The features and forming mechanism of zhuangnan fault zone by menez

Scientific Journal of Earth Science June 2014, Volume 4, Issue 2, PP.75-84

The Features and Forming Mechanism of Zhuangnan Fault Zone Jing Wang1, Qinghua Chen2 1. Department of Petroleum Engineering, Chengde Petroleum College, Chengde 067000, China 2. Faculty of Geo-Resource and Information Collegue, China University of Petroleum, Qingdao 266580, China Corresponding Author Email: jxinwang@sina.com

Abstract Make full use of 3D seismic interpretation data, method to emphasize the combination of the coherent analysis technique and the seismic profile interpretation, to study the geometry characteristics of Zhuangnan fault zone. Through comparative and analysis of the activity of Zhuangnan fault and the boundary faults of Zhanhua Sag, based on the dynamics of geological setting the evolution of Zhuangnan fault zone in the area has been discussed. The result indicates: Zhuangnan fault zone has its special regularity and characteristic which has three-piece planar characterization (eastern part, middle part, western part) and has arches bending in middle part. The study of the formation mechanism explained the special geometry characteristics of Zhuangnan fault zone reasonably. The activity of the two main boundary faults of Zhanhua Sag (Changdi fault, Chengdong fault) influenced the faulting evolution of the eastern and western sides of Zhuangnan fault zone. The significant differences between the activity intensity of the two main boundary faults of Zhanhua Sag also have important impacts on the arches bending in middle part of Zhuangnan fault zone. Keywords: Zhuangnan Fault Zone; Three-piece Planar Characterization; Arches Bending; Boundary Faults; Formation Mechanism

INTRODUCTION With the popularization and renewal of technique and methods for 3-D seismic data interpretation are applied in oilfield production in recent years, the research of structure characteristics of complex fault block area have become increasingly detailed and accurately .However, there are very few exploration and discussion in our country take the formation mechanism of some special geotectonic phenomena seriously. In fact, the problem of ignoring the study of formation mechanism, which restricts the solution of the problems of oil distribution and the exploration of favorable blocks to further, etc. For this reason, take Zhuangnan fault zone for example, this paper discussed and analyzed the features of Zhuangnan fault zone through geometry, kinematics and dynamics. We has summarized two special regularity and characteristic of Zhuangnan fault zone which has three-piece planar characterization (eastern part, middle part, western part) and has arches bending in middle part of mainly faults in Zhuangnan fault zone (CHEN Qinghua et al,2004. DAI Junsheng et al, 1998). Based on the dynamics of geological setting the evolution of Zhuangnan fault zone in the area has been discussed .The paper expatiated the controlling role of the evolution of Zhuangnan fault zone by the activity of boundary faults in Zhanhua sag. Finally, the formation mechanism of the special geotectonic phenomena of Zhuangnan fault zone has been reasonably explained.

1 REGIONAL GEOLOGICAL OUTLINE Zhuangnan fault zone is a secondary structural belt of Zhanhua sag in Jiyang depression. Zhuangnan fault zone was composed of a nearly west-east strike listric normal fault and the sub-faults derived from main fault.Zhuangnan fault zone bordered by Chengdong faultl on the west, and the Wuhaozhuang fault and Changdi fault in the east (CHEN Qinghua et al, 2004. DAI Junsheng et al, 1998, TAN Mingyou et al, 2002) (Figure 1).Four Oilfields developed in - 75 http://www.j-es.org

Zhuangnan fault zone, which were Zhuangxi Oilfield; Wuhaozhuang Oilfield; Changdi Oilfield; Laohekou Oilfield. Those oilfields consist of many oil bearing strata, which distributed in the Archaeozic, Palaeozoic, and Mesozoic and included some reservoirs buried deeply.Paleozoic buried hill reservoir is the most important oil pool types in Zhuangxi oilfield. Major exploration target of Wuhaozhuang Oilfield, Changdi Oilfield and Laohekou Oilfield are reserviors in the Cenozoic (CEHN et al, 2004.LIN et al, 2003.HOU et al, 2005).

