J. Cent. South Univ. Technol. (2010) 17: 169-179

DOI: 10.1007/s11771-010-0027-1                                                                                                                

Diagenesis and reservoir quality of the fourth member sandstones of Shahejie formation in Huimin depression, eastern China

ZHANG Jin-liang(张金亮)^{1, 2}, LI De-yong(李德勇)¹, JIANG Zhi-qiang(江志强)³

1. College of Marine Geo-science, Ocean University of China, Qingdao 266100, China;

2. College of Resources Science and Technology, Beijing Normal University, Beijing 100875, China;

3. China National Offshore Oil Corporation, Shanghai 200030, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2010

Abstract: Petrographic analysis combined with various techniques, such as scanning electron microscopy and X-ray diffraction, was used to assess the timing of growth and original mineral cements, the controls on reservoir and reservoir quality of the fourth member sandstones of Shahejie formation (Es4). The Es4 sandstones are mostly arkose and lithic arkose, rarely feldspathic litharenite, with an average mass fraction of quartz 51.6%, feldspar 33.8% and rock fragments 14.6% (Q_51.6F_33.8R_14.6). They have an average framework composition (mass fraction) of quartz 57.10%, K-feldspar 5.76%, sodium-calcium feldspar 13.00%, calcite 5.77%, dolomite 5.63%, siderite 0.95%, pyrite 0.30%, anhydrite 0.04%, and clay mineral 11.46%. The diagenentic minerals typically include kaolinite, illite-smectite (I/S), illite, chlorite, authigenetic quartz and feldspar, and carbonate and pyrite. Es4 sandstone has undergone stages A and B of eodiagenesis, and now, it is experiencing stage A of mesodiagenesis. Reservoir quality is predominantly controlled by the mechanical compaction, for example, 45.65% of the original porosity loss is related to compaction. The original porosity loss related with cementation is only 26.00%. The reservoir quality is improved as a result of dissolution of feldspar, rock fragment and so forth. The porosity evolved from dissolution varies from 3% to 4%.

Key words: sandstone diagenesis; kaolinite; reservoir quality; compaction; Huimin depression

1 Introduction

Huimin depression, until now, in which eight oil fields have been found, contained plentiful hydrocarbon resources. But the major oil productive series were almost shallow layer above the fourth member succession of the Shahejie formation (Es4). Assessing the oil producing capacity of the Es4 remained a problem. Diagenesis and reservoir evolution research of Tertiary sandstone in Shengli oilfield were brought into operation long time ago [1-2]. ZHU et al [3] studied the diagenesis of Paleogene sandstone in Huimin depression. ZHANG et al [4-6] did much work in connection with physical property, development of secondary pores and evaluation on controlling factors of Paleogene sandstones reservoir in Huimin depression. But no one particularly picked the Es4 sandstones as a research target. It was urgent to investigate the Es4 reservoir.

Diagenesis comprised a broad spectrum of physical, chemical and biological post-depositional processes that played an important role in the reservoir quality. The discovery of the secondary pores that were generated due to sandstone diagenesis especially attracted attention of researchers [7-11]. Understanding diagenesis of the Es4 sandstone was important for future exploration, appraisal and production. For example, the effect of burial depth on reservoir quality might supply a limit of depth, above which production from the Es4 reservoir would generate economic profit. Also, when a working reservoir was discovered in a basin, it would be of great use to know the permeability of the reservoir at different burial  depths, which might help to forecast undrilled prospects. Finally, the types and distribution of diagenetic clays might determine the type of drilling fluids to be used, as well as any future well simulation strategy. Therefore, this work was undertaken to enhance understanding of diagenesis and porosity of the Es4 sandstone in Huimin depression, with a focus on the controlling factors of porosity and the origin and timing of formation of the porosity modifying cement.

2 Geological setting

Huimin depression lay in the southwest of Jiyang depression in Bohai bay basin, east of China. It bordered on Shenxian depression in the west, Dongying depression in the east, and was limited to the north and south by Chengning and Luxi uplifts, respectively. As shown in Fig.1, the breadth of Huimin depression, reaching up to 90 km in latitudinal direction and 70 km from north to south, acted as the biggest subsidiary depression in Jiyang depression.

