J. Cent. South Univ. Technol. (2011) 18: 898-908

DOI: 10.1007/s11771-011-0779-2

Recognition and depiction of special geologic bodies of Member 3 of Dongying Formation in Littoral Slope Zone, Qikou Sag

CHEN Si(陈思)^{1, 2}, WANG Hua(王华)², ZHOU Li-hong(周立宏)³, HUANG Chuan-yan(黄传炎)^{1, 2},

REN Pei-gang(任培罡)⁴, WANG Jia-hao(王家豪)², LIAO Yuan-tao(廖远涛)²,

XIANG Xue-mei(向雪梅)⁵, XIA Cun-yin(夏存银)⁶

1. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education,

China University of Geosciences, Wuhan 430074, China;

2. Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China;

3. PetroChina Dagang Oilfield Company, Tianjin 30028, China;

4. Department of Geology and Logging, Jiangsu Petroleum Exploration Bureau, Yangzhou 225000, China;

5. Sinopec Geophysical Research Institute, Nanjing 210014, China;

6. PetroChina Fushan Oilfield Company, Guangzhou 510240, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2011

Abstract: The purpose of this study is to forecast the profitable oil and gas reservoir, which is the key of finding hydrocarbon, based on the conception of special geologic bodies. With the guiding methodology of the research thinking of integration of point-line-surface by using the methods and techniques of logging, seismic, seismic attribute, and logging constrained inversion in 3D data volume, the special geologic bodies of Member 3 of Dongying Formation in Littoral Slope Zone of Qikou Sag, which has important hydrocarbon exploration potential, are recognized and described under the constraint of sequence stratigraphic framework. As a result, the developed scale, geometric shape and space distribution feature of the special geologic bodies are forecasted; the inner structure and sequence structure patterns of the geologic bodies are also ascertained. From the lowstand system tract (LST) and lacustrine expanding system tract (EST) to the highstand system tract (HST), the geologic bodies have evolved from relative centralization of lake basin reducing period to three relative dispersive isolated parts of broad lake basin period. According to the relevance and regularity of the development of geologic bodies, the conclusions can be obtained that three types of potential profitable reservoir traps, including the lithologic lens traps, lithologic updip pinchout traps and structural-lithologic composite traps, are forecasted. In addition, scientific basis for further hydrocarbon exploration in new area (few-well area and no-well area) is offered in the guidance of sequence stratigraphic model.

Key words: Qikou Sag; Littoral Slope Zone; Dongying Formation; special geologic bodies

1 Introduction

The key of finding favorable hydrocarbon reservoir is to find the favorable oil and gas reservoir and analyze its subtle trap, so the recognition and meticulous depiction of favorable reservoir has been always focused on [1-3]. According to the statistics of Baum in 1995, 86% reserves of most oil and gas fields in the world were hosted in the lowstand system tract (LST). The statistics and analyses of some oil test interval in Song-Liao basin by YI et al in 1997, showed that 72% oil and gas interval distributed in the lowstand system tract (LST), and 24% oil and gas interval distributed in the highstand system tract (HST). The exploration of Neogene in Mexico Basin since 1980s showed that basin floor fan system with thick sandstone, slope fan, and foreset wedge block were rich in oil and gas, and the high quality reservoir sandstone of foreset wedge block distributed in turbidite facies and delta front facies. Therefore, it has important significance to recognize and forecast the favorable reservoir under the constraint of the sequence stratigraphic framework [4-6]. Taking the Member 3 of Dongying Formation in Littoral Slope Zone in Qikou Sag for example, the recognition and depiction of special geologic bodies in this region are described in this  work.

JIANG and WANG [7] put forward the conception of special lowstand system tract (LST) geologic bodies. The special geologic bodies mentioned in this work refers to the lowstand system tract (LST) slope fan which developed above the sequence boundary, the basin floor fan in the center of lake basin, the highstand system tract (HST) turbidite sandstone body, and wedge block on slope. They are always sandstone sedimentary body, and have typical reflectance signature in seismic section, which are mainly reflected as bidirectional downlap (onlap) or unilateral downlap (onlap), and present humped shape, moundy disorder reflection or foreset, aggradation structure. Their lithologic characteristic, bore log characteristic, and geophysical signature are all different from surrounding rock.

