An organic geochemical investigation of crude oils from Shanganning, Jianghan, Chaidamu and Zhungeer Basins, People's Republic of China

Org. Geochem. Vol. 14, No. 4, pp. 447--460, 1989 Printed in Great Britain. All rights reserved

0146-6380/89 $3.00+0.00 Copyright 1989 MaxwellPergamon Macmillanpie

An organic geochemical investigation of crude oils from Shanganning, Jianghan, Chaidamu and Zhungeer Basins, People's Republic of China R. P. PHILP*, LI JINGGUIt and C. A. LEWIS School of Geology and Geophysics, University of Oklahoma, Norman, OK 73019, U.S.A. (Received 22 May 1987; accepted 20 January 1989)

Abstract--Eighteen oils from four different oil producing basins in China have been characterized using a variety of organic geochemical techniques. The source materials for these oils were deposited under a variety of environmental conditions and this study investigates the use of geochemical parameters in differentiating the resultant oils. The parameters discussed are based on relative amounts and distributions of both individual and groups of biomarkers, including organosulphur compounds, and the carbon isotopic composition of the aliphatic and aromatic fractions. Examination of the oils showed that those from the Zhungeer basin, thought to be derived from source rocks formed in a brackish lacustrine environment, possessed high concentrations of tricyclic terpanes, relative to pentacyclic hopanes, and a predominance of 24-methyl- and 24-ethylcholestanes (C2s and C29, respectively) over cholestanes (C27), which were virtually absent. Oils believed to be derived from source rocks deposited in the saline depositional environment of the Jianghan basin were characterized by a large amount of gammacerane, relative to 17~t,21fl-30-homohopanes (C3~; 22R and 22S). In general, those oils considered to be derived from source rocks deposited in fresh water lacustrine environments (Shanganning and Chaidamu basins) had small amounts of tricyclic terpanes, relative to pentacyclic hopanes, the presence of an unknown C30 pentacyclic terpane, small amounts of extended hopanes (C3~--C35),relative to 17~t,21fl-hopane and relatively large amounts of 24-ethylcholestanes (20R and 20S), relative to total steranes. It appears, therefore, that the distribution of biomarkers may be used to differentiate oils derived from different lacustrine environments. Key" words--biomarkers, Chinese oils, Shanganning, Jianghan, Chaidamu, Zhungeer, isotopes, GC-MS

INTRODUCTION China is the oldest producer of petroleum in the world. The appearance of oil as surface seepages in various parts of the country has been noted by several historians and the oldest petroleum field is in Sichuan Province at Chi-liu-ching. It is recorded that in 211 B.C. bamboo drillstrings were used to drill the wells in this field. Other oil fields, near Shanghai and at Yuang Ping in the Shanganning basin (Ordos basin) were worked in the 12th century. However, apart from these instances, there was little further exploration in China until the end of the 19th century (Min, 1981). China's petroliferous regions are associated with transverse trends in the central and eastern parts of the country; however, in western China the basins are generally aligned along the length of the principal tectonic regions. Almost all of the interior basins generated oil from nomarine source rocks which accumulated in nonmarine sandstones, deposited mainly in large inland seas under low-energy sedimentary conditions (Fan Pu et al., 1980a, b). The geology and hydrocarbon potential of two Chinese *Author to whom reprint requests should be addressed. tPermanent address: Lanzhou Institute of Geology, Academia Sinica, Lanzhou, China. 447

and two Australian basins were recently investigated by Moore et al. (1986) in order to compare factors affecting the generation, migration and entrapment of hydrocarbons. In all four basins studied, hydrocarbons were generated from nonmarine source rocks of lacustrine and fluvial-overbank origin. The greater abundance of oil in the Chinese nonmarine basins was explained in terms of tectonic and palaeoclimatic factors yielding thicker and better quality source rocks along with rapid juxtaposition of these source rocks and good quality reservoirs. Relatively little detailed geochemical data has appeared in the literature on the oils and suspected source rocks from Chinese basins (however, see for example Shi Ji-Yang et al., 1982; Yang Wanli et al., 1985; Brassell et al., 1986, 1988; Fu Jiamo et al., 1986; Li Taiming et al., 1987; Philp and Fan Zhaoan 1987; Liu et al., 1988; Zeng et al., 1988; Fan Pu et al., in press). Probably the most comprehensive paper published to date is that by Shi Ji-Yang et al. (1982) on oils and source rocks from the Shengli oil field. There is considerable interest, at the present time, in assigning specific biomarkers to particular types of depositional environments (Moidowan et al., 1985; Mello et al., 1988). A number of biomarkers have already been suggested as being characteristic of certain environments, for example; 2,6,10,14,18-pen-

R. P. PHILP et al.

448

Table 1 Sample No.