Fig.1 Location map of Huimin depression in Bohai bay basin, eastern China, showing main fields and faults in this area

After Yanshan movement, the northern China area went through a short lived time, free of tectonic movement. At the end of Cretaceous period, some districts rose and denuded. When it came to middle-late Palaeocene Epoch, the interesting region evolved a Caenozoic depression filled with terrigenous clastic sediments. At Cenozoic, in Huimin depression a system of fluvial and lacustrine deposit developed, consisting of predominate sandstones and mudstones reaching up to  3-5 km in thickness. A set of fluvial, delta, fan-delta, turbidite-fan, lake shore and deep or semi-deep lake sediments developed at the Paleogene. The Es4 formation could be divided into the bottom part (Es4x) and the top part (Es4s). The main part of the Es4x formation was composed of red claystones and red siltstones. The Es4s formation was chiefly made up of grey and dust claystones and sandstones in the western area and brown oil shale in the Shanghe area, as shown in Fig.2.

Fig.2 Generalized stratigraphy of Huimin depression (a) and lithologic character of Es4 formation in well Y12 (b)

The structures in this area were complex and diverse, containing some large, essentially NE—SW trending faults that divided Huimin depression into several important pieces, including Linnan sub-depression, Yangxin sub-depression, Lizezhen sub-depression, central uplift, southern gentle slope and Linfanjia bulge. The dominating faults were Zizhen, Yangxin and Qihe faults that controlled the depositional and structural character of Huimin depression.

3 Samples and methods

The identification of diagenetic products and interpretation of diagenetic sequence required various petrographic techniques. This work included data from the analysis of core material of 34 wells of Huimin depression with a present depth of 1.6-3.9 km. Thin sections impregnated with red epoxy were examined using backscattered electron petrographic microscope. The amount of detrital and diagenetic components and pore types, as well as the textural model grain size and sorting parameters, were determined by counting 300 points in each of 161 selected representative thin sections. Carbonate and other cements were stained for identification with alizarin red-S and potassium ferricyanide.

Twenty-nine samples from 28 wells were examined with a HITACHI S-530 scanning electron microscope (SEM) equipped with a HPILIPS 9100 energy dispersive X-ray spectrometer (EDS), a secondary electron (SE) and a high-resolution solid-state four-quadrant back- scattered detector, for interpreting the morphology and textural relationships among minerals. The EDS detector had an ultra-thin window to allow the detection of low energy X-rays emitted from light elements. In order to identify the clay minerals present in sandstones, X-ray diffraction (XRD) analysis of the clay fraction with size less than 2 μm was performed in a Rigaku RU 200 diffractometer. The samples used for XRD were air-dried, ethylene glycol-saturated and heated at 490 ℃ for 4 h. Polished thin sections were prepared for cathodoluminescence (CL). The CL analysis was carried out using a Technosym model CCL 8200 mk3a luminoscope stage to study calcite cement. The CL analysis was based on the following standard operating conditions: 16 kV accelerating voltage and 600 mA gun current. At the same time, an EPMA-1600 electron microprobe (EMP) was also used for qualitative elementary analysis.

4 Results

4.1 Sandstone texture and detrital mineralogy

Es4 sandstones are classified as fine to medium grained, moderately to well sorted. The diameter of grains generally extents from 0.05 to 0.25 mm and most of sandy grains are subangular. The level of clastation ranges from middle to feeble. Detrital grains’ touch is mainly point and point-line contact, and the relationship between grains and cement is fundamentally contacted cement and porous cement, partly basal cement.

Point counting data show that clastic grains include quartz with an average mass fraction of 51.6%, feldspar of 33.8% and rock fragments of 14.6% (Q_51.6F_33.8R_14.6), indicating that Es4 sandstones are mostly arkose and lithic arkose, rarely feldspathic litharenite [12], as shown in Fig.3. Quartz grains account for 20%-78% of the rock volume, feldspar 11%-45% and rock fragment 2%-57%. The types of rock fragments are igneous, metamorphic, sedimentary rock and glimmer. The Es4 sandstones have an average mass fraction of quartz 57.1%, K-feldspar 5.76%, sodium-calcium feldspar 13.00%, calcite 5.77%, dolomite 5.63%, siderite 0.95%, pyrite 0.30%, anhydrite 0.04% and clay mineral 11.46%.