2 Regional geological setting

Qikou Sag locates at the middle part of Huanghua Depression, and it is the sedimentary sag which has developed since Oligocene epoch (Fig.1). Qikou Sag is encircled by southern Chengning uplift, northern Beidagang salient, and western Koudian salient [8]. Later Oligocene epoch Dongying period is the decay period of fault depression activity in the study area, and the rate of sedimentation is more than the subsidence rate [9]. It presents the characteristics of deep faulted depression, great subsidence amplitude and great sedimentary thickness. Paleogene Dongying Formation is divided into three third order sequences which are Member 1 of Dongying Formation, Member 2 of Dongying Formation and Member 3 of Dongying Formation. It is in the fault-depressed diversionary period of decay period of fault depression activity. The typical sedimentary system and type of sedimentary facies are lake delta. Dongying Formation is one of the important oil and gas exploration intervals in Qikou Sag.

Littoral Slope Zone locates at the northeast corner of 3D region in the peripheral region of Qikou deep sag, which is controlled by both Xingang structure and coastal low uplift. Affected by the Xingang-Haihe Fault and decollement structure in north west, the Littoral Slope Zone develops rollover anticline and fault. Controlled by the coastal low uplift structural belt, it develops the gentle slope structure in the southern anticline. This area is commonly controlled by several provenances (including northern Yanshan provenance, north west provenance which develops in Cangxian, provenance from Beidagang buried hill, southern Chengning uplift provenance, and probable Shaleitian provenance), which makes this area a concourse place of several provenances. Because of the abundant provenance, it is inevitable to develop some special geologic bodies on the gentle slope around the lake basin and in the delta front zone[10].

Fig.1 Structure outline map of 3D seismic region of Qikou Sag

3 Research thoughts and methods

The special geologic bodies are controlled by tectonic setting and material provenance system. They distribute in a certain tectonic position and sedimentary environment and develop corresponding special sequence pattern, which present corresponding reflectance signature in seismic section, and their geophysical signature is different from surrounding rock.

Controlled by each material provenance system, the study area (Littoral Slope Zone) mainly deposits delta front sub-facies and develops front turbidite body in slope front from the Member 1 of Shahejie Formation to Dongying Formation (JIAO Yang-quan, et al, 2008, Internal data of research project of sedimentary system in Qikou rich hydrocarbon sag).

Theory and basis to recognize and divide the special geologic bodies mainly embody the following items:

1) The special geologic bodies mentioned in this work include the lowstand system tract (LST) slope fan, basin floor fan, highstand system tract (HST) turbidite body and slope wedge block.

2) The reflectance signature in seismic section of the special geologic bodies is: the special geologic bodies develop onlap, updip pinchout or downlap, downdip pinchout or bidirectional onlap in the line and trace direction, or one side of the sand body controlled by fault is plugged by fault at the downthrown side of fault, as the other side develops onlap and updip pinchout. They present reflectance signature of lens of bidirectional downlap sometimes.

3) The recognition of special geologic bodies should combine with well drilling, logging, reflectance signature in seismic section. The position, scale and space distribution feature of the special geologic bodies are recognized according to the seismic attribute，as the inner structure and sedimentation pattern of the geologic bodies are recognized accurately according to the logging constrained inversion.

According to the research thinking of integration of point-line-surface, the features of special geologic bodies are depicted systematically, which include the space location, scale, range of distribution, thickness, seismic reflectance signature, inner structure, etc [11-12] (Fig.2).

4 Recognition and depiction of special geologic bodies of Member 3 of Dongying Formation in Littoral Slope Zone

4.1 Drilling and logging data analysis

4.1.1 Single well analysis

4.1.1.1 Sequence and sedimentary facies analysis of A4 hole (Fig.3(a))

The lithology of A4 hole (see Fig.1 for the plane position of this well) was mainly siltstone and mudstone, with a little silty mudstone in the Member 3 of Dongying Formation, which presented the structure of sand and mud interbed in general, and had hydrocarbon show. The SP curve presented the feature of strong amplitude and bayonet-like shape. The apparent resistivity curve presented the feature of moderate amplitude and frequency dentation. This sequence developed highstand system tract (HST) and lowstand system tract (LST). The thickness of lacustrine expanding system tract (EST) was relatively small, which was about 50 m. This sequence was conjectured to be the lake delta front sediments in general. The mouth bar and distal bar sand body mainly developed in lowstand system tract (LST). The sand body of highstand system tract (HST) was less than the lowstand system tract (LST), which developed mouth bar and distal bar, and the interlaid deposit seized major area.