Location Reservoirages and strata Basin (province) (depositionalenvironments)a % Ali % Aro % Pol Ningxia Late Triassic, Yanchang Series (Fr.W) 68 31 I 1 Shangannmg 51 47 2 2 Shangannmg Ningxia Late Triassic, Yanchang Series (Fr.W) 79 19 2 3 Shangannmg Ningxia MiddleJurassic, Zhiluo Group (Fr.W) 71 26 3 4 Shangannmg Ningxia Early Jurassic, Yanan Group (Fr.W) Late Triassic, Yanchang Series (Fr.W) 64 33 3 5 Shangannmg Shansi 6 Shangannmg Shansi Late Triassic, Yanchang Series (Fr.W) 69 27 3 81 18 2 7 Shangannmg Shansi EarlyJurassic, Yanan Group (Fr.W) 76 20 4 8 Shangannmg Shansi EarlyJurasic, Yanan Group (Fr.W) 84 14 2 9 Shangannmg Gansu EarlyJurassic, Yanan Group (Fr.W) 80 17 3 12 Shangannmg Ningxia Early Jurassic, Yanan Group (Fr.W) Hubei Eogene(Paleogene), Qianjiang Group (S) 65 23 12 13 Jianghan Hubei Eogene(Paleogene), Qianjiang Group (S) 48 48 4 14 Jianghan 15 Chidamu Qinghai Tertiary(S) 91 7 2 16 Chidamu Qinghai Jurassic(Fr.W) 73 22 5 17 Zhungeer Xinjiang Middle Triassic (Fr.Br) 86 11 3 18 Zhungeer Xinjiang Early Triassic (Fr.Br) 81 17 2 Xinjiang Early Triassic (Fr.Br) 79 14 7 19 Zhungeer 20 Zhungeer Xinjiang MiddleTriassic (Fr.Br) 81 15 4 °Fr.W = Fresh-water lacustrine; S = Saline Lacustrine; Fr.Br = Fresh-brackish lacustrine. tamethyleicosane, saline lagoonal-type environment (Waples et al., 1974); botryococcane, brackish water environment (Moldowan and Seifert, 1980); gammaccrane and 2,3-dimethyl-5-(2,6,10-trimethylundecyl)thiophene, hypersaline environment (Ten Haven et aL, 1988). The present study represents an attempt to extend the correlation of biomarkers with depositional environment and to investigate the nature of biomarkers in a number of previously unreported oils from four different basins in China. These oils are taken from the Shanganning, Jianghan, Chaidamu and Zhungeer Basins and represent various geologic ages and depositional environments including fresh to brackish and saline lakes (Table 1). GEOLOGICAL BACKGROUND

In the Permian southern China consisted of a marine environment whilst at the same time many continental lakes and basins were formed in northern China. During the Jurassic the sea receded, with the subsequent formation of continental lakes and basins in the south and in the Tertiary, only small regions in Taiwan and the far west of China were under marine influence (Fan et al., 1980a, b). Mesozoic and Cenozoic continental basins in China typically have a long period of continued subsidence and sedimentation. Thus, they are favorable to oil formation on account of the thick accumulation of superimposed source beds. The continental source rocks contain abundant fresh and brackish water biota, such as Charaphytes, Gastropods, Pelecopods, Ostracods, and various species of algae. Only in some intercallations of the Tertiary source rocks in north China are marine forms such as Foraminifera and Polychaeta found (Fan et al., 1980a, b). The salinity of the continental basins was controlled by climatic and replenishment conditions and fluctuates mostly in the fresh to brackish water range. The U p p e r Triassic and Lower Jurassic source rocks have a low chloride content and are classified as

Alkane range (CMAx) Pr/Ph Pr/n-Ci7 Ci2-(Ci,)-C3s 1.82 0.57 ---CI2~(C15)-C29 1.64 0.57 Ct2-(C~5)--C3~ 1.68 0.52 CI4~(CI7)--C29 1.46 0.41 Ci3-(Cts)--C31 1.25 0.49 Ci2-(Cis)-C31 1.19 0.45 CI4-(CI7)-C3t 1.17 0.45 CI(-(CI6)--C3o 1.43 0.54 CI2-(CI4)-C31 1.54 0.47 CI3-(CI5)-C33 0.62 0.91 CI2~(CI5)-C34 0.53 1.22 C1:-(CI3)--C26 1.37 0.54 Cl1-(C)3)-C27 3.00 0.33 CI2-(C~5)-C26 1.62 0.60 CI2-(CIs)-C2s 1.26 1.15 CIz-(CI5)-C34 1.58 0.42 CIj-(C~5)-C30 1.14 1.14

having been deposited in a fresh water environment. The chloride content of the Tertiary source rocks is higher, i.e. the depositional environment was typically brackish lacustrine and occasionally saline lacustrine. The U p p e r Jurassic and Lower Cretaceous rocks were deposited under conditions of intermediate salinity. Hence, one of the major factors differentiating these source rocks appears to be the salinity of the time of deposition. The four basins selected for this study are described briefly below.

(i) Shanganning Basin The Shanganning basin is a large sedimentary basin composed mainly of Mesozoic terrestrial strata with an area of 320,000 km 2 (Moore et al., 1986). It is the second largest petroliferous basin in China, and was the site of the first major commercial discovery and utilization of oil and gas in the country (Moore et al., 1986). Oil resources in the basin have been estimated as high as 3 billion barrels (Moore et al., 1986). This basin was located on a stable platform and developed into a huge and flat inland lacustrine basin in the U p p e r Triassic which produced sediments up to 2400 m thick by the Middle Jurassic. The major Mesozoic source rocks are thought to be the second and third members of the Triassic Yanchang Series (T3Y2, T3Y3, respectively), deposited in response to foredeep basin development, as the T a r i m Sinokorean and South China-Southeast Asia plates collided (Moore et al., 1986). The major source rocks are 300-500 m thick and the organic matter is composed predominantly of terrestrial plant detritus deposited in fresh water. In contrast to this, organic matter in the source rocks of the Early Jurassic Yanan G r o u p is composed predominantly of humictype material deposited in a lacustrine-brackish swamp environment and is approximately 40-120 m thick. The distribution of oil shows and fields in the Triassic is closely related to facies variations, with a

An organic geochemical investigation of crude oils

449

Table l~continued 28,30 Bisnorhop Gain/ G~0 hop ----------0.76 1.24 0.26 -0.45 0.52 0,31 0,43

T, frm 2,24 2.58 2.80 2.90 1.06 0.79 1.00 1,00 1.19 2.64 0,28 0.18 0.88 1.57 0.52 1,50 0.67 0.62

CutC~,

C./C~, hop -0.16 0.17 -0.17 0.18 0.21 0.12 --0.51 0.73 -0.13 --0.60 --

hop 0.34 0.33 0.32 0.24 0.23 0.20 0.33 0.34 0.26 0.53 0.53 0.93 0.34 0.46 0.57 0.58 0.60 0.31

