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Distribution and properties of high molecular weight hydrocarbons in crude oils and oil reservoir of Shengli oil field, China
Journal of Petroleum Science and Engineering 48 (2005) 227 – 240 www.elsevier.com/locate/petrol
Distribution and properties of high molecular weight hydrocarbons in crude oils and oil reservoir of Shengli oil field, China Changzheng Zhou a, Xiuyun Li b, Shengxiang Jiang a,* a
Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, Lanzhou, Gansu Province, 730000, China b Shengli Oil Field of SINOPEC, Dongying, Shangdong Province, 257000, China Received 29 October 2004; received in revised form 30 May 2005; accepted 1 June 2005
Abstract With the development of high temperature gas chromatography (HTGC), it is possible to examine high molecular weight hydrocarbons (HMWHCs) up to about C120. Utilizing the difference in solubility between HMWHCs and middle-low molecular weight ones, HMWHCs were separated from other components and concentrated. In order to keep the comparability of indies between conventional methods and HTGC, all samples from Shengli oil field of China were analyzed by several kinds of instruments. The contents and distribution characteristics of HMWHCs in various types of oils and the ability of antibiodegradation of HMWHCs were summarized. The ubiquitous occurrence of HMWHCs was found. The ability of antibiodegradation of HMWHCs is approximately equivalent to that of regular steranes. Dense sampling and analyses with HTGC and conventional instruments revealed the low mobility and heterogeneity of HMWHCs in oil reservoir of Gaoxie 73 well. The contents of HMWHCs in oil layers were obviously more than that in oils produced. D 2005 Elsevier B.V. All rights reserved. Keywords: HTGC; HMW hydrocarbons; Biodegradation; GC-MS; Fluidity
1. Introduction Petroleum geochemistry studies mainly the chemical compositions, structures and characteristics of biological ashes of natural organisms in earth, the regularities of distribution, evolution and transition of organic matters and their products under different * Corresponding author. Tel.: +86 931 4968271; fax: +86 931 827 7088. E-mail address: [email protected] (S. Jiang). 0920-4105/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2005.06.001
circumstances. Over a long period of time, the objects of petroleum geochemistry are the compositions of middle-low molecular weight since they are widely distributed and easily detected by conventional gas chromatography–mass spectrometry (GC–MS) (Peters and Moldowan, 1992). Most of them are derived from complex HMW compositions (Del Rio and Allen, 1992), so they are indirect and inaccurate in indices and characteristics to explain the generation of petroleum and its ancestors (Li and Lu, 1999). In the past, limited by hardware technologies, the studies on
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HMWHCs in exploited petroleum were few. Another reason was the drawbacks in pretreatment technologies, with which most of HMWHCs were lost. Recently, the appearance of HTGC leads to the extension of hydrocarbons which can be detected (Lipsky and Duffy, 1986a, b) and that makes it possible to study HMW compositions (above C40, up to C120) through a new route (Del Rio and Philp, 1992). As an offset of organic geochemistry, oil–gas reservoir geochemistry utilizes analytical methods of geochemistry to study fluids within oil and gas reservoirs; their origin and interaction with the reservoir medium at all scales. From researches, people found that HMW compositions are one of the important causes that hinder the exploitation of reservoir (Cubitt and England, 1995). These substances may remain in the migration pathway and reservoir rocks or precipitate in the drill pipe due to their low mobility (Burger et al., 1981; de Aquino Neto et al., 1994). HMW compositions in petroleum include bitumen, HMWHCs and some non-hydrocarbons (Carnahan, 1989). The study on these compositions is conducive to oil exploitation, reservoir assessment and oil field management. In fact, the formation and distribution of HMW compositions are controlled by oil source, sediment conditions, oil migration and aggregation (Hsieh and Philp, 2001). Through analyzing these substances, people can acquire more geological and oil reservoir information. HMWHCs found in crude oils are mainly longchain aliphatic, cycloalkyl and alkyl-aromatic hydrocarbons, and they represent a significant fraction in whole oil (Barker, 1995; Hsieh and Philp, 2001). Those compositions are related to the long-chain and HMW aliphatic structural moieties in source rocks (Lipsky and Duffy, 1986a,b; Mueller and Philp, 1998; Killops et al., 2000; Tuo and Philp, 2003; Huang et al., 2003). The existence of HMWHCs in oils of many regions has been demonstrated (Carlson et al., 1993; Philp et al., 1995). Crude oil can be categorized with oil condensate, light oil, ordinary oil and viscous oil according to viscosity, density and other physical and chemical parameters. There are many kinds of epigenesis that can affect the characteristics of crude oils. After accumulating in the reservoir, the evolution of oil does not stop. Actions such as thermal evolution, oxidation, water washing and bacteria degradation all change the original char-
acteristics of crude oils. Simulation experiments and actual samples demonstrate that the bacteria degradation starts from low molecular hydrocarbons and HMWHCs have the ability of anti-biodegradation (Setti et al., 1993; Heath et al., 1997). Utilizing HTGC, we can find out the existing information of HMWHCs in various kinds of crude oils and the influence of biodegradation to HMWHCs. All these are very meaningful in the aspect of enriching and perfecting the recognition to crude oils. In this study, on the basis of a HTGC method for HMWHCs in crude oils and oil sands, representative samples according to density and viscosity in Shengli oil field of eastern China were analyzed. Some interesting distribution regularities of HMW alkanes were summarized. The rational deduction on their ability of anti-biodegradation was discussed. One of the difficult to exploit wells was carefully studied. Significant quantity of HMWHCs were found in reservoir and ascertained as one of the main causes that led to very poor output.
2. Geological background Shengli oil field is the second largest oil-producing base in Shandong Province, eastern China (Fig. 1). It is a large scale Cenozoic petroliferous basin, mainly including Jiyang, Changwei, Jiaolai and Linqing Depressions. Its main body lies in the Yellow River Delta; the working area is on the both sides of the Yellow River mouth, covering some 44,000 sq. km. The main source rocks in this region are mudstone and shale of lake and fluvial facies in the third (Es3), fourth (Es4) and first (Es1) members of the Eocene–Oligocene Shahejie Formation and Dongying formation under fresh to brackish water river to lacustrine conditions. Shahejie, Dongying and Miocene Guantao formation are main reservoir stara. There has been more than 40-yr history of exploration and exploitation in Shengli oil field. Nowadays, with the development of oil production and appearance of new situations, there are many new problems to be solved in exploration and exploitation. HTGC is one of the powerful tools to better understand the origin and occurrence of HMW compositions in crude oils, source rocks and oil reservoirs. That is favorable for revealing the
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Fig. 1. Sketch map of the location of Shengli oil field in China.
mechanism of oil formation, exploiting new oil reservoirs, reducing operation costs and improving oil production (Tuttle, 1983; Escobedo and Mansoori, 1992; Leontaritis, 1996).
3. Experimental
3.2. Oil sand extraction 100 g of oil sand was broken into pieces then immersed in dichloromethane + methanol (93 : 7) and extracted by a hot Soxhlet-extractor at 80 8C for about 24 h. After the solvent was removed by warm heating, the extracts of oil sand was acquired and weighted precisely.
3.1. Samples 3.3. TLC/FID analysis In order to represent various kinds of samples, oils, sand stones and source rocks were selected from Chengdao, Kendong, Dongxin, Gaoqing, Gunan, Shanjiasi, Zhengjia, Linnan, Bamianhe and Chexi region in Shengli oil fields, and Dagang oil fields in Tianjin City, China. All these oil fields have been exploited and gone into production for many years. The properties of the oils in these oil fields were very clear, such as viscosity, density, maturity, sediment circumstances, kerogen types of source rocks, etc. The types of oils were from oil condensate to superviscous oil and the maturities of them were between middle mature and mature (oil condensates and light oils were highly mature). Representative oil samples and their basic data are listed in Table 1.
