a 17,21- secohopane series. The former is more source-specific than the la t ter and p r o b a b l y is re la ted to the presence o f oleanane. A number o f pentacycl ic terpanes have been detected, many o f which may be source-specific for ter r igenous source mater ials . Oleanane , de-A- o leanane /u r sane and de -A lupane can be used as indica tors for ang iosperms which have evolved since the late Cretaceous .

Sterane d is t r ibu t ions are domina t ed by the C29 steranes, including regular and rear ranged steranes indica t ing the ter r igenous organic ma t t e r in the shale as the oil source. Both the hopane and sterane isomers show incomple te i somer iza t ion which is consis tent with the observa t ions o f G r a n t h a m (1986) for oils f rom Ter t i a ry basins, indica t ing that insufficient t ime has e lapsed for these oils to reach equi l ibr ium values o f the b i o m a r k e r i somerizat ion.

Geochemica l studies on oil samples f rom var ious areas in Ta iwan show that the oils f rom Hsinchu offshore a rea have a more significant ter r igenous source mate r ia l than the oils f rom the Chinshui onshore field. The THS-1 oil f rom the middle Ta iwan area and the Chiahs ien Creek oil seep f rom south Ta iwan area are b iodegraded .

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I

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APPENDIX I

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geochemical characteristics of oils from taiwan

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