Com. X/ C~9-hop 0,5 0.7 1.0 0.7 0.2 0.05 0.05 0.30 0.25 0.66 ---1.0 -----

C~9-hop 0.4 0.4 0.4 0.3 0.1 0.2 0.2 0.2 0.1 0.3 ---0.2 -----

l a r g e l a c u s t r i n e s y s t e m d e v e l o p i n g in t h e s o u t h e r n c e n t r a l p a r t o f t h e b a s i n . S o u r c e r o c k s u p to s e v e r a l h u n d r e d m e t e r s in t h i c k n e s s , w i t h d a r k s h a l e s p r e d o m i n a n t , a c c u m u l a t e d in this d e p r e s s i o n (Yin, 1985). M a n y o f t h e oilfields a l r e a d y d i s c o v e r e d a r e l o c a t e d in t h e s h a l l o w e r p o r t i o n s o f t h e s e d e p r e s s i o n s o r in a d j a c e n t , f l a n k i n g s t r u c t u r e s . Oil h a s n o t b e e n e n c o u n t e r e d in t h e n o r t h e r n p a r t o f t h e b a s i n , b e y o n d D i n b i a n , w h e r e t h e s e q u e n c e is m a r g i n a l l y m a t u r e a n d is d o m i n a t e d by fluvial facies.

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0.99 0.96 1.17 1.15 0.92 0.85 0.99 0.86 0.87 1.42 1.14 1.29 1.10 1.01 0.29 0.31 0.43 0.38

0.74 0.75 0.68 0.85 0.94 0.70 0.64 0.78 0.68 0.70 0.67 0.57 0.93 0.56 0.83 0.92 0.85 1.04

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-31.68 -32.84 -33.48 -33.10 - 32.25 -32.54 -32.78 -32.54 -32.43 - 33.27 -28.85 -28.22 -24.31 -31.43 - 29.29 - 29.94 -30.94 -30.01

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0.49 0.51 0.51 0.59 0.58 0.48 0.51 0.52 0.49 0.54 0.43 0.32 0.26 0.49 0.47 0.48 0.47 0.38

(ii) Jianghan Basin

/'('"~"i~ ..le "" i';;:~"~":', •1'

0.73 0.72 0.79 0.97 0.87 0.59 0.64 0.67 0.59 1.08 0.76 0.73 1.07 0.56 0.24 0.28 0.37 0.39

T h i s b a s i n is a C r e t a c e o u s - T e r t i a r y t i f t - d e p r e s s i o n w i t h a n a r e a o f 27,920 k m 2 a n d is s i t u a t e d to t h e s o u t h e a s t o f t h e S h a n g a n n i n g B a s i n (Fig. 1). T h e major source rocks are the Tertiary Qianjiang Group (3rd a n d 4 t h m e m b e r s ) J i a n g J i g o n g , 1985; P h i l p a n d F a n Z h a o a n , 1987), w h i c h were d e p o s i t e d in a saline lacustrine environment (Jiang Jigong and Zhang Q i a n , 1982). T h e r e f o r e , this s e q u e n c e is ideally s u i t e d to s t u d y i n g t h e e v o l u t i o n o f a terrestrial saline,

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Fig. 1. A m a p of China indicating the location of the samples that were collected and analysed as part o f this study. Locations are also summarized in Table 1.

450

R . P . PHILP et al.

possibly hypersaline, lacustrine environment and the generation of oils from such facies. The Qianjiang Group covers 2500 km 2, with an average thickness of 1300-2000 kin. The evolution of this basin is extremely interesting since the hot and dry palaeoclimate and high salinity of the aqueous medium had a profound effect on the abundance and nature of organic matter in the sediments. The major kerogen in the Qianjiang Group is a mixture of sapropelic and humic types whose preservations was favored by the high salinity in the basin. Philp and Fan Zhaoan (1987) have proposed that the major source rocks for the oils in this basin are in the second section of the Qianjiang Formation. It was also noted by Philp and Fan Zhaoan (1987) that the oils from the Qianjiang and the Xingouzui Formations were derived from different formations.

(iii) Chaidamu Basin The Chaidamu Basin is located to the west of the Shanganning basin in Qinghai Province (Fig. 1) and contains both Jurassic and Tertiary source rocks. The Jurassic source rocks were deposited in a fresh water lacustrine environment and the Tertiary sequence was deposited in a saline lacustrine environment. A reconstruction of the Tertiary depositional environment of the Chaidamu basin clearly shows the numerous rivers and deltas responsible for the input of the terrestrial organic material into the basin (Fan et al., 1980a, b).

(iv) Zhungeer Basin The Kelamayi oil field is located in the northwestern part of the Zhungeer basin and is elongated in a NE-SW direction. Basement of this area is slightly metamorphosed Lower Carboniferous rocks with overlying Permian, Triassic, Jurassic, and Lower Cretaceous deposits. The Permian system to the east of the field, composed of grayish-green and brownishred conglomerates is believed to be a major source for the oil. The Lower Triassic in the eastern part of the field is composed almost entirely of conglomerates and conglomeratic sandstones. The Upper Triassic is composed of fine-grained sandstone in the lower part and mudstone in the upper part. The Lower Jurassic has a layer of conglomerate and grades upward into coal-bearing sediments. EXPERIMENTAL

Eighteen oils from the four basins were examined and are listed in Table 1 along with their location, stratigraphic age of their reservoir formation and the general nature of the proposed depositional environment of their source rocks.