TLC/FID (Iatroscan MK-5, Japan) and Chromarod S3 were used to analyze the group components of oil and oil sand extracts. Method was standard (Number: SY/T 6338-1997, enterprise standard of China). Oil samples and oil sand extracts were dissolved in trichloromethane to make up solution of known concentration, then 1 AL of solution was dropped onto the origin of chromarod. Developers were n-hexane, dichloromethane + hexane (2 : 1) and dichloromethane + methanol (95 : 5), respectively. The group components of alkanes, aromatics, non-hydrocarbons and asphaltene were obtained. External standard method was used for quantitation. An oil or oil sand extract in a batch was selected as standard sample. The group components of the sample were quantified by conven-
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Well
Oil field
Formation
Depth (m)
Alkane content (wt. %)
Chengbei 303
Chengdao
Ar
3598.20
Ban 821
Dagang
Es1
Gunan 131
Gunan
Gao 12
a
HMW alkanes (wt. %)
Density (g/cm3)
Viscosity (mPad s, 50 8C)
52.51
0.63
0.8066
2609.35
43.74
0.94
Es3
3404.00
36.20
Gaoqin
Ek2
974.00
Zheng 36 Xia 70
Zhengjia Linnan
Ng Es3
Ken 108
Kendong
Shan 10-9 Shan 6-11 a
Depositional environment
Oil source
1.19
Marine
0.7881
1.18
Fresh water lacustrine
1.75
0.8548
9.15
Fresh water lacustrine
24.72
0.49
0.9614
1635.68 3294.20
18.53 35.94
almost none 1.31
1.0251 0.8419
Ng
1328.64
26.76
0.38
0.9842
537
Fresh water lacustrine
Shanjiasi
Ng
1094.18
18.68
almost none
0.9729
7352
Fresh water lacustrine
Shanjiasi
Ng
1176.31
17.1
almost none
1.1096
64,000
Fresh water lacustrine
Mudstone in Es3, Kerogen type II Mudstone in Es1, Kerogen type II2 Mudstone and oil shale in Es3, Kerogen type II1–III Mudstone in Es4, Kerogen type I–II1 Mudstone in Es4, Oil shale in Es3, Kerogen type I–II1 Mudstone in Es3, Kerogen type II2–III Mudstone in Es3, Kerogen type II2–III Mudstone in Es3, Kerogen type II2–III
Conventional column chromatographic method (SY/T5119-1995, enterprise standard of China).
286 5944 11.71
Saline lacustrine Marine Fresh water lacustrine
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Table 1 Representative oil samples and their basic data
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tional column chromatographic method (SY/T51191995, enterprise standard of China) and compared with the corresponding peak area of TLC, the associated quality of unit area of every chromatographic peak was determined, then the absolute quality of the group components of every sample was obtained. 3.4. Precipitation of HMW alkanes Concentration of HMW alkanes referred to the description of Cubitt and England (1995). Crude oil sample (10–20 g) or extracted substances (2–5 g) of oil sand were weighed precisely and dissolved in 50 mL of cyclohexane (oil sample) or 25 mL of cyclohexane (extracted substances), then shaken in an ultrasonator to the complete dispersion and left for about 30 min, the ambient temperature was 25 8C. The solution was centrifuged and the supernatant liquid was carefully removed, the precipitation was warmly heated to eliminate solvent, then 25 ml of tetrahydrofuran was added into the precipitation. The solution was heated in water bath at 60 8C to dissolve the precipitation. After that, the tetrahydrofuran solution was refrigerated to 0 8C for about 12 h and precipitation appeared again. After centrifugation and dryness, the residue was HMW alkanes and weighed precisely again. 3.5. HTGC analysis of HMW alkanes A HP6890 GC (with EPC) (Agilent, USA) equipped with a HT-5 aluminum plated fused-silica capillary column (25 m 0.32 mm 0.10 Am, SGE, Australia) and an on-column injector were used in HTGC analysis. The initial temperature of the column oven was 35 8C and kept for 3 min, then it was risen to 420 8C at the speed of 8 8C/min and kept for 50 min. A FID detector at 400 8C was used. The flow rate of H2 was 30 mL/min and that of air was 350 mL/min. The carrier gas was He with a flow rate of 30 mL/min. Before injection, HMW alkanes were dissolved in a little warm toluene. If necessary, standard substances (nC32 , Chemservice, USA) were mixed into HMW compositions and analyzed together to determine carbon numbers. A wax sample from oil refinery (nC15– nC70, the content of group components was 68.2% of alkanes, 18.3% of aromatics, 5.6% of non-hydrocarbons and 3.9% of asphaltenes. Petroleum University of East China, China) was also used for qualification.
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3.6. Whole oil GC analysis Whole oil GC is a common type of analysis for oil samples. The analysis will give a complete picture of the hydrocarbons present in the oil and also of the sulphur-containing compositions. It is important to have good resolution for both the light and heavy components. Here a Varian 3400 GC (Varian, USA) with a HP-1 fused silica capillary column (60 m 0.25 mm 0.22 Am, Agilent/J and W, USA) was used. Temperature of inlet and FID detector was all at 300 8C. The oven temperature was programmed from 35 8C for 10 min, then to 110 8C at 2 8C/min and followed by an increase to 280 8C at 5 8C/min and kept for 20 min. The flow rate of H2 was 30 mL/min and that of air was 350 mL/min. The carrier gas was He with a flow rate of 30 mL/min; split ratio was 50 : 1. 3.7. GC–MS analysis Oil samples were analyzed on a HP58902/5970B GC–MS. Column was SE-54 fused silica capillary column (30m 0.25mm 0.22Am, Agilent/J and W, USA). The oven temperature was programmed from 60 8C for 2 min, then heated to 300 8C at the rate of 2 8C/min. Temperature of injector was 300 8C. The carrier gas was He and its velocity was 30 cm/s. The MS was operated in full scan mode at 50–600 amu. Ionization mode was EI and Ionization energy was 70eV. Time for each scan was 0.35 s.
4. Results and discussion 4.1. Distributions of HMW alkanes in various types of crude oils Through conventional GC, the information of middle-low molecular weight hydrocarbons in various kinds of crude oils is familiar to researchers. Utilizing HTGC, we can find out the existing information of HMWHCs in various kinds of crude oils. In the process of HTGC analysis, the quantities of oil samples and their concentrated HMW alkanes are weighed precisely. Representative whole oil GC and HTGC chromatograms of oil samples are shown in Fig. 2. Some obvious characteristics by two kinds of GC are
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Fig. 2. Whole oil GC and HTGC analyses of various kinds of oils. Ar = Archaeozoic, Ek = Kongdian formation of Eocene, Ng = Guantao member of Neocene.
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listed in Table 2 as comparison. Regularities are obvious and meaningful. In former times, because of its high maturity, oil condensate was supposed to be absent from HMWHCs. In conventional GC, its highest peak is always before nC10 and components after nC25 are hardly observed. Light oil is similar. Its dominant peak is commonly less than nC15. But with HTGC, HMW alkanes are found in all of highly mature crude oils. Their original sources are complex. Perhaps they came from their source rocks, or were dissolved by light oils in the process of oil migration. In fact, HMW alkanes spread widely in various kinds of crude oils, but in most crude oils, their absolute amounts are small and generally less than 2.0 wt.%. This conclusion is in agreement with some recent researches (Heath et al., 1997; Thanh et al., 1999; Hsieh et al., 2000; Hsieh and Philp, 2001), in which HMWHCs were found as ubiquitous components in crude oils. As kerogen types of their source rocks are analogous, the amounts of HMW alkanes are mainly correlated with maturity and alkane contents of the crude oils. Usually, there are 0.5–2.0 wt.% of HMW alkanes in mature oils with more than 30 wt.% of alkane contents (except high-wax crude oils). Highly mature crude oils such as oil condensates also have a little amount of HMW alkanes, but their absolute contents are always less than 0.7 wt.%. In viscous crude oils, usually there is also less than 0.5 wt.% of HMW alkanes. With HTGC, it is found that the distribution of HMW n-alkanes is regular. In oil condensates, the carbon numbers of HMW n-alkanes can surprisingly extend to more than nC65, but between nC35 and nC65, peak profiles are obviously inclined toward light portion. That is to say, the highest peaks are always in the
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front and the height of subsequent peaks decrease rapidly. The carbon numbers of HMW n-alkanes in most of light oils also extend to nC65, the highest peaks usually appear near nC40 and the amounts of heavier hydrocarbons increase evidently. In ordinary mature crude oils, the peak profiles of HMW alkanes are approximately symmetric on both sides of the highest peak, sometimes they are even low in the front and high in the rear. The carbon range can extend to nC70, but the highest peaks change in dependence with the types of oil source, maturity, and so on. The amounts of wax in viscous oils are always low, and the amounts of HMW alkanes decrease quickly according to the increase of density, viscosity and degree of degradation. The highest peaks are mostly more than nC50, and the peak profiles are fairly low in the front and high in the rear. To most superviscous crude oils, almost no HMWHCs are found in the samples. 4.2. Influences of biodegradation on HMW alkanes Biodegradation is a main cause that makes oil quality worse. Many original characteristics of components are also changed by biodegradation (Palmer, 1984; Peters and Moldowan, 1992). Combining whole oil GC, GC–MS and HTGC, we can obtain the information about the ability of anti-biodegradation of HMW alkanes. In order to have comparability with the indices widely used nowadays, biodegradation of oil samples was first classified by the amounts of middle-low molecular weight hydrocarbons, isoprenoids, steranes, and terpanes. Then the analytical results of HMW alkanes were contrasted. Typical schematic diagram of biodegradation degrees is shown in Fig. 3 (Connan, 1983). Typical whole oil
Table 2 Main characteristics of whole oil GC and HTGC of HMW alkanes in various types of crude oils Sample
Whole oil GC Highest peak
Range of carbon number
Peak profile
Highest peak
Range of carbon number
Peak profile
Chengbei 303 Ban 821 Gunan 131
nC8 nC15 nC23
nC1–nC25 nC6–nC30 nC8–nC35
nC37 nC41 nC53
nC35–nC65 nC35–nC70 nC35–nC70
Gao 12 Zheng 36
n.d.a absent
nC20–nC35 absent
front crest front crest approximate symmetric n.d.a n.d.a
n.d.a absent
nC40–nC70 absent
front crest front crest approximate symmetric back crest n.d.a
a
n.d. = not discriminated.