Fig.3 Ternary diagram showing various types of sand found in Es4 sandstones

4.2 Diagenesis and authigenetic minerals

The Es4 sandstones in Huimin depression have undergone numerous diagenetic processes that exert a strong influence on reservoir quality. The main diagenetic processes that affect the Es4 sandstones include mechanical and chemical compaction, the precipitation of clay minerals, carbonate, pyrite, authigenesis of quartz and feldspar, and dissolution.

4.2.1 Clay minerals

According to the identification of X-ray diffraction, the clay cements in this area include chlorite, illite, kaolinite, and mixed-layer illite-smectite (I/S). The content and type of clay minerals evolve as burial depth changes.

(1) Kaolinite

Kaolinite is the most common clay mineral of the Es4 sandstones. Its relative amount ranges from 1% to 87%, averaging 29.98% of the clay volume. Under SEM, monocrystalline kaolinite is usually hexagonal tablet. Its polymer regularity shows vermicular, rarely book- textural, as shown in Fig.4(a). In samples from well Qx141, the surface of kaolinite is contaminated by organic material containing asphalt, as shown in Fig.4(b). Kaolinite occurs as pore-filling or replacement of feldspar, as shown in Fig.4(c). Although pores are filled by kaolinite, they are not completely occluded, with many relic pores and micropores between kaolinite crystals, as shown in Fig.4(a). Kaolinite will become friable when meeting water, which plays a key role in the permeability of reservoir.

(2) Mixed-layer illite-smectite

Mixed-layer I/S is the most clay mineral with an average mass fraction of 33.91%. When it is present in substantial quantities, it will appear as pore-filler, as well as grain-coating cement, forming typical type of flocculated structure, as shown in Fig.4(d). In the diagenetic process, smectite is gradually transformed into I/S as the burial depth increases. The mixed-layer ratio ranging from 20% to 30% demonstrates that the Es4 sandstones have gone into stage A of mesodiagenesis.

(3) Illite

Illite accounts for 1%-80% of clay minerals. Precipitation of illite is a late stage and a deep burial diagenetic event, which is widespread in Huimin depression. Illite presents constantly a flaky structure forming a illite-bridge contacting one grain with others to separate pores, as shown in Fig.4(e). This illite obstructs the migration of oil and pore fluid, resulting in a fast reduction of reservoir permeability.

(4) Chlorite

Chlorite is observed in approximately every sample, which occupies 1%-55% of the clay volume. Chlorite occurs typically as grain-coat around detrital grains, as shown in Fig.4(f). The thickness of chlorite-coat ranges from 1 to 3 μm. Other chlorite mainly acts as pore-filler that grows tangentially to the surface of detrital grains forming rosette texture, as shown in Fig.4(g). Chlorite grows before the overgrowth of quartz and usually generates with authigenetic quartz or appears in plagioclase emposieu [13], as shown in Fig.4(h) and Fig.4(f). The development of grain-coating chlorite preserves some porosities by preventing the secondary growth of quartz, but instead of contributing to the improvement of reservoir quality, its growth damages the reservoir.

Fig.4 Scanning electron micrographs showing character of authigenetic minerals: (a) Vermicular kaolinite polymer filling in pores

4.2.2 Authigenesis and overgrowth of quartz

Quartz cement occurs as overgrowth and crystallite that are seen frequently in this area, as shown in Fig.5(a). The mass fraction of quartz cement ranges from 0.5% to 6%. Authigenetic quartz crystal is nearly found in each sample. The quartz grains resume regular geometric pattern as a result of secondary growth, as shown in Fig.5(b). The width of overgrowth quartz is usually narrower than 50 μm, but in some samples from wells J203 and Ps3, it amounts to 100 μm. Overgrowth quartz and relic pores and micropores between kaolinite crystals; (b) Kaolinite contaminated by asphalt; (c) Dissolution of feldspar and kaolinite occurring as replacement of feldspar; (d) Grain-coating I/S cement; (e) Flaky illite-bridge cutting pores; (f) Grain-coating chlorite around grains and appearance of chlorite associating with albite; (g) Rosette like chlorite filling in pores; (h) Quartz overgrowth and relationship between overgrowth quartz and chloriteis often interrupted because flanking clay minerals develop. Actually, the precipitation of quartz cement is temperature-dependent and largely relates to time- temperature history of the rock buried [14]. The overgrowth level of quartz is usually in degrees Ⅱ-Ⅲ, as shown in Fig.5(c). Overgrowth quartz accompanied with kaolinite indicates that the dissolution of feldspar is a potential way to offer silicon dioxide source for quartz cementation, as shown in Fig.5(d). Mass balance calculations show that abundant silicon dioxide is released during the dissolution, which provides basic material for quartz cementation in the lightly cemented sandstones. The unstable feldspar dissolution can be described as follows:

2K[AlSi₃O₈]+16H₂O→2K⁺+2Al³⁺+8OH^?+6H₄SiO₄→

(K-feldspar)

2Al[SiO₅](OH)₄+4SiO₂+2K⁺+2OH^?+13H₂O  (1)

(Kaolinite)  (Quartz)

According to the quantified amount of feldspar, the dissolution of an average volume fraction of 2% of K-feldspar will produce 4% of quartz.

4.2.3 Carbonate cement

Carbonate cement is widespread in Es4 sandstones. Its total volume varies from 1% to 44%. Calcite and dolomite are the main part of carbonate cement. Carbonate cement can generally be classified as early stage cement and late stage cement. Early carbonate cement is composed of calcite and dolomite, which are generated primarily in the late stage of early diagenesis. The early carbonate chiefly exhibits as coarse xenomorphic crystal and mosaic cementation that seriously damages the quality of Es4 sandstone reservoir, as shown in Fig.5(e). Late carbonate including ferroan calcite and ferroan dolomite, is primarily generated in stage A of mesodiagenesis after authigenetic quartz, as shown in Fig.5(f). In some samples, as shown in Fig.5(g), calcite is generally encircled and metasomatized by ferroan calcite or ferroan dolomite, which approves the genetical order of early and late carbonate. The ferroan dolomite that is usually associated with ferroan calcite and detrital grain, is often metasomatized by carbonate. In reality, a special quantity of carbonate contributes to the property improvement of reservoir since it not only prevents significant mechanical compaction but also supplies dissoluble material for secondary porosity.

Fig.5 Scanning electron and cast thin section micrographs showing character of authigenetic minerals:(a)Scanning electron micrograph(SEM)showing authigenetic quartz crystallite;(b) SEMshowing quartz grains resuming regular geomtric pattern as result of secondary growth;(c)Cast thin section micrograph showing overgrowth level of detrital quartz;(d) SEM showing associated relationship bdtween overgrowth quartz and kaolinite:(e) Cast thin section micrograph showing calcite appearing as basal cement;9f0 SEM showing authigenetic quartz generating before dolomite;(g) Cast thin section micrograph showing anlerite overgrowing and metasomatizing ferroan calcite;(h) SEM showing authigenetic feldspars 

4.2.4 Authigenesis and overgrowth of feldspar

Feldspar cement is frequently seen in Es4 sandstones with a total mass fraction of less than 1.5%. As shown in Fig.5(h), it is mainly automorphic crystal, and rarely acts as the secondary enlargement feldspar. Conditions with sufficient silica, natrium, potassium and reasonable pressure and temperature in pore fluid are conducible for the generation of albite. Albitization of plagioclase is ubiquitous among Es4 sandstones. The reaction equation is described as follows:

NaAlSi₃O₈·CaAl₂Si₂O₈ +3SiO₂+2H₂O＋Na⁺→

(Sodium-calcium feldspar)

NaAlSi₃O₈+0.5CaAl₂Si₄O₂·4H₂O+0.5Ca²⁺         (2)

(Albite)    (Montmorillonite)

4.2.5 Pyrite

Pyrite is a minor diagenetic component. It accounts for 2%-5% of the rock volume as detected in very few samples. The monocrystalline pyrite represents octahedral structure, and when polymerized together, it acts as pellet. Under EMP, ferrum and sulfur respectively account for 53.85% and 45.72% of pyrite. The mass ratio of S to Fe is 0.85, which indicates that the pyrite generates in reducing environment.

5 Discussion and interpretation

5.1 Diagenetic evolution of sandstones

Due to the variation of palaeogeothermal, burial history, vitrinite reflectance, inclusion homogenization temperature and mixed clay mineral facies [15], the Es4 reservoir has undergone stages A and B of eodiagenesis, and now, it is experiencing stage A of mesodiagenesis, as shown in Fig.6.