Fig.2 Research procedure and methods of depiction of special geologic bodies

Fig.3 Well to well correlation sections of typical wells of Member 3 of Dongying Formation in north of 3D region of Qikou Sag (see Fig.1 for plane position of these sections): (a) A6-A5-A4-A2-A1 correlation section; (b) A3-A2-A1 correlation section

4.1.1.2 Sequence and sedimentary facies analysis of A2 well (Figs.3(a) and (b))

The lithology of A2 hole (see Fig. 1 for the plane position of this well) was mainly siltstone and mudstone, with a little silty mudstone and fine sandstone in the Member 3 of Dongying Formation. It had hydrocarbon show. The SP curve presented moderate amplitude. The apparent resistivity curve presented the feature of strong amplitude and dentation. The thickness of highstand system tract (HST) was relatively large in this sequence, and accounted for more than half of the total thickness. The content of sand bodies in the highstand system tract (HST) was low, and they presented a few sand body sediments in argillaceous setting, which was conjectured to be mouth bar and distal bar. The thickness of the monolayer sand body in the mouth bar was large, while the sand layers were few. The thickness of the monolayer sand body in distal bar was small, while the sand layers were multiple, which presented the structure of sand and mud interbed. Sand bodies including mouth bar sediments and underwater distributary channel sediments with the dentation box-shaped resistivity curve developed in the lowstand system tract (LST). This sequence was conjectured to be front sediments of lake delta in general.

4.1.1.3 Sequence and sedimentary facies analysis of A6 well (Fig.3(a))

The lithology of A6 hole (see Fig.1 for the plane position of this well) was mainly mudstone, siltstone, silty mudstone, sandy mudstone and a little fine sandstone in the Member 3 of Dongying Formation. It had hydrocarbon show. The SP curve presented straight line in general and moderate-strong amplitude salient partly. The apparent resistivity curve presented the feature of moderate-strong amplitude sawtooth. The thickness of this sequence was relatively small, which was about 130 m and was conjectured to be front sediments of the lake delta in general. The apparent resistivity curve and lithology of both the lowstand system tract (LST) and highstand system tract (HST) clearly presented the feature of progradation. The sedimentary micro-facies were mainly mouth bar, distal bar and interlain.

4.1.2 Well to well correlation sections analysis

4.1.2.1 A6-A5-A4-A2-A1 well to well correlation section (Fig.3(a))

This section (see Fig.1 for the plane position of this section) distributed in the slope of Qikou deep sag along the littoral zone, which showed the recognition feature, section structure and space distribution pattern of reservoir sand in well control area. The correlation of provenance system, which was between A4 hole and A2 hole, was presented in this section. According to the analysis of well to well correlation section, northern provenance was relatively decreased in the Member 3 of Dongying Formation, which didn’t reached the A4 hole. The A4 hole was controlled by the northwestern provenance, which was mainly siltstone and was probably distal sediments of delta front. The A2 hole was mainly siltstone and probably distal sediments of delta front controlled by the northern provenance.

4.1.2.2 A3-A2-A1 well to well correlation section (Fig.3(b))

This section (see Fig.1 for the plane position of this section) crossed the northern slope of Qikou deep sag which was controlled by both coastal low uplift and southern rollover anticline. The A3 hole and A2 hole were controlled by different provenances in the Member 3 of Dongying Formation. The A3 hole was controlled by the northwestern Cangxian provenance, while the A2 hole received the northern provenance. The A3 hole deposited gritstone in lowstand system tract (LST), which was proximal sediments of delta front. The A2 hole mainly deposited siltstone, argillaceous siltstone, and a little fine sandstone, which was distal sediments of delta front. The A3 hole and A2 hole were controlled by different provenance systems in the lowstand system tract (LST). The geologic bodies of distal sedimented overlay mutually. Sediments which developed respectively in highstand system tract (HST) were retreated integrally. The sediments of two wells separated gradually, and this feature also presented in the thickness of special geologic bodies. The A2 hole and A1 hole were controlled by the same provenance system in general.