HTGC
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Fig. 3. Typical schematic diagram of biodegradation degrees.
GC and HTGC chromatograms of oil samples in different biodegradation degrees are shown in Fig. 4 and representative GC–MS chromatograms of the same samples (m/z 191 and m/z 217) are shown in Fig. 5, respectively. Obviously, in the un- and low-biodegradated oils (Degree 1 to 3, sample Xia 70 in Figs. 4 and 5), the middle-low molecular weight hydrocarbons are obviously the majority and spread in a wide range of carbon numbers. Isoprenoids are complete, regular steranes are dominant, the amounts of rearrangement steranes are low and terpanes, especially macromolecular pentacycloterpanes, are not biodegradated. In HMW region, HMW alkanes from nC35 to nC70 are
complete. The lower carbon number compositions have relative higher abundance and no any absence. In the middle biodegradated oils (Degree 4 to 5, sample Ken 108 in Figs. 4 and 5), the loss of low and most middle molecular weight hydrocarbons happens. Part of isoprenoids still exists, but sometimes they disappear. Regular steranes and terpanes are hardly affected by biodegradation. The amounts of rearrangement steranes are still low. In HMW region, the distribution of HMW alkanes becomes narrow. The lower carbon number components begin to be degradated, the highest peak moves toward heavier part and the peak profile is evidently low in the front and high in the rear. In the highly and seriously
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Whole oil GC
235
HTGC
Xia 70, Es3
0
20
40
60
80
100
120
0
5
10
Retention Time (min)
15
20
25
30
35
40
45
Retention Time (min)
Un_light Degradation Ken 108, Ng Ph Pr
0
20
40
60
80
100
120
0
10
Retention Time (min)
20
30
40
50
Retention Time (min)
Middle Degradation, Degradation Degree 4
Shan 10_9, Ng
0
20
40
60
80
100
120
Retention Time (min)
0
10
20
30
40
50
Retention Time (min)
Heavy Degradation, Degradation Degree 6 Fig. 4. Whole oil GC and HTGC analyses of oil samples in different biodegradation degrees.
biodegradated oils (Sample Shan 10-9 and Shan 6-11 in Figs. 4 and 5), almost all middle-low molecular weight hydrocarbons are degradated. Chromatograms of whole oils show many small and unknown disordered peaks. Regular steranes decrease so much that they can hardly be distinguished but the relative amounts of rearrangement steranes are marked because of their ability of anti-biodegradation. The relative amounts of C25-norhopane are from remarkable (Sample Shan 10-9) to absent (Sample Shan 6-11), but tricycloditerpanes, tetracyclotriterpanes and gammacerane become more and more outstanding. HMW
alkanes are mostly damaged and it is even difficult to collect them by concentration. Only minimal residual concentrated middle molecular weight hydrocarbons appear. In fact, the conjectures and reports that HMWHCs have the ability of anti-biodegradation are believable (Setti et al., 1993; Heath et al., 1997). Here we draw some more detailed conclusions. The tendency of biodegradation on HMWHCs first happens to the lower molecular weight components and then to the higher molecular weight ones. The ability of antibiodegradation of HMWHCs becomes stronger with
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Fig. 5. GC–MS analyses of oil samples in different biodegradation degrees.
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237
Fig. 6. Sketch map of the tectonic location of Gaoqing oil field.
the increase of carbon numbers. The stability of HMW alkanes is equivalent to that of regular steranes, i.e., when regular steranes are extensively biodegradated, HMW alkanes are basically degradated and even disappeared. In general, when the biodegradation degree is more than Class 5 (serious degradation), all HMW alkanes are degradated.
4.3. Heterogeneity description of reservoir fluid in Es3 and Es4 of Eocene in Gaoxie 73 well In oil reservoir, the connectivity of sand body is not equal to that of fluid. There are many organic partition beds with different causes of formation in oil reservoir. These partition beds lead to the phe-
Fig. 7. The quantitative distributions of organic matter and polar compounds from 1970.00 to 2060.00 m. TLC plate: Chromarods III silica gel plate; Developers: Hexane, Dichloromethane + Hexane (2 : 1), isoamyl Alcohol + Hexane (1 : 9); Detector: FID; Temperature: 25 8C.
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nomena of connectivity of reservoir but separation of oil layers. It is reported that HMW compositions spread widely in reservoir (Cubitt and England, 1995). Most of them are bitumen, HMWHCs and part of non-hydrocarbons (Carnahan, 1989). Describing reservoir with conventional instruments and HTGC can give detailed and accurate information about the residue of the reservoir and positions of partition beds, which is helpful to improve secondary and tertiary oil recovery. Gaoxie 73 well lies in Gaoqing County of Shandong Province, China (Fig. 6). It lies in Gaoxie 73 block in north of Qingcheng Arch of Jiyang depression, where the thick sediment of sand body and good oil appearance are found. Source rocks of the oil are also found in the east Boxing SEG. In comprehensive explanations of mud logging and electric logging, there are thick oil layers there. But after exploitation,
the oil output of that well is very low. The density of oil produced is between 0.89 and 0.92 g/cm3. Viscosity is between 40 and 190 mPad s. Using TLC, whole oil GC and HTGC, more than 80 oil sand samples in depth from 1970.00 to 2060.00 m were analyzed. In HTGC analyses, the concentration and quantity of extracted substances injected of all oil sand samples were the same. The quantitative distributions of total extracted organic matter and polar components (non-hydrocarbons + bitumen) in the samples are shown in Fig. 7. Combined with the average porosity of the sand stone, oil properties and whole oil GC data, 25 mg of organic matter per gram extracted substances was ascertained as the lowest limit of oil layer in reservoir of Gaoxie 73 well. There are three layers of oil in the range of depth; others are mainly layers of oil immersion, oil-water and poor oil. In three oil layers, the
Whole Oil GC
HTGC
Poor Oil Layer
0
20
40
60
80
100
Retention Time (min)
0
10
20
30
40
50
40
50
40
50
Retention Time (min)
Oil Layer
0
20
40
60
80
100
0
10
20
30
Retention Time (min)
Retention Time (min)
Oil-water Layer
0
20
40
60
Retention Time (min)
80
100
0
10
20
30
Retention Time (min)
Fig. 8. Typical HTGC and whole oil GC analyses of the poor oil layer, the oil layer and the oil–water layer.