Sample Preparation Each oil (c. 3 g) was initially treated to remove asphaltenes by precipitation with n-pentane. Approx-

imately 100 mg of the n-pentane soluble fraction was further separated by silica-alumina column chromatography into fractions containing aliphatic hydrocarbons, aromatic hydrocarbons and polar components. These fractions were obtained by elution with cyclohexane, chloroform and methanol, respectively. Removal of n-alkanes was achieved by treating the aliphatic hydrocarbon fraction with S115 molecular sieve (Union Carbide Corp.) in 2,2,4trimethylpentane.

Gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS) Aliphatic hydrocarbon fractions were analyzed by GC using a Hewlett-Packa.rd 5890 gas chromatograph for the saturated hydrocarbons or a Varian 3300 gas chromatograph for the aromatic hydrocarbons and organosulphur compounds. Fused silica capillary columns used were coated with either DB-I or DB-5 (c. 25 m x 0.25 mm i.d.; 0.25/~m film thickness) and a temperature program from 40 to 150°C at 20°C min 1 and 150 to 300°C at 4°C min -~ was applied. Injection was performed in the split/splitless mode and injector and detector temperatures were held at 300°C. GC-MS analyses were performed using a Hewlett-Packard 5890 instrument interfaced directly to a Finnigan-MAT Series 700 Ion Trap Detector. A fused silica capillary column coated with DB-5 (c. 30 m x 0.25 mm i.d.; 0.25/~m film thickness) was temperature programmed from 40 to 140°C at 10°C min-1 and then to 300°C at 4°C min-1. The injector and transfer line temperatures were maintained at 300°C and c. 290°C, respectively. Helium (c. 1 ml min-J) was used as the carrier gas. The ITD manifold temperature was 230°C and electron current and energy were 80/~A and 70 eV, respectively. Extensive use was made of single ion monitoring to determine

Table 2. Peak identities of components in the rn/z 191 chromatograms Peak No. I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23-26

Identification C~9 Tricyclic terpane C20 Tricyclic terpane C2~ Tricyclic terpane C23 Tricyclic terpane C24 Tricyclic terpane C25 Tricyclic terpanes C24 Tetracyclic terpane C26 Tricyclic terpanes C2~ Tricyclic terpanes C:9 Tricyclic terpanes 17~tH-Trisnorhopane 18:t H-Trisnorhopane 17:t H,18:t14,21flH-28,30-Bisnorhopane 17¢tH,21 fl l-l-30-Norhopane 18ctH,21 fl l-t-30-Norhopane (?) C30 Terpane 17flH,21 a H-30-Norhopane 18~tH-Oleanane 17~tH,21 fl H-Hopane 17fl H,21 ctl-l-Hopane 17~tH,21fll-l-Homohopanes (22S + 22R) Gammacerane C32-C3~-ExtendedHopanes (22S + 22R)

An organic geochemical investigation of crude oils the distribution of long-chain isoprenoids (m/z 183), sequi-(123), di-(191) and triterpanes (191), steranes (217), phenanthrenes (178 and 192), and mono (253)and triaromatic (231) steroid hydrocarbons. 13C Isotopic Composition

The aliphatic and aromatic hydrocarbon fractions were statistically combusted according to the method of Sorer (1980). Following combustion, the tubes were opened on a vacuum line (2 x 10 -3 torr) and water was removed by trapping at - 7 0 ° C (Neslab Cryocool CC-60 cooling unit). The CO 2 was collected in a gas sampling ampoule at - 196°C (liquid N2) and then transferred directly to the inlet system of the VG 602C Micromass mass spectrometer equipped with a 90 ° sector magnetic deflector. The stable carbon isotopic composition is expressed as 13C(~)O) =

[Rsample/Rstandard-1] ×

103

where R is the abundance ratio of the heavy to light isotope of carbon (13C/~:C). The values are expressed relative to PDB carbonate (0%0) and are corrected for ]70 contribution. R E S U L T S AND D I S C U S S I O N

For the most part the oils were predominantly aliphatic in nature, with varying amounts of aromatic hydrocarbons and extremely small amounts of polar components (Table 1). With the exception of oil 2, from the Shanganning Basin, the aliphatic hydrocarbon chromatograms were dominated by alkanes in the C~3-C30 range (see examples in Fig. 2 and Table l). Oil 2, on the basis of its aliphatic hydrocarbon chromatogram, appears to have been extensively biodegraded. Oils 13 and 14 from the Jianghan saline lake environment had a noteable increase in concentrations of the higher molecular weight alkanes in the C:3--C30 region. The CPI values (not tabulated) for

451

virtually all of the oils examined were approximately l, although oils 13 and 14 showed a slight even-overodd carbon number predominance similar to that previously observed by Fu et al., 1986. Pristane/phytan¢ and pristane/n-C~7 ratios (Pr/Ph and Pr/n-C]7, respectively) were measured for all the oils, except oil 2 which is biodegraded (Table 1). Pr/Ph ratios for most of the oils were in the 1.1-1.8 range with the noteable exception of oils 13, 14 and 16 (0.62, 0.53 and 3.0 respectively). Pr/Ph ratios have been used for some time as indicators of depositional environments. Although the ranges of values for different types of depositional environments are relatively well accepted (Brooks et al., 1969; Powell and McKirdy, 1973), some caution needs to be exercised in their use. This is particularly true in the light of the alternative sources proposed for pristane (Goossens et al., 1984) and various other isoprenoids (Aibaiges et al., 1985). ten Haven et al. (1987) recently indicated the need to exercise caution when interpreting Pr/Ph ratios and applying them to the characterization of depositional environments. While not refuting the suggestion that sediments deposited under strictly anoxic conditions are characterized by low Pr/Ph ratios ten Haven et aL (1987) suggested that in hypersaline environments, productivity of halophilic bacteria is high. These bacteria contain lipids with the phytanyl moiety and hence can give rise to low Pr/Ph ratios in hypersaline environments. The two oils from the more saline, and possibly hypersaline, Jianghan basin (Nos 13 and 14) had relatively low values of 0.62 and 0.53, respectively, and agree with the proposal often Haven et al. (1987). However, the sample from the saline formation in the Chaidamu Basin has a relatively high Pr/Ph value of 3. This in turn may reflect differences in the salinity or source inputs of these two environments. The majority of Pr/n-C]7 values are in the 0.3-0.6 range which according to Didyk et al. (1978) would be indicative of a substan-

4

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Fig. 2. Selected chromatograms showing the n-alkanes for representative samples of the oils. Note that sample 2 has apparently been biodegraded and samples 13 and 14 are characterized by their relatively high concentration of phytane relative to pristane.