4.2 Recognition of reflectance signature in typical seismic sections

The 3D seismic data of high precision provided evidence for the studying of conjecture from well area to few-well area and no-well area. According to the analysis of typical seismic reflectance signature in certain line and trace direction, the lowstand system tract (LST) slope fan, basin floor fan, highstand system tract (HST) turbidite body and slope wedge block developed in the Littoral Slope Zone. In the seismic section of line direction (Fig.4(a)), slope fan developed in lowstand system tract (LST) and lacustrine expanding system tract (EST). The sand body developed lateral accretion along the gentle slope and formed lateral onlap, updip pinchout, developed downlap, and downdip pinchout in the side of deep lake, which possessed the conditions of forming sandstone lens traps [13]. Highstand system tract (HST) delta and turbidite body developed in the highstand system tract (HST). One side of the sand body controlled by fault was plugged by faults at the downthrown side of fault, while the other side developed onlap, and updip pinchout along the gentle slope. In the seismic section of trace direction (Fig.4(b)), slope fan developed in the lowstand system tract (LST) and lacustrine expanding system tract (EST). The sand body formed downlap, and downdip pinchout along the slope. In the highstand system tract (HST), delta developed, while the sand body formed downlap, and downdip pinchout, which easily formed lithologic traps. Therefore, the structural- lithologic hydrocarbon reservoir and lithologic subtle hydrocarbon reservoir were beneficial to developing in the Littoral Slope Zone.

4.3 Thickness and space distribution feature of special geologic bodies

According to the tracing and closure of the special geologic bodies which were recognized in the seismic section, the top and bottom interface, boundary, distribution range of special geologic bodies and approximate contour in plane were portraited. The thickness isoline map of lowstand system tract (LST), lacustrine expanding system tract (EST) and highstand system tract (HST) were obtained, with the computation of the top and bottom interface (Fig.5).

Fig.4 Typical seismic structure features of some special geologic bodies of Member 3 of Dongying Formation in 3D region of Qikou Sag: (a) Typical seismic structure feature of line direction; (b) Typical seismic structure feature of trace direction

Fig.5 Thickness chorisograms of some special geologic bodies of Member 3 of Dongying Formation in 3D region of Qikou Sag (Unit: m): (a) LST and EST; (b) HST

The thicknesses of lowstand system tract (LST) and lacustrine expanding system tract (EST) were relatively concentrated, and the concourse and mutual overlay of provenances are presented from different directions. The scale of the northwestern provenance was relatively large, whose maximum thickness was about 110 m, which presented that the lake basin reduced in this period, and sediments were centralized. The lake basin enlarged in highstand system tract (HST). The geologic bodies were retreated integrally and deposited dispersedly on the slope. With the separation of the provenances from different directions, the thickness of geologic bodies dispersed and formed three thickness centers, whose scale and thickness decreased integrally. The scale of the northeastern provenance was relatively large, whose maximum thickness was about 100 m. From the lowstand system tract (LST) to the highstand system tract (HST), the thickness presented the evolution process from the lake basin reducing to broad lake basin.

4.4 Seismic attribute analysis

The geophysical method can provide reliable basis for the prediction of few-well area and no-well area. Based on the analysis of thickness, the seismic attributes of the special geologic bodies were extracted and calibrated with the drilling, logging data and seismic sections, so as to portray the distribution scale and boundary of the special geologic bodies in plane, and to present the relationship between different geologic bodies. With a series of comparison, screening and calibration, the root mean square amplitude (RMS amplitude) attribute along the layer which could well reflect the special geologic bodies were chosen at last.

According to the sand body morphology, the root mean square amplitude (RMS amplitude) attributes (Fig.6(a)) of the lowstand system tract (LST) and lacustrine expanding system tract (EST) of the Member 3 of Dongying Formation were extracted along layer by shifting the sequence bottom interface of the Member 3 of Dongying Formation upward 40 ms and setting the time window as 40 ms (Fig.7). The RMS amplitude of the special geologic bodies was different from the surrounding rock, which presented continuity and stability relatively. The northwestern and northern provenances, and probable western provenances developed in this area. The RMS amplitude attribute graph coincided with the thickness graph of the special geologic bodies which was traced in the seismic sections. The white lines in the graph present the boundary and range of the plane distribution. The gray arrows indicate the approximate provenance directions. In the same way, the RMS amplitude attribute of highstand system tract (HST) was extracted along layer (Fig.6(b)) with the time window of 40 ms, and calibrated with the loggings and seismic sections. The northwestern provenances and lobe bodies from northeastern developed in this area, with probable southeastern provenances. The RMS amplitude attribute presented the strong amplitude reflection characteristics along the downthrown side of the Qizhong Fault. The attribute graph coincided with the thickness graph of the special geologic bodies.