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quantities of unmovable oils (non-hydrocarbons + bitumen) account for more than 50 wt.% of the total extracted organic matter, maximum is 63.42 wt.%. That shows that the fluidity of oils is very bad and not easily exploited. It is found that HMWHCs spread widely in the reservoir and the abundance is rather rich. The typical chromatograms of HTGC and whole oil GC analyses of poor oil layer, oil layer and oil–water layer are shown in Fig. 8. From HTGC analyses, it is found that HMWHCs are also non-homogeneous in the reservoir. To the three samples above, HMW alkanes in the sample of oil layer are 0.81mg/g (2.84 wt.% in 28.50 mg/g of extracted substances). In the sample of oil–water layer they are 0.67 mg/g (4.74 wt. % in 14.14 mg/g of extracted substances). In the poor oil layer sample they are 0.09 mg/g (1.26 wt.% in 7.14 mg/g of extracted substances) and in empty layer there are hardly any HMW alkanes. To HMW alkanes in extracted substances of oil layers, the range of carbon numbers (nC47–nC70, with highest peak at nC57) is higher than that of ordinary oils. The contents of HMW alkanes are also higher than those of ordinary oils. That shows that HMWHCs are more easily retained in the reservoir, but not in the oil produced. Therefore, the reason for the low oil output in Gaoxie 73 well is mainly because of the high contents of non-hydrocarbons and bitumen (unmovable oils). The wide existence of HMW alkanes with high carbon range and high contents should be another important reason for the difficult exploitation of the oil. In the studied depth, there is no obvious interface of organic partition bed (tar bed). The reservoir is connective in vertical direction but the heterogeneity is obvious, the fluidity of oil is very bad and oil in the well is difficult to form an industrial oil flow. In order to raise the percentage recovery of this well, the tertiary oil recovery methods probing at bitumen and HMWHCs should be studied.
5. Conclusions The application of HTGC combined with other conventional instruments in Shengli oil field of China has revealed some regulations about the existence of HMWHCs in crude oils. The wide distribu-
239
tion of them was demonstrated, even in oil condensate and light oils. The carbon ranges, peak profiles and contents of HMW alkanes in different oils were summarized. Maturity and alkane content were the main factors that estimated the content of HMW alkanes. The anti-biodegradation ability of HMW alkanes was found and approximately equivalent to that of regular steranes. The content of HMWHCs in oil reservoir was obviously more than that in oils produced. In Gaoxie 73 well, they, together with other HMW compositions, even influenced the oil output seriously. Another meaningful deduction was that HMWHCs were easily retained in reservoir and it was a reason why they produce less oil and ignored by researchers for such a long time.
Acknowledgements We would like to thank Prof. Guangjia Zhou for his instruction and suggestions, Dr. Linye Zhang for her comments. We are grateful to Shengli Oil Company for their samples used in this study.
References de Aquino Neto, F.R., Cardoso, J.N., dos Santos Pereira, A., Zupo Fernandez, M.C., Caetano, C.A., de Castro Machado, A.L., 1994. Application of high temperature high resolution gas chromatography to paraffinic deposits in petroleum production pipelines. J. High Resolut. Chromatogr. 17, 259 – 263. Barker, A., 1995. The chromatographic analysis of refined and synthetic waxes. In: Adlard, E.R. (Ed.), Chromatography in the Petroleum Industry, Journal of Chromatography Library Series, vol. 56, pp. 55 – 93. Burger, E.D., Perkins, T.K., Striegler, J.H., 1981. Studies of wax deposition in the trans Alaska pipeline. J. Pet. Technol. 33, 1075 – 1086. Carlson, R.M.K., Teerman, S.C., Moldowan, J.M., Jacobson, S.R., Chan, E.I., Dorrough, K.S., Seetoo, W.C., Mertani, B., 1993. High temperature gas chromatography of high-wax oils. 22nd Annual Convention Proceedings. Indonesian Petroleum Association, Jakarta, Indonesia, pp. 483 – 507. Carnahan, N.F., 1989. Paraffin deposition in petroleum production. J. Pet. Technol. 41, 1024 – 1025. Connan, J., 1983. Petroleum biodegradation in oil reservoir. Advance in Petroleum Geochemistry. Petroleum Industry Press, Beijing, pp. 296 – 307. Cubitt, J.M., England, W.A. (Eds.), The Geochemistry of Reservoirs, Geological Society of London Special Publication, vol. 86, pp. 15 – 18.
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Del Rio, J.C., Allen, J., 1992. Nature and geochemistry of high molecular weight hydrocarbons (above C40) in oils and solid bitumens. Org. Geochem. 18, 541 – 553. Del Rio, J.C., Philp, R.P., 1992. High molecular weight hydrocarbons: a new frontier in organic geochemistry. Trends Anal. Chem. 11, 187 – 189. Escobedo, J., Mansoori, G.A., 1992. Heavy organic deposition and plugging of wells (Analysis of Mexico’s experience). SPE 23696, 349 – 362. Heath, D.J., Lewis, C.A., Rowland, S.J., 1997. The use of high temperature gas chromatography to study the biodegradation of high molecular weight hydrocarbons. Org. Geochem. 26, 769 – 785. Hsieh, M., Philp, R.P., 2001. Ubiquitous occurrence of high molecular weight hydrocarbons in crude oils. Org. Geochem. 32, 955 – 966. Hsieh, M., Philp, R.P., del Rio, J.C., 2000. Characterization of high molecular weight biomarkers in crude oils. Org. Geochem. 31, 1581 – 1588. Huang, H.P., Larter, S.R., Love, G.D., 2003. Analysis of wax hydrocarbons in petroleum source rocks from the Damintun depression, eastern China, using high temperature gas chromatography. Org. Geochem. 34, 1673 – 1687. Killops, S.D., Carson, R.M.K., Peters, K.E., 2000. High-temperature GC evidence for the early formation of C40+ n-alkanes in coals. Org. Geochem. 31, 589 – 597. Leontaritis, K.J., 1996. Offshore asphaltene and wax deposition: problems/solutions. World Oil 217, 57 – 63. Li, X.J., Lu, S.N., 1999. High temperature gas chromatography and the analysis of petroleum. Advance in Earth Sciences 2, 193 – 196. Lipsky, S.R., Duffy, M.L., 1986a. High temperature gas chromatography: the development of new aluminum clad flexible fused silica glass capillary columns coated with thermostable nonpolar
phases: Part 1. J. High Resolut. Chromatogr. Chromatogr. Commun. 9, 376 – 382. Lipsky, S.R., Duffy, M.L., 1986b. High temperature gas chromatography: the development of new aluminum clad flexible fused silica glass capillary columns coated with thermostable nonpolar phases: Part 2. J. High Resolut. Chromatogr. Chromatogr. Commun. 9, 725 – 730. Mueller, E., Philp, R.P., 1998. Extraction of high molecular weight hydrocarbons from source rocks: an example from weight hydrocarbons from source rocks. Org. Geochem. 28, 625 – 631. Palmer, S.E., 1984. Effect of water washing on C15+ hydrocarbon fraction of crude oils from northwest Palawan, Philippines. AAPG Bull. 68, 137 – 149. Peters, K.E., Moldowan, J.M., 1992. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, NJ. Philp, R.P., Bishop, A.N., del Rio, J.C., Allen, J., 1995. Characterization of high molecular weight hydrocarbons (NC40) in oils and reservoir rocks. In: Cubitt, J.M., England, W.A. (Eds.), The Geochemistry of Reservoir Rocks, Geological Society Special Publication, London, vol. 86, pp. 71 – 85. Setti, L., Lanzarini, G., Pifferi, P.G., Spagna, G., 1993. Further research into aerobic degradation of n-alkanes in a heavy oil by a pure culture of a Pseudomonas spp. Chemosphere 26, 1151 – 1157. Thanh, N.X., Hsieh, M., Philp, R.P., 1999. Waxes and asphaltenes in crude oils. Org. Geochem. 30, 119 – 132. Tuo, J.C., Philp, R.P., 2003. Occurrence and distribution of high molecular weight hydrocarbons in selected non-marine source rocks from the Liaohe, Qaidam and Tarim Basins, China. Org. Geochem. 34, 1543 – 1558. Tuttle, R.N., 1983. High pour-point and asphaltic crude oils and condensates. J. Pet. Technol. 35, 1192 – 1197.