452

R.P. PHILP et al. 11~. 14

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Fig. 3 The relative concentrations of the tricyclic terpanes vary depending upon the nature of their original depositional environment. These chromatograms are selected to show variations in relative proportions of tricyclic to pentacyclic terpanes for representative samples (see Table 2 for peak identifications). tial input from marine source material. However, it would appear from the results of the current study that a similar observation may be applicable to saline or hypersaline lacustrine environments. Pr/n-Cl7 values greater than 1 would be generally indicative of a terrestrial input or source material to the samples (Didyk et al., 1978) although this parameter can also be affected by maturity. Biomarker Distributions It was observed that none of the oils contained detectable amounts of long chain isoprenoids ( > C:0), or diterpanes. This is significant since it tends to signify the probable absence of archaebacterial (Moldowan and Seifert, 1979), and resin containing higher plant (Noble et al., 1985) source material, respectively. (a) Terpanes and carotanes On the basis of the tricyclic, tetracyclic and pentacyclic terpane distribution described below, the oils could be divided into five major groups: (I) oils 1-5 and 8, 9 and 12 from the Shanganning basin and oil 16 from the Chaidamu basin, (II) oils 6 and 7 from the Shanganning basin, (III) oils 13 and 14 from the Jianghan basin, (IV) oil 15 from the Chaidamu basin, and (V) oils 17-20 from the Zhungeer basin. These groups roughly correspond to the divisions based on the suspected depositional environment of their source rocks (Table 1). A characteristic distribution

of the terpanes for each of the five groups is shown in Fig. 3. Groups I and II comprise oils suspected to be derived from source rocks deposited under a fresh water lacustrine environment. The terpane distributions of these two groups are somewhat similar but are generally distinct from the other three groups (III, IV and V). Group II is characterised by a distribution having low amounts of tri- and tetracyclic terpanes, relative to pentacyclic terpanes (Fig. 3). In addition, an unknown terpane, possibly 18~,21fl-30-norneohopane, (peak 15) eluting just after 17~,21fl-30-norhopane (peak 14) is present, along with the tentative presence of 17~,18~,21fl-28,30-bisnorhopane (peak 13), an unknown C30 terpane (peak 16; Fig. 3), previously referred to as compound X by Philp and Gilbert, (1986), and possibly gammacerane, all in relatively low concentrations. Group I is again characterised by low amounts of tri- and tetracyclic terpanes, relative to pentacyclic terpanes (Fig. 3) and the presence of the unknown terpane (peak 15) eluting after 17~,21fl-30-norhopane (peak 14). This group is also characterised by the presence of the unknown C~0 terpane (peak 16; Fig. 3) in amounts typically c. 50% that of 17c¢,21fl-30-norhopane (peak height; m/z 191 chromatogram) and 17~,18~,21fl28,30-bisnorhopane (peak 13) in amounts no more than c. 30% that of 17~,21fl-30-norhopane (peak height; m/z 191 chromatogram). Two oils comprising group III (Nos 13 and 14) are

An organic geochemical investigation of crude oils suspected to be derived from source rocks deposited in a saline lacustrine environment. An important characteristic of their terpane distribution is the presence of gammacerane (peak 22) in an amount c. 2-3 times that of the 17ct,21/~;-30-homohopanes (C3~; 22R and 22S peak 21) and for oil 14 it is the most abundant terpane (gammacerane/17~,21 fl-hopane ratio, 1.24; Table 1; Fig. 3). One of the earliest reports of gammacerane was in the Green River shale (Henderson et al., 1969) where it was proposed to be derived from tetrahymanol. Although the precise source organism (or organisms) remains unclear, it is apparent that a large amount of gammacerane, relative to other terpanes, is commonly associated with a saline and more probably a hypersaline environment (Fu et al., 1986; ten Haven et al., 1987). Moldowan et al., (1985) claimed, from a world-wide survey of 45 oils, that gammacerane was an ubiquitous component and, thus, was not an unequivocal indicator of depositional environment. However, when gammaccrane is present in as large a relative amount as in the group III oils (Fig. 3), a specific, although unknown, source (or sources) is associated with a saline to hypersaline depositional environment (Mello et al., 1988). Another characteristic feature of the group III terpane distribution is the presence of C34 and C35 extended hopanes (peaks 25 and 26) in high concentrations, relative to the lower molecular weight pseudohomologues (e.g. C31; Table 1). A similar situation has been observed in oils derived from carbonate source rocks (e.g. Palacas et al., 1984); however, in those cases it was often accompanied by a 17ct,21fl30-norhopane/17~t,21fl-hopane ratio of c. unity, which is not the case with the group III oils. Moldowan et al. (1989) has attributed such a distinction to a strongly reducing depositional environment. As for the group I and II oils, the tri- and tetracyclic terpanes are present only in low abundance, relative to the pentacyclic terpanes (Fig. 3). Other features of the group III oils are the presence of 17fl,21~,-hopanes (moretanes; C29 and C30 peaks 17 and 20) and of a small amount of 18ct-oleanane (peak 18). Although all the oils studied herein are derived from source rocks thought to be deposited by lacustrine environments, which most probably also experienced an input of higher plant material, 18~t-oleanane was present in detectable amounts only in the group III oils. The presence of 18~- oleanane has previously been associated with the evolution of angiosperms during the Cretaceous and Tertiary (Ekweozer et al., 1979 a and b; Philp and Gilbert, 1986). Group IV consists of a single oil (No. 15), from a Tertiary reservoir of the Chaidamu basin and suspected to be derived from source rocks deposited under saline lacustrine conditions. Tri- and tetracyclic terpanes are present in low amounts, relative to pentacyclic terpanes (Fig. 3), although they occur in slightly larger amounts than the oils discussed above (groups I, II and III) Gammacerane (peak 22) is