FIG.1 REGIONAL STRUCTUAL POSITION OF ZHUANGNAN FAULT ZONE

All the four oilfields are the typical complicated fault-block oilfields in east china. They have more styles, more complex forming causation types. At present, Zhuangxi oilfield relies mainly on structure reservoir, Wuhaozhuang oilfield is dominated in composite structure-lithology reservoir. Changdi oilfield is complicated by many faults, belongs to anticline reservoir having multilayer of sands, which is divided into two tectonic types: Paleozoic buried hill structure and Mesozoic draping structure. The main accumulation model of Laohekou oilfield in upper Guantao formation is the lithologic oil and gas pools (CHEN, 2004, LIN et al., 2003, HOU et al, 2005).

2 STRUCTURE TYPES Applying three-dimensional seismic data, geological data and logging data, we analyze and describe the structural feature and fracture feature of Zhuangnan fault zone. Geoframe 4.3 seismic interpretation software has been applied to seismic layer labeling, make synthetic seismogram, coherent data analysis and seismic explanation. By making synthetic seismic record of well data of test wells a fine horizon calibration for the target strata. By the accurately interpreted with a 3D coherence analysis and seismic section in plain, section and space, the geometry characteristics of Zhuangnan fault zone have preliminarily been understood. Application of coherent data volume to interpret fault blocks in complex fault block area is a very effective tool. The time slice of coherent data body can unveil the direction of the Zhuangnan fault zone generally, the configurations of faults in both areal and vertical directions, and - 76 http://www.j-es.org

guidance-find the secondary fracture. Considering most of the faults in Zhuangnan fault zone are nearly west-east strike, so the east-west interpretation section should be one that supplements a focus on the south-north interpretation section.

2.1 Vertical fault assemblage The closure of intersecting geophysical information has been being an important problem in geophysical data interpretation. By the closure interpretation of 43 south-north interpretation sections and 24 east-west interpretation sections, the article summarized faults growth in Zhuangnan fault zone below. Zhuangnan fault zone had developed on the tectonic background of Zhuangnan buried hill. The fault zone have the characteristic of plastic uplifting by reason of the spreading function and antithetic drag of the main faults in Zhuangnan fault zone. The study found that Zhuangnan fault zone had the characteristics of more active faults developed on both sides of the fault zone than middle part of the fault zone. And the faults were concentrated developed in the north of the area. From that we divided Zhuangnan fault zone into three-parts: western part, middle part, eastern part. Western part: The evolution of Zhuangnan fault zone experiences three stages. Four listric faults trending NEE, dipping S developed of early evolution, those faults arranged in equivalence and parallelism. Faults styles are mainly step fault. Those faults have long fault displacement, which can cut the overlying formations and basement strata. In median period of development, four secondary fractures which dipping is north were developed. Those faults ending on the listric faults dipping S which located on the southernmost part of the four listric faults. Those secondary fractures arranged in equivalence and parallelism. Faults styles are mainly step fault. The median period subordinate structure fault and the early mainly faults display “y” character form fault array in profile, Faults styles are mainly graben and step fault. Those secondary fractures also have long fault displacement, which mainly cut the overlying formations. The late development faults grown on the four secondary fractures of median period development, which displayed “anti-y” character form fault array with median development faults in profile. The late development faults have short fault displacement, which only cut the shallow formation (figure2, table1). TABLE1 MAJOR FAULTS ELEMENTS OF THE WESTERN PART Fault

Fault strike

Dip directions

Fault dip

Fault displacement(m)

Extended length(Km)

Section shape

NEE

35-70

500

Listric

NEE

45-60

200-300

Platelike

40-50

300-400

Listric

NWW

40-50

100-200

Listric

NEE

45-60

100-200

2.5

Platelike

NEE

45-60

100-200

2.5

Platelike

F10

NEE

40-50

100

Platelike

Middle part: Most early development faults gradually cease their activity, only cut the basement strata. The activity of Zhuangnan fault is always persisted. Zhuangnan fault becomes the most important fault in dipping S which section shape is listric. The gradient of Zhuangnan fault slackens from the top down. Zhuangnan fault cut the basement strata. The subsidiary shear fractures complicate the overlying formations. The major “y” character form fault array in the studied area was composed of Zhuangnan fault and one secondary fracture in dipping N. Three stages of secondary fracture evolved between them. The combinations of the faults of different phases are “y” character form and “anti-y” character form. Over all, the middle part of structure possesses the characteristic of faults in the overlying formations are more complex than in the basement strata. Eastern part: the activity of early development faults enhancement. Three-fifths of early development faults cut the overlying formations and basement strata and two-fifths of early development faults cut the basement strata, not cut - 77 http://www.j-es.org

the overlying formations. Four secondary fractures dipping north were developed in Cenozoic strata. There is a strong symbiotic relationship between mainly faults of F1, F3, F5 and secondary fault of F2. F2 fault was cut by F1 fault, displayed “y” character form fault array in profile with F1 and F3 faults.F4 fault was the late development fault, displayed “y” character form with mainly faults. TABLE 2 MAJOR FAULTS ELEMENTS OF THE MIDDLE PART Fault