Distribution and properties of high molecular weight hydrocarbons in crude oils and oil reservoir of Shengli oil field, China Changzheng Zhou a, Xiuyun Li b, Shengxiang Jiang a,* a
Key Laboratory for Natural Medicine of Gansu Province, Lanzhou Institute of Chemical Physics, Graduate School of the Chinese Academy of Sciences, Lanzhou, Gansu Province, 730000, China b Shengli Oil Field of SINOPEC, Dongying, Shangdong Province, 257000, China Received 29 October 2004; received in revised form 30 May 2005; accepted 1 June 2005
Abstract With the development of high temperature gas chromatography (HTGC), it is possible to examine high molecular weight hydrocarbons (HMWHCs) up to about C120. Utilizing the difference in solubility between HMWHCs and middle-low molecular weight ones, HMWHCs were separated from other components and concentrated. In order to keep the comparability of indies between conventional methods and HTGC, all samples from Shengli oil field of China were analyzed by several kinds of instruments. The contents and distribution characteristics of HMWHCs in various types of oils and the ability of antibiodegradation of HMWHCs were summarized. The ubiquitous occurrence of HMWHCs was found. The ability of antibiodegradation of HMWHCs is approximately equivalent to that of regular steranes. Dense sampling and analyses with HTGC and conventional instruments revealed the low mobility and heterogeneity of HMWHCs in oil reservoir of Gaoxie 73 well. The contents of HMWHCs in oil layers were obviously more than that in oils produced. D 2005 Elsevier B.V. All rights reserved. Keywords: HTGC; HMW hydrocarbons; Biodegradation; GC-MS; Fluidity
1. Introduction Petroleum geochemistry studies mainly the chemical compositions, structures and characteristics of biological ashes of natural organisms in earth, the regularities of distribution, evolution and transition of organic matters and their products under different * Corresponding author. Tel.: +86 931 4968271; fax: +86 931 827 7088. E-mail address: [email protected] (S. Jiang). 0920-4105/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2005.06.001
circumstances. Over a long period of time, the objects of petroleum geochemistry are the compositions of middle-low molecular weight since they are widely distributed and easily detected by conventional gas chromatography–mass spectrometry (GC–MS) (Peters and Moldowan, 1992). Most of them are derived from complex HMW compositions (Del Rio and Allen, 1992), so they are indirect and inaccurate in indices and characteristics to explain the generation of petroleum and its ancestors (Li and Lu, 1999). In the past, limited by hardware technologies, the studies on
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HMWHCs in exploited petroleum were few. Another reason was the drawbacks in pretreatment technologies, with which most of HMWHCs were lost. Recently, the appearance of HTGC leads to the extension of hydrocarbons which can be detected (Lipsky and Duffy, 1986a, b) and that makes it possible to study HMW compositions (above C40, up to C120) through a new route (Del Rio and Philp, 1992). As an offset of organic geochemistry, oil–gas reservoir geochemistry utilizes analytical methods of geochemistry to study fluids within oil and gas reservoirs; their origin and interaction with the reservoir medium at all scales. From researches, people found that HMW compositions are one of the important causes that hinder the exploitation of reservoir (Cubitt and England, 1995). These substances may remain in the migration pathway and reservoir rocks or precipitate in the drill pipe due to their low mobility (Burger et al., 1981; de Aquino Neto et al., 1994). HMW compositions in petroleum include bitumen, HMWHCs and some non-hydrocarbons (Carnahan, 1989). The study on these compositions is conducive to oil exploitation, reservoir assessment and oil field management. In fact, the formation and distribution of HMW compositions are controlled by oil source, sediment conditions, oil migration and aggregation (Hsieh and Philp, 2001). Through analyzing these substances, people can acquire more geological and oil reservoir information. HMWHCs found in crude oils are mainly longchain aliphatic, cycloalkyl and alkyl-aromatic hydrocarbons, and they represent a significant fraction in whole oil (Barker, 1995; Hsieh and Philp, 2001). Those compositions are related to the long-chain and HMW aliphatic structural moieties in source rocks (Lipsky and Duffy, 1986a,b; Mueller and Philp, 1998; Killops et al., 2000; Tuo and Philp, 2003; Huang et al., 2003). The existence of HMWHCs in oils of many regions has been demonstrated (Carlson et al., 1993; Philp et al., 1995). Crude oil can be categorized with oil condensate, light oil, ordinary oil and viscous oil according to viscosity, density and other physical and chemical parameters. There are many kinds of epigenesis that can affect the characteristics of crude oils. After accumulating in the reservoir, the evolution of oil does not stop. Actions such as thermal evolution, oxidation, water washing and bacteria degradation all change the original char-
acteristics of crude oils. Simulation experiments and actual samples demonstrate that the bacteria degradation starts from low molecular hydrocarbons and HMWHCs have the ability of anti-biodegradation (Setti et al., 1993; Heath et al., 1997). Utilizing HTGC, we can find out the existing information of HMWHCs in various kinds of crude oils and the influence of biodegradation to HMWHCs. All these are very meaningful in the aspect of enriching and perfecting the recognition to crude oils. In this study, on the basis of a HTGC method for HMWHCs in crude oils and oil sands, representative samples according to density and viscosity in Shengli oil field of eastern China were analyzed. Some interesting distribution regularities of HMW alkanes were summarized. The rational deduction on their ability of anti-biodegradation was discussed. One of the difficult to exploit wells was carefully studied. Significant quantity of HMWHCs were found in reservoir and ascertained as one of the main causes that led to very poor output.
2. Geological background Shengli oil field is the second largest oil-producing base in Shandong Province, eastern China (Fig. 1). It is a large scale Cenozoic petroliferous basin, mainly including Jiyang, Changwei, Jiaolai and Linqing Depressions. Its main body lies in the Yellow River Delta; the working area is on the both sides of the Yellow River mouth, covering some 44,000 sq. km. The main source rocks in this region are mudstone and shale of lake and fluvial facies in the third (Es3), fourth (Es4) and first (Es1) members of the Eocene–Oligocene Shahejie Formation and Dongying formation under fresh to brackish water river to lacustrine conditions. Shahejie, Dongying and Miocene Guantao formation are main reservoir stara. There has been more than 40-yr history of exploration and exploitation in Shengli oil field. Nowadays, with the development of oil production and appearance of new situations, there are many new problems to be solved in exploration and exploitation. HTGC is one of the powerful tools to better understand the origin and occurrence of HMW compositions in crude oils, source rocks and oil reservoirs. That is favorable for revealing the
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Fig. 1. Sketch map of the location of Shengli oil field in China.
mechanism of oil formation, exploiting new oil reservoirs, reducing operation costs and improving oil production (Tuttle, 1983; Escobedo and Mansoori, 1992; Leontaritis, 1996).
3. Experimental
3.2. Oil sand extraction 100 g of oil sand was broken into pieces then immersed in dichloromethane + methanol (93 : 7) and extracted by a hot Soxhlet-extractor at 80 8C for about 24 h. After the solvent was removed by warm heating, the extracts of oil sand was acquired and weighted precisely.
3.1. Samples 3.3. TLC/FID analysis In order to represent various kinds of samples, oils, sand stones and source rocks were selected from Chengdao, Kendong, Dongxin, Gaoqing, Gunan, Shanjiasi, Zhengjia, Linnan, Bamianhe and Chexi region in Shengli oil fields, and Dagang oil fields in Tianjin City, China. All these oil fields have been exploited and gone into production for many years. The properties of the oils in these oil fields were very clear, such as viscosity, density, maturity, sediment circumstances, kerogen types of source rocks, etc. The types of oils were from oil condensate to superviscous oil and the maturities of them were between middle mature and mature (oil condensates and light oils were highly mature). Representative oil samples and their basic data are listed in Table 1.