453

present in approximately the same amount as 17~t,21fl-30-homohopanes (22R and 22S peak 20). In addition, 17fl,21a-hopanes (C29 and C30 peaks 17 and 20) are present, with the C30 pseudohomologue being more abundant than the C29 pseudohomologue, as well as the unknown terpane (peak 15) eluting just after 17~t,21fl-30-norhopane (Fig. 3, peak 14; cf. groups I and II). This group bears a superficial resemblance to group III oils (Nos 13 and 14) in that, qualitatively, the same components are present; however, the relative amounts of the characteristic components such as tricyclic and pentacyclic terpanes including gammacerane is different from each group. Group V is comprised of four oils (Nos 17-20) thought to be derived from source rocks deposited under fresh-to-brackish lacustrine conditions. Tricyclic terpanes are present in amounts typically approximately equal to 17ct,21fl-hopane (Fig. 3). Gammacerane (peak 22) is present in amounts equal to or greater than 17~t,21fl-30-homohopanes (peak 21; 22R and 22S). Relatively small amounts of 17fl,21ct-hopanes (Peaks 17 and 20; C29 and C30) are also present. The relatively large amount of tricyclic terpanes in the group V oils implies that the environmental conditions prevalent during the deposition of these oil's source rocks were conducive to the growth of an organism (or organisms) responsible for the primary input of their precursors. In a previous publication it has been noted that the tricyclic terpanes are generally absent or at least not detectable in oils derived from terrestrial sources of organic material (Philp and Gilbert, 1986). However, the tricyclic terpanes are generally predominant features of the m/z 191 chromatograms from marine-derived oils and marine source rock extracts suggesting that these compounds are indicative of marine algae or bacteria (Philp and Gilbert, 1986). In the present study these compounds are present in varying amounts in the oils thought to be derived from the saline and fresh-brackish depositional environments (i.e. Groups III, IV and V). However, it is noteworthy that the relative concentration of pentacyclic terpanes is greatest in the Group V oils from the fresh-brackish depositional environments and not the oils of Groups III and IV derived from the saline depositional environment. This leads us to suggest the possibility that the occurrence of these tricyclic terpanes is indeed salinity controlled and that the environments that have been classified as being saline are of such a salinity that growth of the organisms producing the tricyclics, or their precursors is inhibited. Similarly the fresh-brackish type environments are proposed to have salinities similar to that of a more marine type environment thus promoting the growth of these organisms. A similar proposal was made in the recent paper by Mello et al., (1988). The high concentration of gammacerane further supports the idea that the depositional environment for oils 13 and 14 was hypersaline.

R.P. PI.'IILPet al.

454

OIL 4,17 BRANCHED/CYCLIC FRACTION

Pr

--INCREASING

RETENTION TIME

Fig. 4. Gas chromatograms for the branched/cyclic hydrocarbons from the Zhungeer sample No. 17. This sample, which contained relatively high concentrations of the tricyclic terpanes was characterized by the abundant presence of y- and fl-carotanes, marked here as peaks * and ** respectively. Unfortunately it was not possible for us to measure the concentrations of halite or gypsum in the original source rocks for these oils which would give us an indication of the salinity of the original depositional environment. However, it is suggested that in future studies such observations along with other inorganic geochemical measurements are made and an attempt is undertaken to correlate these observations with tricyclic terpane concentrations. This in turn could lead to the development of a salinity scale based on tricyclic terpane concentrations. Another interesting feature of the group V oils is the relatively high concentration of carotanes (Fig. 4). Carotenoids, although undoubtedly ubiquitous in the biosphere, are not commonly observed in ancient geochemical samples, especially oils. However, among other samples, they have been detected in the Green River shale (e.g. Murphy et al., 1967; Anders and Robinson, 1971), Tertiary shales from the Shengli oilfield, People's Republic of China (Shi Ji-Yang et al., 1982), certain oils from the People's Republic of China (e.g. Jiang and Fowler, 1986; Jiang Zhusheng et al., 1988) and various Brazilian oils (Mello et al., 1988). It seems likely that the environmental conditions under which the precursors of carotanes are preserved are rare and, hence, their detection in oil may be characteristic for this type of environment. However, at present we are unable to say what these are except that they appear to be related to a fresh water brackish type environment similar to that proposed for the Green River shale (Murphy et al., 1967; Anders and Robinson, 1971).