4.5 Logging constrained inversion analysis

Based on the analysis of plane distribution morphology of the special geologic bodies, the logging constrained inversion was used in the study area, which could study the internal structure of the special geologic bodies and verify the analysis results of logging, seismic method and attribute [14-15]. The logging constrained inversion can combine the advantage of both high vertical resolution of logging and good horizontal continuity of seismic method [16-17]. The impedance inversion used in this work had a definite physical significance, which was the deterministic method of reservoir lithology prediction and reservoir feature description, and had achieved significant geological effect in practical application [18-19].

Fig.6 Along-surface attribute of root mean square amplitude of some special geologic bodies of Member 3 of Dongying Formation in 3D region of Qikou Sag: (a) LST and EST; (b) HST

Fig.7 Cartogram of extracting along-surface seismic attribute of Member 3 of Dongying Formation

The inversion of Littoral Slope Zone depicted the internal structure and deposit pattern of the special geologic bodies with the seismic and logging data. According to continuous modification of the initial impedance model and wavelet, repeated manufacture of synthetic seismogram, which had the best correlatability with the near-well seismic traces, the inversion impedance of the seismic section was gotten at last.

The range of threshold value of sandstone was    7 450-9 000 (g/cm³?m/s) in the impedance inversion section (Fig.8(a)) of line direction of the Member 3 of Dongying Formation (see Fig.5, which corresponded to the seismic section in Fig.4(a)). The impedance of sandstone was greater than that of surrounding rocks. In the inversion section, the lowstand system tract (LST) slope fan developed lateral accretion along the gentle slope and formed onlap and updip pinchout, with the other side developed downlap, and downdip pinchout in deep lake. The highstand system tract (HST) delta developed downlap and downdip pinchout. The root of the highstand system tract (HST) turbidite body controlled by fault was plugged by faults, while the other side developed onlap along the gentle slope and formed the updip pinchout. The range of threshold value of sandstone was 7 450-9 000 (g/cm³?m/s) in the impedance inversion section (Fig.8(b)) of trace direction (see Fig.5, which corresponded to the seismic section in Fig.4(b)). In the inversion section, the lowstand system tract (LST) slope fan developed lateral downlap and downdip pinchout. The highstand system tract (HST) delta developed lateral downlap and downdip pinchout, which coincided with the typical reflection feature of seismic section.

5 Sequence pattern and profitable reservoir trap forecasts of Member 3 of Dongying Formation in Littoral Slope Zone

According to the tectonic setting, provenance system and sedimentary system feature in the study area, the sequence construction patterns of the Littoral Slope Zone were summarized. The special geologic bodies located at the deep lake gentle slope of broad lake basin (Fig.9). In the lowstand system tract (LST) and lacustrine expanding system tract (EST) of the Member 3 of Dongying Formation, supplied by provenances from three directions (the northwestern Cangxian provenance, the northern provenance, and southwestern provenance), the geologic bodies developed at A3 hole and A2 hole were controlled by different provenances. The geologic bodies controlled by the northwestern provenances mainly developed at the slope near the A3 hole, which spread to the A4 hole, whose maximum thickness was about 110 m. The seismic reflection feature presented continuous moderate-frequency and weak amplitude. The scale of the geologic bodies which were controlled by the northern provenances was relatively small. Some thin layer turbidite body developed in the downthrown side of Qizhong Fault. In the highstand system tract (HST), supplied by three provenances (the northwestern Cangxian provenance, the northeastern provenance, and the southern provenance), the geologic bodies were integrally retreated and reciprocally separated, and the scale and thickness of the geologic bodies integrally decreased. The scale of the geologic bodies which were controlled by the northern provenance was relatively large. Highstand system tract (HST) thin layer turbidite body developed in downthrown side of Qizhong Fault (Fig.10).