TLC/FID (Iatroscan MK-5, Japan) and Chromarod S3 were used to analyze the group components of oil and oil sand extracts. Method was standard (Number: SY/T 6338-1997, enterprise standard of China). Oil samples and oil sand extracts were dissolved in trichloromethane to make up solution of known concentration, then 1 AL of solution was dropped onto the origin of chromarod. Developers were n-hexane, dichloromethane + hexane (2 : 1) and dichloromethane + methanol (95 : 5), respectively. The group components of alkanes, aromatics, non-hydrocarbons and asphaltene were obtained. External standard method was used for quantitation. An oil or oil sand extract in a batch was selected as standard sample. The group components of the sample were quantified by conven-
230
Well
Oil field
Formation
Depth (m)
Alkane content (wt. %)
Chengbei 303
Chengdao
Ar
3598.20
Ban 821
Dagang
Es1
Gunan 131
Gunan
Gao 12
a
HMW alkanes (wt. %)
Density (g/cm3)
Viscosity (mPad s, 50 8C)
52.51
0.63
0.8066
2609.35
43.74
0.94
Es3
3404.00
36.20
Gaoqin
Ek2
974.00
Zheng 36 Xia 70
Zhengjia Linnan
Ng Es3
Ken 108
Kendong
Shan 10-9 Shan 6-11 a
Depositional environment
Oil source
1.19
Marine
0.7881
1.18
Fresh water lacustrine
1.75
0.8548
9.15
Fresh water lacustrine
24.72
0.49
0.9614
1635.68 3294.20
18.53 35.94
almost none 1.31
1.0251 0.8419
Ng
1328.64
26.76
0.38
0.9842
537
Fresh water lacustrine
Shanjiasi
Ng
1094.18
18.68
almost none
0.9729
7352
Fresh water lacustrine
Shanjiasi
Ng
1176.31
17.1
almost none
1.1096
64,000
Fresh water lacustrine
Mudstone in Es3, Kerogen type II Mudstone in Es1, Kerogen type II2 Mudstone and oil shale in Es3, Kerogen type II1–III Mudstone in Es4, Kerogen type I–II1 Mudstone in Es4, Oil shale in Es3, Kerogen type I–II1 Mudstone in Es3, Kerogen type II2–III Mudstone in Es3, Kerogen type II2–III Mudstone in Es3, Kerogen type II2–III
Conventional column chromatographic method (SY/T5119-1995, enterprise standard of China).
286 5944 11.71
Saline lacustrine Marine Fresh water lacustrine
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Table 1 Representative oil samples and their basic data
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tional column chromatographic method (SY/T51191995, enterprise standard of China) and compared with the corresponding peak area of TLC, the associated quality of unit area of every chromatographic peak was determined, then the absolute quality of the group components of every sample was obtained. 3.4. Precipitation of HMW alkanes Concentration of HMW alkanes referred to the description of Cubitt and England (1995). Crude oil sample (10–20 g) or extracted substances (2–5 g) of oil sand were weighed precisely and dissolved in 50 mL of cyclohexane (oil sample) or 25 mL of cyclohexane (extracted substances), then shaken in an ultrasonator to the complete dispersion and left for about 30 min, the ambient temperature was 25 8C. The solution was centrifuged and the supernatant liquid was carefully removed, the precipitation was warmly heated to eliminate solvent, then 25 ml of tetrahydrofuran was added into the precipitation. The solution was heated in water bath at 60 8C to dissolve the precipitation. After that, the tetrahydrofuran solution was refrigerated to 0 8C for about 12 h and precipitation appeared again. After centrifugation and dryness, the residue was HMW alkanes and weighed precisely again. 3.5. HTGC analysis of HMW alkanes A HP6890 GC (with EPC) (Agilent, USA) equipped with a HT-5 aluminum plated fused-silica capillary column (25 m 0.32 mm 0.10 Am, SGE, Australia) and an on-column injector were used in HTGC analysis. The initial temperature of the column oven was 35 8C and kept for 3 min, then it was risen to 420 8C at the speed of 8 8C/min and kept for 50 min. A FID detector at 400 8C was used. The flow rate of H2 was 30 mL/min and that of air was 350 mL/min. The carrier gas was He with a flow rate of 30 mL/min. Before injection, HMW alkanes were dissolved in a little warm toluene. If necessary, standard substances (nC32 , Chemservice, USA) were mixed into HMW compositions and analyzed together to determine carbon numbers. A wax sample from oil refinery (nC15– nC70, the content of group components was 68.2% of alkanes, 18.3% of aromatics, 5.6% of non-hydrocarbons and 3.9% of asphaltenes. Petroleum University of East China, China) was also used for qualification.
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3.6. Whole oil GC analysis Whole oil GC is a common type of analysis for oil samples. The analysis will give a complete picture of the hydrocarbons present in the oil and also of the sulphur-containing compositions. It is important to have good resolution for both the light and heavy components. Here a Varian 3400 GC (Varian, USA) with a HP-1 fused silica capillary column (60 m 0.25 mm 0.22 Am, Agilent/J and W, USA) was used. Temperature of inlet and FID detector was all at 300 8C. The oven temperature was programmed from 35 8C for 10 min, then to 110 8C at 2 8C/min and followed by an increase to 280 8C at 5 8C/min and kept for 20 min. The flow rate of H2 was 30 mL/min and that of air was 350 mL/min. The carrier gas was He with a flow rate of 30 mL/min; split ratio was 50 : 1. 3.7. GC–MS analysis Oil samples were analyzed on a HP58902/5970B GC–MS. Column was SE-54 fused silica capillary column (30m 0.25mm 0.22Am, Agilent/J and W, USA). The oven temperature was programmed from 60 8C for 2 min, then heated to 300 8C at the rate of 2 8C/min. Temperature of injector was 300 8C. The carrier gas was He and its velocity was 30 cm/s. The MS was operated in full scan mode at 50–600 amu. Ionization mode was EI and Ionization energy was 70eV. Time for each scan was 0.35 s.
4. Results and discussion 4.1. Distributions of HMW alkanes in various types of crude oils Through conventional GC, the information of middle-low molecular weight hydrocarbons in various kinds of crude oils is familiar to researchers. Utilizing HTGC, we can find out the existing information of HMWHCs in various kinds of crude oils. In the process of HTGC analysis, the quantities of oil samples and their concentrated HMW alkanes are weighed precisely. Representative whole oil GC and HTGC chromatograms of oil samples are shown in Fig. 2. Some obvious characteristics by two kinds of GC are
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Fig. 2. Whole oil GC and HTGC analyses of various kinds of oils. Ar = Archaeozoic, Ek = Kongdian formation of Eocene, Ng = Guantao member of Neocene.
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listed in Table 2 as comparison. Regularities are obvious and meaningful. In former times, because of its high maturity, oil condensate was supposed to be absent from HMWHCs. In conventional GC, its highest peak is always before nC10 and components after nC25 are hardly observed. Light oil is similar. Its dominant peak is commonly less than nC15. But with HTGC, HMW alkanes are found in all of highly mature crude oils. Their original sources are complex. Perhaps they came from their source rocks, or were dissolved by light oils in the process of oil migration. In fact, HMW alkanes spread widely in various kinds of crude oils, but in most crude oils, their absolute amounts are small and generally less than 2.0 wt.%. This conclusion is in agreement with some recent researches (Heath et al., 1997; Thanh et al., 1999; Hsieh et al., 2000; Hsieh and Philp, 2001), in which HMWHCs were found as ubiquitous components in crude oils. As kerogen types of their source rocks are analogous, the amounts of HMW alkanes are mainly correlated with maturity and alkane contents of the crude oils. Usually, there are 0.5–2.0 wt.% of HMW alkanes in mature oils with more than 30 wt.% of alkane contents (except high-wax crude oils). Highly mature crude oils such as oil condensates also have a little amount of HMW alkanes, but their absolute contents are always less than 0.7 wt.%. In viscous crude oils, usually there is also less than 0.5 wt.% of HMW alkanes. With HTGC, it is found that the distribution of HMW n-alkanes is regular. In oil condensates, the carbon numbers of HMW n-alkanes can surprisingly extend to more than nC65, but between nC35 and nC65, peak profiles are obviously inclined toward light portion. That is to say, the highest peaks are always in the
233
front and the height of subsequent peaks decrease rapidly. The carbon numbers of HMW n-alkanes in most of light oils also extend to nC65, the highest peaks usually appear near nC40 and the amounts of heavier hydrocarbons increase evidently. In ordinary mature crude oils, the peak profiles of HMW alkanes are approximately symmetric on both sides of the highest peak, sometimes they are even low in the front and high in the rear. The carbon range can extend to nC70, but the highest peaks change in dependence with the types of oil source, maturity, and so on. The amounts of wax in viscous oils are always low, and the amounts of HMW alkanes decrease quickly according to the increase of density, viscosity and degree of degradation. The highest peaks are mostly more than nC50, and the peak profiles are fairly low in the front and high in the rear. To most superviscous crude oils, almost no HMWHCs are found in the samples. 4.2. Influences of biodegradation on HMW alkanes Biodegradation is a main cause that makes oil quality worse. Many original characteristics of components are also changed by biodegradation (Palmer, 1984; Peters and Moldowan, 1992). Combining whole oil GC, GC–MS and HTGC, we can obtain the information about the ability of anti-biodegradation of HMW alkanes. In order to have comparability with the indices widely used nowadays, biodegradation of oil samples was first classified by the amounts of middle-low molecular weight hydrocarbons, isoprenoids, steranes, and terpanes. Then the analytical results of HMW alkanes were contrasted. Typical schematic diagram of biodegradation degrees is shown in Fig. 3 (Connan, 1983). Typical whole oil
Table 2 Main characteristics of whole oil GC and HTGC of HMW alkanes in various types of crude oils Sample
Whole oil GC Highest peak
Range of carbon number
Peak profile
Highest peak
Range of carbon number
Peak profile
Chengbei 303 Ban 821 Gunan 131
nC8 nC15 nC23
nC1–nC25 nC6–nC30 nC8–nC35
nC37 nC41 nC53
nC35–nC65 nC35–nC70 nC35–nC70
Gao 12 Zheng 36
n.d.a absent
nC20–nC35 absent
front crest front crest approximate symmetric n.d.a n.d.a
n.d.a absent
nC40–nC70 absent
front crest front crest approximate symmetric back crest n.d.a
a
n.d. = not discriminated.