(b) Steranes The eighteen oils can be delineated into six groups on the basic of their sterane distribution (m/z 217 chromatogram). These groups differ slightly from those based on the m/z 191 terpane chromatograms. Hence to avoid unnecessary confusion with the grouping due to the terpane distribution, that due to the steranes has been designated as A through F; (A) oils 1, 2 and 5-9 from the Shanganning basin, (B) oils 3, 4 and 12 from the Shanganning basin and oil 16 from the Cbaidamu basin, (C) oil 13 from the Jiang-

han basin, (D) oil 14 from the Jianghan basin, (E) oil 15 from the Chaidamu basin, and (F) oils 17-20 from the Zhungeer basin (see Fig. 5). The oils classified into groups A and B are suspected to be derived from source rocks deposited under a fresh water environment. As with group I and II (oils 1-5, 8, 9, 12 and oils 6, 7 respectively), delineated by the terpane distribution, groups A and B are similar to each other but do not correspond exactly to those comprising I and II. Group A oils are characterized by relative amounts of steranes in the order C29 > C2s > C27 [peak height; Fig. 5(A)]. For the 24-ethylcholestane pseudohomolognes (C29), the 5~t,14fl,17fl isomer (20R and 20S) predominate over the 5~,14~t,17~ isomers (20R and 20S). Diasteranes are present although not in significant amounts, relative to non-rearranged steranes, as evidenced by 2OR- and 20S-13fl,17~t-diacholestane [Fig. 5(A); oil 7]. Group B oils are characterised by relative amounts of steranes in the order C29 ~ C27 > C2s [Fig. 5(B); oil 3]. As for group A oils, 2OR- and 20S-50t,14fl, l 7flethylcholestane predominate over the 5ct,14~,17~t isomers (20R and 20S). Diasteranes are present, in slightly larger relative abundance to the steranes than the group A oils. Groups C, D and E each comprise a single oil (Nos 13, 14 and 15, respectively), which are suspected to be derived from source rocks deposited under a saline lacustrine environment [Figs 5(C), 5(D) and 5(E) respectively]. The group C oil (No. 13) has a more complex sterane distribution than group D and E oils. It is characterised by relative amounts of steranes in the order C29 ~ C27> C2s. The 5~t,14fl,17fland 5~t,14ct,17ct-24-ethylcholestanes (20R and 20S) are present in approximately equal amount. A lower content of diasteranes is present, relative to non-rearranged steranes. Group D (No. 14) has the simplest sterane distribution of all the oils. In fact the distribution is reminiscent of those observed in many immature source rocks. The 5~t,14~t,17~t-steranes (20R and 20S) are present in relative amount in the order C29 ~ C27 > C2s. The 5~t,14fl,17fl-24-ethyicholestanes (20R and 20S) are present only in low abundance, relative to the 5et,14et,17~-isomers (20R and 20S) and it also appears to contain the 5fl,14et,17et-C29 sterane co-eluting with the 5~t,14fl,17fl-20R-ethylcholestane. This observation, in conjunction with a 20S/(20R + 20S) ratio of 0.32 for 5ct,14ct,17~t-24-ethylcholestane emphasises the idea that this oil looks immature with respect to its sterane distribution. In the past, the "anomalously" low values for biomarker maturity parameters, such 20S/(20R + 20S), in certain oils has been difficult to explain; however, it has been proposed that these ratios can depend on the heating rate during burial (Grantham, 1986; see also Mackenzie and McKenzie, 1983; Alexander et al., 1986) and, thus, oils may be generated which look "immature" with respect to the biomarker maturity parameters.

An organic geochemical investigation of crude oils

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Shi Ji-Yang et al. (1982) also noted that one oil from the Shengli oil field was immature based on biomarker measurements. There were no detectable diasteranes in this group. The sterane distribution of the group E oil (No. 15) shows some similarities to both group C and D (Nos 13 and 14, respectively). The distribution is again relatively simple but is slightly more complex than that for group D. The increased complexity is due to the presence of relatively low concentrations of diasteranes (cf. group C), which are absent from group D. The 5ct isomers (20R and 20S) are present in relative abundance in the order C27 ~ C28 ~ C:9. Again the 5ct,14fl,17fl-24-ethylcholestanes (20R and 20S) are present in lower abundance, relative to the 5~t,14~t,17or-isomers (20R and 20S). (20R)58,14ct,170t-24-ethylcholestane is present in larger amount than the (20S)-5ct isomer (cf. group D). Group F oils (Nos 17-20) are suspected to be derived from source rocks deposited under a fresh-tobrackish water environment. They are characterised by relative amounts of steranes in the order C29 ~ C28 > C27. There is a slight predominance of the 5ct-14fl,17fl-24-ethylcholestanes (20R and 20S) over the 5ct,14ct,17~ isomers (20R and 20S). The 20S/ (20R + 20S) ratio (5~,14ct,17~t-24-ethylcholestane) is 0.38-0.49, considerably lower than the generally accepted equilibrium value (0.54; e.g. Mackenzie, 1984 and reference therein). Diasteranes were not detected in group F oils. The presence of diasteranes is commonly associated with the presence of clay minerals and acid catalysed rearrangement of sterenes (Rubenstein et al., 1975). However, it is of interest to note that in this group of samples the diasteranes were absent despite the presence of clay minerals in the original depositional environment. Similarly the concentrations of diasteranes were also low or absent in the oils of group C, D and E which were also deposited under saline or hypersaline conditions. Whilst it is impossible to suggest at the present time that the formation of diasteranes is controlled by salinity it must be remembered that factors such as maturity will also play a role in determining the ratio of diasteranes/regular steranes (Seifert and Moldowan, 1978). (c) Aromatic hydrocarbons and sulphur containing organic compounds

The aromatic fractions of all the oils contained appreciable amounts of naphthalene and phenanthrene isomers. Methyl phenanthrene indices were calculated for the oils as previously described by Radke et al. (1982) but since it was noted that there was little correlation, if any, between the indices and reservoir depth the values will not be discussed in detail herein. The only useful observation that could be made was that within a basin the values were fairly consistent. Thus for example, the Shanganning Basin oils all appeared to be less mature than the oils from other basins and oils from the Zhungeer Basin consis-