Fig.8 Logging constrained inversion sections of some special geologic bodies of Member 3 of Dongying Formation in 3D region of Qikou Sag (see Fig.5 for plane position of this section): (a) Line direction (Variable: impedance; Unit: g/cm³·m/s); (b) Trace direction (Variable: impedance; Unit: g/cm³·m/s)

Fig.9 Palaeogeomorphology of depositional stage of Member 3 of Dongying Formation in Littoral Slope Zone of Qikou Sag

On the basis of the above analysis, the slope fans of the lowstand system tract (LST) of the Member 3 of Dongying Formation widely developed in the Littoral Slope Zone，most of which were isolated or mutual overlying sandstone body (reservoir body). They were easy to form the lithologic lens traps, and could form isolated sandstone lens hydrocarbon reservoir of self-generation and self-accumulation if the sandstone lens were encircled by deep lake mudstone [20]. On the slope, geologic bodies developed onlap and easily formed lithologic updip pinchout traps, whose lower part had deep lake mudstone which could be provenance rocks, with the upper part having EST mudstone which could be covered. This provenance-reservoir-cap assemblage type was favorable for hydrocarbon accumulation [21-22]. The highstand system tract (HST) delta was favorable to form onlap delta sandstone reservoir [23] (Fig.11). One side of the highstand system tract (HST) turbidite body developed onlap, with the other side plugged by faults, which easily formed structural-lithologic composite traps and fault sealing hydrocarbon reservoir (Fig.11(b)). This area was commonly controlled by several provenances (including northern Yanshan provenance, northwestern provenance which developed in Cangxian, provenance from Beidagang buried hill, southern Chengning uplift provenance, and probable Shaleitian provenance), which provided necessary conditions for hydrocarbon reservoir. These conditions which allocated with the depositional setting of delta front constituted the favorable developed area of hydrocarbon reservoir of Littoral Slope Zone [24-25].

Fig.10 Patterns of some special geologic bodies of Member 3 of Dongying Formation in Littoral Slope Zone of Qikou Sag: (a) LST and EST; (b) HST

Fig.11 Reservoir pattern sections of Member 3 of Dongying Formation in 3D region of Qikou Sag (see Fig.5 for plane position of this section): (a) Line direction; (b) Trace direction

6 Conclusions

1) According to the research thinking of point-line- surface, with the research method of binding and corroborating the log and seismic data, the distribution morphology and range feature of the special geologic bodies were obtained. From the lowstand system tract (LST) and lacustrine expanding system tract (EST) to the highstand system tract (HST), the thickness of special geologic bodies had evolved from the relative centralization of lake basin reducing period to the relative decentralization of wide lake basin period. The thickness centers had evolved from one to three, with their thickness and scale reduced. The provenances had evolved from concourse and mutual overlying to mutual separation.

2) The root mean square amplitude attributes present the distribution morphology and approximate boundary of the special geologic bodies. From the lowstand system tract (LST) and lacustrine expanding system tract (EST) to the highstand system tract (HST), the geologic bodies had evolved from concentration to three relative dispersive isolated parts, whose morphology feature coincided with the thickness.

3) Logging constrained inversion sections present the inner structure of special geologic bodies. The sandstone developed onlap and downlap, and was plugged by faults, which coincided with the reflection characteristics of the seismic section.

4) The probable favorable traps are forecasted according to the high resolution sequence pattern. The lowstand system tract (LST) slope fans in study area could easily form lithologic lens traps and lithologic updip pinchout traps. The highstand system tract (HST) delta and turbidite body were favorable to forming the onlap delta sandstone reservoirs and structural-lithologic composite traps, which had favorable provenance- reservoir-cap assemblage and reservoir forming conditions.

Acknowledgements

This work was supported by the research institute exploration and development, PetroChina Dagang Oilfield Company. The authors thank for their help in work.

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(Edited by PENG Chao-qun)

Foundation item: Project(40872077) supported by the National Natural Science Foundation of China; Project(2008CDA098) supported by the Key Natural Science Foundation of Hubei Province, China; Project(TPR-2010-01) supported by the Open Project Foundation of Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China

Received date: 2010-05-11; Accepted date: 2010-06-21

Corresponding author: CHEN Si, PhD candidate; Tel: +86-15926294840; E-mail: cc_720@126.com