HTGC
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Fig. 3. Typical schematic diagram of biodegradation degrees.
GC and HTGC chromatograms of oil samples in different biodegradation degrees are shown in Fig. 4 and representative GC–MS chromatograms of the same samples (m/z 191 and m/z 217) are shown in Fig. 5, respectively. Obviously, in the un- and low-biodegradated oils (Degree 1 to 3, sample Xia 70 in Figs. 4 and 5), the middle-low molecular weight hydrocarbons are obviously the majority and spread in a wide range of carbon numbers. Isoprenoids are complete, regular steranes are dominant, the amounts of rearrangement steranes are low and terpanes, especially macromolecular pentacycloterpanes, are not biodegradated. In HMW region, HMW alkanes from nC35 to nC70 are
complete. The lower carbon number compositions have relative higher abundance and no any absence. In the middle biodegradated oils (Degree 4 to 5, sample Ken 108 in Figs. 4 and 5), the loss of low and most middle molecular weight hydrocarbons happens. Part of isoprenoids still exists, but sometimes they disappear. Regular steranes and terpanes are hardly affected by biodegradation. The amounts of rearrangement steranes are still low. In HMW region, the distribution of HMW alkanes becomes narrow. The lower carbon number components begin to be degradated, the highest peak moves toward heavier part and the peak profile is evidently low in the front and high in the rear. In the highly and seriously
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Whole oil GC
235
HTGC
Xia 70, Es3
0
20
40
60
80
100
120
0
5
10
Retention Time (min)
15
20
25
30
35
40
45
Retention Time (min)
Un_light Degradation Ken 108, Ng Ph Pr
0
20
40
60
80
100
120
0
10
Retention Time (min)
20
30
40
50
Retention Time (min)
Middle Degradation, Degradation Degree 4
Shan 10_9, Ng
0
20
40
60
80
100
120
Retention Time (min)
0
10
20
30
40
50
Retention Time (min)
Heavy Degradation, Degradation Degree 6 Fig. 4. Whole oil GC and HTGC analyses of oil samples in different biodegradation degrees.
biodegradated oils (Sample Shan 10-9 and Shan 6-11 in Figs. 4 and 5), almost all middle-low molecular weight hydrocarbons are degradated. Chromatograms of whole oils show many small and unknown disordered peaks. Regular steranes decrease so much that they can hardly be distinguished but the relative amounts of rearrangement steranes are marked because of their ability of anti-biodegradation. The relative amounts of C25-norhopane are from remarkable (Sample Shan 10-9) to absent (Sample Shan 6-11), but tricycloditerpanes, tetracyclotriterpanes and gammacerane become more and more outstanding. HMW
alkanes are mostly damaged and it is even difficult to collect them by concentration. Only minimal residual concentrated middle molecular weight hydrocarbons appear. In fact, the conjectures and reports that HMWHCs have the ability of anti-biodegradation are believable (Setti et al., 1993; Heath et al., 1997). Here we draw some more detailed conclusions. The tendency of biodegradation on HMWHCs first happens to the lower molecular weight components and then to the higher molecular weight ones. The ability of antibiodegradation of HMWHCs becomes stronger with
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Fig. 5. GC–MS analyses of oil samples in different biodegradation degrees.
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237
Fig. 6. Sketch map of the tectonic location of Gaoqing oil field.
the increase of carbon numbers. The stability of HMW alkanes is equivalent to that of regular steranes, i.e., when regular steranes are extensively biodegradated, HMW alkanes are basically degradated and even disappeared. In general, when the biodegradation degree is more than Class 5 (serious degradation), all HMW alkanes are degradated.
4.3. Heterogeneity description of reservoir fluid in Es3 and Es4 of Eocene in Gaoxie 73 well In oil reservoir, the connectivity of sand body is not equal to that of fluid. There are many organic partition beds with different causes of formation in oil reservoir. These partition beds lead to the phe-
Fig. 7. The quantitative distributions of organic matter and polar compounds from 1970.00 to 2060.00 m. TLC plate: Chromarods III silica gel plate; Developers: Hexane, Dichloromethane + Hexane (2 : 1), isoamyl Alcohol + Hexane (1 : 9); Detector: FID; Temperature: 25 8C.
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nomena of connectivity of reservoir but separation of oil layers. It is reported that HMW compositions spread widely in reservoir (Cubitt and England, 1995). Most of them are bitumen, HMWHCs and part of non-hydrocarbons (Carnahan, 1989). Describing reservoir with conventional instruments and HTGC can give detailed and accurate information about the residue of the reservoir and positions of partition beds, which is helpful to improve secondary and tertiary oil recovery. Gaoxie 73 well lies in Gaoqing County of Shandong Province, China (Fig. 6). It lies in Gaoxie 73 block in north of Qingcheng Arch of Jiyang depression, where the thick sediment of sand body and good oil appearance are found. Source rocks of the oil are also found in the east Boxing SEG. In comprehensive explanations of mud logging and electric logging, there are thick oil layers there. But after exploitation,
the oil output of that well is very low. The density of oil produced is between 0.89 and 0.92 g/cm3. Viscosity is between 40 and 190 mPad s. Using TLC, whole oil GC and HTGC, more than 80 oil sand samples in depth from 1970.00 to 2060.00 m were analyzed. In HTGC analyses, the concentration and quantity of extracted substances injected of all oil sand samples were the same. The quantitative distributions of total extracted organic matter and polar components (non-hydrocarbons + bitumen) in the samples are shown in Fig. 7. Combined with the average porosity of the sand stone, oil properties and whole oil GC data, 25 mg of organic matter per gram extracted substances was ascertained as the lowest limit of oil layer in reservoir of Gaoxie 73 well. There are three layers of oil in the range of depth; others are mainly layers of oil immersion, oil-water and poor oil. In three oil layers, the
Whole Oil GC
HTGC
Poor Oil Layer
0
20
40
60
80
100
Retention Time (min)
0
10
20
30
40
50
40
50
40
50
Retention Time (min)
Oil Layer
0
20
40
60
80
100
0
10
20
30
Retention Time (min)
Retention Time (min)
Oil-water Layer
0
20
40
60
Retention Time (min)
80
100
0
10
20
30
Retention Time (min)
Fig. 8. Typical HTGC and whole oil GC analyses of the poor oil layer, the oil layer and the oil–water layer.