tently appeared to be the most mature of all the oils examined. The distributions of organosulphur compounds in crude oils, as determined by gas chromatography using a flame photometric detector (FPD), were used by Hughes (1984) to differentiate oils derived from carbonate and siliciclastic sources. Oils presumed to be derived from carbonate sources contained an abundance of benzothiophenes, a similar concentration of the substituted dibenzothiophenes and a characteristic distribution of methyldibenzothiophenes. On the contrary, oils derived from siliciclastic sources commonly possessed low concentrations of benzothiophenes, decreasing amounts of substituted dibenzothiophenes and a distribution of methyldibenzothiophene isomers which differed considerably from that of carbonate derived oils. In the present study the aromatic fractions of most of the oils from the four different basins were examined by GC using a FPD (Fig. 6). From these chromatograms a number of comments can be made on variations within these distributions and their relationship to deposition environments. First, oils 17-20 which have already been distinguished on their sterane and terpane distributions showed a virtual absence of organosulphur compounds. All of the oils proposed to be derived from the fresh water lacustrine depositional environments (1-12 and 16) had similar organosulphur compound distributions dominated by the dimethyldibenzothiophenes and methyldibenzothiophenes. Benzothiophenes were virtually absent from all of these samples which according to the results of Hughes (1984) would be consistent with a non-carbonate source for the oils. The two oils (13 and 14) from the Jianghan Basin thought to have been generated from a saline, lacustrine source facies, had the most characteristic distribution of organosulphur compounds (Fig. 6). These two oils have already been distinguished by their low Pr/Ph ratios, low T s / T m ratios, high gammacerane/hopane ratio and the relatively low concentrations of tricyclic terpanes and the sterane distributions. All of these parameters could be rationalized by the nature of the saline, and in all probability hypersaline, depositional environments (Mello et al., 1988). The relatively high abundance and characteristic distribution of the organosulphur compounds in these two oils supports a hypersaline depositional environment. Many of the compounds present in these two oils are similar to those recently described by Sinninghe Damst6 et al. (1987) and isolated from hypersaline depositional enviroments. Oil 15, from the Chaidamu Basin, whilst also proposed to be derived from a saline lacustrine environment, does not appear to possess such high concentrations of the organosulphur compounds thought to be characteristic of hypersaline environments. Similarly, this oil also had a higher Pr/Ph ratio and lower gammacerane/hopane ratio consistent

An organic geochemical investigation of crude oils

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INCREASING

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Fig. 6. Variations in sulphur compounds between samples from the different depositional environments were also quite significant as illustrated in the flame photometric detector traces shown above. Peak A is benzothiophene; peaks B are mcthylbenzothiophene; peaks C are dimethylbenzothiophene and peak D is dibenzothiophene. with a lower salinity in the original depositional environment.

(d) Isotopic composition of aliphatic and aromatic fractions The carbon isotopic composition of the saturate and aromatic hydrocarbon fractions isolated from the eighteen oils were determined and are plotted in Fig. 7. F o r the most part these oils plot on or above the so-called waxy line described by Sorer (1984). More importantly, the isotopic distributions further support a number of the groupings based on various

biomarker distributions. The majority of oils from the Shanganning Basin group together with the exc e p t i o n of oil 12. No explanation for this can be offered at this time since there is nothing in the biomarker data which suggests anything particularly unusual about this sample. (However experimental error due to small sample size may be responsible for this anomaly.) The two oils from the Jianghan Basin have similar isotopic compositions to each other as do the four oils from the Zhungeer Basin. However, the two oils (Nos 15 and 16) from the Chaidamu Basin have widely different isotopic compositions

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458

R.P. PmLP et al.

with oil 15 being the heaviest of all the samples with

Acknowledgements--This work was supported in part by

613C~t = -24.31 and 6t3Carom= -23.55 whereas oil 16 had a much lighter isotopic composition. It is proposed this major difference is due to the fact that oil 15 is Tertiary and from a saline lacustrine environment whereas 16 is from a Jurassic fresh water lacustrine environment. In general the isotopic composition of these oils shows a tendency to become heavier with increasing salinity and those with the highest gammacerane content (with the exception of oil 15) also have the heaviest 613C composition. Hence in summary ~ ~3C values provide an additional tool for grouping these oils on the basis of their original depositional environments. The major groups are very close to those based on the terpane and sterane distributions as illustrated in Fig. 7.

funds from UNOCAL, and the NSF U.S.-China Cooperative Program ( # INT8518810). We also wish to thank Dr M. H. Engel and Mr R. Maynard for the carbon isotopic determinations.

CONCLUSIONS

Eighteen oils from four different basins and three different depositional environments have been characterized using a number of geochemical parameters. From these analyses it was possible to associate certain geochemical parameters with specific types of environments. Oils from the Shannganning Basin, a fresh water lacustrine environment, for the most part had very similar biomarker properties. The terpane distributions were identical for all of the oils apart from two (6 and 7) which contained relatively low concentrations of compound X (peak 16 in m / z 191 chromatogram). This was the only group of oils to contain significant amounts of the bisnorhopane. Oils 3, 4 and 12 had slightly different sterane distributions and contained higher abundances of diasteranes than the other oils within this group. The two oils from the Jianghan Basin possess properties characteristic of hypersaline rather than a saline depositional environment. The characteristic features of these oils were high gammacerane concentrations, virtual absence of diasteranes, low tricyclic terpane concentrations, low Pr/Ph ratio, high C35 hopane concentration, abundant organosulphur compounds of the thiolane type, and relatively hcavy isotopic composition for aliphatic and aromatic fractions. The two oils from Chaidamu clearly showed characteristic differences between the one originating from a sediment deposited in a saline environment versus those originating from a fresh water deposit. This is particularly expressed by the extremely heavy isotopic composition of the former oil. In the Zhungeer Basin, oils originating from sediment deposited in a fresh brackish environment were all very similar to each other. These oils were characterized by a high abundance of

tricyclic terpanes, significant proportion of the C34 hopanes, very high concentrations of fl-carotane, absence of diasteranes, low but detectable proportions of gammacerane, low concentrations of organosuiphur compounds and isotopic compositions which were all slightly lighter than those of oils from the more saline Jianghan Basin.

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