Fig.6 Paragenetic sequence of diagenetic minerals and burial history of Es4 sandstones in Huimin depression

5.1.1 Stage A of eodiagenesis

The detrital materials were loosely packed with abundant initial pores. Clay mineral coats on grains, such as I/S, developed. Smectite, random-mixed clay minerals could be seen frequently in sandstones. The early overgrowth of detrital quartz also generated, but its amount was lower. Mechanical compaction proceeded during this stage. In this stage, Es4 formation burial depth gradually accreted with the time going by. The diagenesis of Es4 formation was mainly controlled by the lake basin depositional environment.

5.1.2 Stage B of eodiagenesis

Burial depth of Es4 formation became larger and larger. Mechanical compaction strengthened and reduced much more initial porosity. Most of early carbonate, especially calcite, precipitated. Quartz overgrowth was also commonly seen. Clay minerals, such as I/S, formed obviously, and a great deal of kaolinite appeared. At the end of this stage, the Es4 source rocks generated hydrocarbons, and the first episode of petroleum migration took place in Huimin depression. Abundant gaseous-liquid hydrocarbon inclusions were present in the overgrowth quartz and alternated feldspar, indicating that petroleum emplacement occurred after the quartz overgrowth, but before the alternative feldspar.

5.1.3 Stage A of mesodiagenesis

At the late Oilgocene series, soon after Es4 formation coming in mesodiagenesis A, the Es4 formation uplifted closing to the land surface. Therefore, Es4 sandstones suffered meteoriclithogenesis, resulting in great quantity of halloysite. Until Neocene, Es4 sandstone once more came to a period of burial diagenetic stage. When it came to stage A of mesodiagenesis, as the product evolved from mixed-layer I/S, chlorite and illite cements precipitated in mass. Overgrowth quartz and feldspar became bigger. At the end of Neocene, the second episode of petroleum migration happened. Hydrocarbons released from source rock of the third member succession of Shahejie formation were accumulated in reservoirs. Kaolinite was present in a large amount as a result of the dissolution of feldspar. Ferroan carbonate and pyrite also appeared. A considerable amount of secondary porosity developed.

5.2 Controls of diagenesis on reservoir quality

Reservoir quality, defined here using porosity and permeability, can be a function of many diagenetic controls including compaction, cementation and dissolution. Secondary porosity is a real benefit to reservoir quality [16].

Mechanical compaction is the most significant porosity, reducing agent in the Es4 reservoir. The degree of compaction of individual sandstones can be calculated as the difference between pre-cement porosity and the initial value [17]. The compaction porosity loss (L) can be estimated by [18]:

L=M-(100N-MN)/(100-N)                     (3)

where  M is the original porosity; N is the intergranular porosity before cementation but after compaction, whose value is the sum of present intergranular porosity plus total cement content [19]. M is calculated using the methodology of BEARD and WEYL [20], which can be expressed as:

M=20.91+22.90/σ                             (4)

where σ is Trask sorting factor. After calculation, the original porosity of Es4 sandstones varies from 30.07% to 38.56%. Therefore, L ranges from 1.00% to 30.48%, averaging 16.32%, as shown in Fig.7. That is to say, 3.33%-79.05%, 45.65% on average, of the original porosity loss links with compaction. Indeed, burial depth is the fide control factor on reservoir quality when compaction happens. Reduction of intergranular pores is enhanced with the increase of burial depth. The relational expression between porosity and burial depth of Es4 sandstones can be expressed as follows:

Φ=87.078exp(-0.000 6H)                      (5)

where Φ is the present porosity and H is the burial depth of Es4 reservoir. With the depth increasing from 2.0 to 2.1 km, the loss of porosity due to compaction is 1.53%. However, the loss of porosity is only 0.84% when burial depth increases from 3.0 to 3.1 km.

Fig.7 Plot showing compaction porosity loss (L) versus cementation porosity loss (K)

Cementation also plays an important role in the reduction of porosity. It coexists with compaction. The cementation porosity loss (K) can be evaluated by the methodology of Lundegard. The equation can be expressed as follows [17]:

K=(M-L)×Q/N                              (6)

where Q is the present cement volume fraction of rock. After calculation, K varies from 1.41% to 21.50%, with an average of 9.29%, as shown in Fig.7, which means that 4.51%-55.81%, averaging 26.00%, of the original porosity is occupied by the cement. The main factors that control K are carbonate, clay and quartz cements. The mass fraction of carbonate ranges from 0.80% to 51.30%, in which calcite occupies 77.29%. When the mass fraction of carbonate cements is higher than 18.40%, the porosity is less than 18.00%. Otherwise, when the mass fraction of carbonate is lower than 12.20%, the porosity may vary from 4.82% to 32.80%. There is not a simple correlation between porosity and carbonate, as shown in Fig.8(a). The reason for this is that though a minor part of carbonate occupies pore space, it also saves many pores by preventing compaction. In order to find the exact function of carbonate on reservoir, some sandstone samples, selected from sand body of the same microfacies at a similar burial depth, are tested. As shown in Fig.8(c) the relationship between porosity and mass fraction of carbonate cements can be expressed as the following equation:

Φ^*=-0.021 3C²+0.210 1C+27.633                (7)

Fig.8 Scatter diagrams showing relationship between porosity and cement: (a) Relationship between porosity and mass fraction of carbonate; (b) Relationship between porosity and mass fraction of calcite cement; (c) Relevant curve and equation between porosity and mass fraction of carbonate cement after compensating other factors’ effect on reservoir quality; (d) Relationship between porosity and mass fraction of clay cement; (e) Relationship between porosity and mass fraction of quartz cement; (f) Relationship between porosity and mass fraction of kaolinite cementwhere  Φ^* is the reservoir porosity; and C is the mass fraction of carbonate cement. As shown in Fig.8(a) and Fig.8(b), the relationship between porosity and mass fraction of calcite cement is similar to the relationship between porosity and mass fraction of carbonate. This indicates that calcite controls the process among all the components of carbonate. Clay cement, tending to fill pores and block pore throats, also plays an important role in reservoir [21]. As shown in Fig.8(d), when the mass fraction of clay cement is less than 1.5% or more than 7.0%, reservoir porosity is mostly lower than 10.0%. Whereas, when the mass fraction of clay varies from 2% to 6%, the porosity reaches up to 25% in some samples. This phenomenon mainly attributes to kaolinite, within which there exist many intercrystal pores, as shown in Fig.8(f). Quartz cement is less important. There is an obvious negative correlation between porosity and the mass fraction of quartz cement: high mass fraction of quartz is related to low porosity, and by contrary low mass fraction of quartz is related to relatively high porosity, as shown in Fig.8(e). The explanation for this is that, quartz is inflexible, helping to prevent compaction to some extent, but large amount of quartz cement may occupy much pore space.

Relating to reservoir quality, dissolution cannot be ignored. Generally, the dissolved intragranular pores, moldic pores and part of the oversized pores are bone with dissolution. So the porosity evolving from dissolution varies from 3% to 4%. The dissolved detrital grains include feldspar, rock fragment, interstitial clay material and carbonate. Reservoir quality is partly improved due to dissolution.

6 Conclusions

(1) The Es4 sandstones are mostly arkose and lithic arkose, belonging to a system of fluvial and lacustrine deposits, which experience mechanical and chemical compaction, cementation and dissolution, all of which significantly affect the reservoir quality. Es4 sandstones have undergone stages A and B of eodiagenesis, and now they are experiencing stage A of mesodiagenesis.

(2) In eodiagensis stage, the diagenetic minerals mainly include grain-coating clay minerals, early micritic carbonate and overgrowth quartz. Early carbonate, especially calcite that seriously damages the reservoir quality, is generated in a large amount. The first petroleum generation and migration happen at the end of stage B of eodiagenesis.

(3) Diagenetic processes in stage A of mesodiagenesis involve precipitation of authigenetic kaolinite, chlorite, illite, quartz, feldspar, ferroan calcite and ankerite, mixed-layer I/S and pyrite, as well as feldspar and detritus dissolution. The second petroleum generation and migration take place in this stage.

(4) Reservoir quality is predominantly controlled by the mechanical compaction, burial depth exactly, with cementation acting as a subordinate control. Calcite, clay minerals and quartz cement are the main elements of cementation that affect Es4 reservoir quality. Carbonate, kaolinite and authigenetic quartz have two-side effect on reservoir quality. Dissolution partly improves the reservoir quality.

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Foundation item: Project(2006AA09Z336) supported by the National High-Tech Research and Development Program of China

Received date: 2009-03-05; Accepted date: 2009-06-28

Corresponding author: ZHANG Jin-liang, PhD, Professor; Tel: +86-13864805481; E-mail: jinliang@ires.cn

(Edited by CHEN Wei-ping)