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quantities of unmovable oils (non-hydrocarbons + bitumen) account for more than 50 wt.% of the total extracted organic matter, maximum is 63.42 wt.%. That shows that the fluidity of oils is very bad and not easily exploited. It is found that HMWHCs spread widely in the reservoir and the abundance is rather rich. The typical chromatograms of HTGC and whole oil GC analyses of poor oil layer, oil layer and oil–water layer are shown in Fig. 8. From HTGC analyses, it is found that HMWHCs are also non-homogeneous in the reservoir. To the three samples above, HMW alkanes in the sample of oil layer are 0.81mg/g (2.84 wt.% in 28.50 mg/g of extracted substances). In the sample of oil–water layer they are 0.67 mg/g (4.74 wt. % in 14.14 mg/g of extracted substances). In the poor oil layer sample they are 0.09 mg/g (1.26 wt.% in 7.14 mg/g of extracted substances) and in empty layer there are hardly any HMW alkanes. To HMW alkanes in extracted substances of oil layers, the range of carbon numbers (nC47–nC70, with highest peak at nC57) is higher than that of ordinary oils. The contents of HMW alkanes are also higher than those of ordinary oils. That shows that HMWHCs are more easily retained in the reservoir, but not in the oil produced. Therefore, the reason for the low oil output in Gaoxie 73 well is mainly because of the high contents of non-hydrocarbons and bitumen (unmovable oils). The wide existence of HMW alkanes with high carbon range and high contents should be another important reason for the difficult exploitation of the oil. In the studied depth, there is no obvious interface of organic partition bed (tar bed). The reservoir is connective in vertical direction but the heterogeneity is obvious, the fluidity of oil is very bad and oil in the well is difficult to form an industrial oil flow. In order to raise the percentage recovery of this well, the tertiary oil recovery methods probing at bitumen and HMWHCs should be studied.
5. Conclusions The application of HTGC combined with other conventional instruments in Shengli oil field of China has revealed some regulations about the existence of HMWHCs in crude oils. The wide distribu-
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tion of them was demonstrated, even in oil condensate and light oils. The carbon ranges, peak profiles and contents of HMW alkanes in different oils were summarized. Maturity and alkane content were the main factors that estimated the content of HMW alkanes. The anti-biodegradation ability of HMW alkanes was found and approximately equivalent to that of regular steranes. The content of HMWHCs in oil reservoir was obviously more than that in oils produced. In Gaoxie 73 well, they, together with other HMW compositions, even influenced the oil output seriously. Another meaningful deduction was that HMWHCs were easily retained in reservoir and it was a reason why they produce less oil and ignored by researchers for such a long time.
Acknowledgements We would like to thank Prof. Guangjia Zhou for his instruction and suggestions, Dr. Linye Zhang for her comments. We are grateful to Shengli Oil Company for their samples used in this study.
References de Aquino Neto, F.R., Cardoso, J.N., dos Santos Pereira, A., Zupo Fernandez, M.C., Caetano, C.A., de Castro Machado, A.L., 1994. Application of high temperature high resolution gas chromatography to paraffinic deposits in petroleum production pipelines. J. High Resolut. Chromatogr. 17, 259 – 263. Barker, A., 1995. The chromatographic analysis of refined and synthetic waxes. In: Adlard, E.R. (Ed.), Chromatography in the Petroleum Industry, Journal of Chromatography Library Series, vol. 56, pp. 55 – 93. Burger, E.D., Perkins, T.K., Striegler, J.H., 1981. Studies of wax deposition in the trans Alaska pipeline. J. Pet. Technol. 33, 1075 – 1086. Carlson, R.M.K., Teerman, S.C., Moldowan, J.M., Jacobson, S.R., Chan, E.I., Dorrough, K.S., Seetoo, W.C., Mertani, B., 1993. High temperature gas chromatography of high-wax oils. 22nd Annual Convention Proceedings. Indonesian Petroleum Association, Jakarta, Indonesia, pp. 483 – 507. Carnahan, N.F., 1989. Paraffin deposition in petroleum production. J. Pet. Technol. 41, 1024 – 1025. Connan, J., 1983. Petroleum biodegradation in oil reservoir. Advance in Petroleum Geochemistry. Petroleum Industry Press, Beijing, pp. 296 – 307. Cubitt, J.M., England, W.A. (Eds.), The Geochemistry of Reservoirs, Geological Society of London Special Publication, vol. 86, pp. 15 – 18.
240
C. Zhou et al. / Journal of Petroleum Science and Engineering 48 (2005) 227–240
Del Rio, J.C., Allen, J., 1992. Nature and geochemistry of high molecular weight hydrocarbons (above C40) in oils and solid bitumens. Org. Geochem. 18, 541 – 553. Del Rio, J.C., Philp, R.P., 1992. High molecular weight hydrocarbons: a new frontier in organic geochemistry. Trends Anal. Chem. 11, 187 – 189. Escobedo, J., Mansoori, G.A., 1992. Heavy organic deposition and plugging of wells (Analysis of Mexico’s experience). SPE 23696, 349 – 362. Heath, D.J., Lewis, C.A., Rowland, S.J., 1997. The use of high temperature gas chromatography to study the biodegradation of high molecular weight hydrocarbons. Org. Geochem. 26, 769 – 785. Hsieh, M., Philp, R.P., 2001. Ubiquitous occurrence of high molecular weight hydrocarbons in crude oils. Org. Geochem. 32, 955 – 966. Hsieh, M., Philp, R.P., del Rio, J.C., 2000. Characterization of high molecular weight biomarkers in crude oils. Org. Geochem. 31, 1581 – 1588. Huang, H.P., Larter, S.R., Love, G.D., 2003. Analysis of wax hydrocarbons in petroleum source rocks from the Damintun depression, eastern China, using high temperature gas chromatography. Org. Geochem. 34, 1673 – 1687. Killops, S.D., Carson, R.M.K., Peters, K.E., 2000. High-temperature GC evidence for the early formation of C40+ n-alkanes in coals. Org. Geochem. 31, 589 – 597. Leontaritis, K.J., 1996. Offshore asphaltene and wax deposition: problems/solutions. World Oil 217, 57 – 63. Li, X.J., Lu, S.N., 1999. High temperature gas chromatography and the analysis of petroleum. Advance in Earth Sciences 2, 193 – 196. Lipsky, S.R., Duffy, M.L., 1986a. High temperature gas chromatography: the development of new aluminum clad flexible fused silica glass capillary columns coated with thermostable nonpolar
phases: Part 1. J. High Resolut. Chromatogr. Chromatogr. Commun. 9, 376 – 382. Lipsky, S.R., Duffy, M.L., 1986b. High temperature gas chromatography: the development of new aluminum clad flexible fused silica glass capillary columns coated with thermostable nonpolar phases: Part 2. J. High Resolut. Chromatogr. Chromatogr. Commun. 9, 725 – 730. Mueller, E., Philp, R.P., 1998. Extraction of high molecular weight hydrocarbons from source rocks: an example from weight hydrocarbons from source rocks. Org. Geochem. 28, 625 – 631. Palmer, S.E., 1984. Effect of water washing on C15+ hydrocarbon fraction of crude oils from northwest Palawan, Philippines. AAPG Bull. 68, 137 – 149. Peters, K.E., Moldowan, J.M., 1992. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, NJ. Philp, R.P., Bishop, A.N., del Rio, J.C., Allen, J., 1995. Characterization of high molecular weight hydrocarbons (NC40) in oils and reservoir rocks. In: Cubitt, J.M., England, W.A. (Eds.), The Geochemistry of Reservoir Rocks, Geological Society Special Publication, London, vol. 86, pp. 71 – 85. Setti, L., Lanzarini, G., Pifferi, P.G., Spagna, G., 1993. Further research into aerobic degradation of n-alkanes in a heavy oil by a pure culture of a Pseudomonas spp. Chemosphere 26, 1151 – 1157. Thanh, N.X., Hsieh, M., Philp, R.P., 1999. Waxes and asphaltenes in crude oils. Org. Geochem. 30, 119 – 132. Tuo, J.C., Philp, R.P., 2003. Occurrence and distribution of high molecular weight hydrocarbons in selected non-marine source rocks from the Liaohe, Qaidam and Tarim Basins, China. Org. Geochem. 34, 1543 – 1558. Tuttle, R.N., 1983. High pour-point and asphaltic crude oils and condensates. J. Pet. Technol. 35, 1192 – 1197.