Fault strike

Dip directions

Fault dip

Fault displacement(m)

Extended length(Km)

Section shape

35-70

500

Listric

45-60

200-300

Platelike

40-50

300-400

Platelike

NWW

40-50

100-200

Platelike

45-60

100-200

Platelike

TABLE 3 MAJOR FAULTS ELEMENTS OF THE EASTERN PART Fault

Fault strike

Dip directions

Fault dip

Fault displacement(m)

Extended length(Km)

Section shape

NEE

35-70

500

Listric

NWW

45-60

200-300

Platelike

NWW

40-50

100-200

Platelike

NEE

45-60

100-200

Listric

2.2 Planar fault assemblage Three breaking with near E-W strike through the Zhuangnan fault zone have horizontally stretching distance. They are arranged parallel to each other, and have arches bending in middle part. Zhuangnan fault (F1) is the most important fault as the southern boundary fault of Zhuangxi buried hill, and is also an important oil-controlling fault. From the collection and processing of data, the cause of Zhuangnan fault throw increased from bottom to top was identified. it is because the dip directions of Zhuangnan fault steeper above Zhuangxi buried hill. Four synthetic faults are associated with Zhuangnan fault. There are (F2, F3, F9, F10). They are arranged parallel to each other and evenly spaced. In those four faults, F2 and F3 faults have longer horizontally stretching distance than F9 and F10 faults. F9 and F10 faults mainly distribute in the west area of the Zhuangnan fault zone. Except to the associated faults many secondary fractures derived from F1 fault and F2 fault were developed. The secondary fractures derived from F1 fault extended along the tangent to the bend deformation occurs on the middle part of Zhuangnan fault zone(F5 tangent to main fault along the east and F7 tangent to main fault along the west). And shaped the “入” Chinese character form fault array with Zhuangnan fault. Three secondary fractures derived from F2 fault developed between F2 and F3, They are arranged in echelon to each other and evenly spaced. The secondary fractures join with the F2 fault in bipenniform. The order of these secondary fractures is F6 to F4 and then F8 from east to west (figure 2). Two sets of faults provide the condition of shelter for hydrocarbon accumulation .The “入” Chinese character form fault array promoted formation of faults screened structure types of favorable traps from hydrocarbon accumulation perspective. A large structural trap from the southern of F2 fault is made up of buried-hill draping structural belt and a sealing fault cutting the draping anticline. The analysis of the geometrical characteristics of Zhuangnan fault zone leads out the following rule: the structural pattern of Zhuangnan fault zone is normal fault assemblage. Zhuangnan fault zone has two main section structure patterns, stair-step shape structure pattern and composite “y” character form structure pattern. Parallel is the main - 78 http://www.j-es.org

plane structure pattern, bipenniform structure pattern characterized by strike slip is the important supplement plane structure pattern. The evolution of Zhuangnan fault zone is under the extensional and shear stress field environment. We need for an in-depth study of geological evolution and dynamics of Zhuangnan fault zone on the basis of geometrical characteristics study.

FIG.2 FAULTS COMBINATION STYLES IN PLAIN AND SECTION OF T0 REFLECTION HORIZON

3 TECTONIC EVOLUTION AND REGIONAL DYNAMICS Zhuangnan fault zone is an important west-east strike structural belt of Zhanhua sag. The geological evolution and dynamics of it is closely related to the evolution and dynamics of Zhanhua sag. The activities of Tan-Lu fault zone have a significant impact on east of Zhanhua sag. The extending direction of faults in Zhuangnan fault zone is consistent with the strike of major faults of the western - 79 http://www.j-es.org

branches of Tan-Lu fault zone and Lai-xi fault. The strike slip structural style occurs vertically as echelon faults in the east of Zhuanghua sag like Changdi fault, Gudong fault and Kendong fault and faults in the west like Chengdong fault, Yidong fault and Yinan fault. Fault strike is NNE or nearly NE. Those faults controlled the distribution of Meso-Cenozoic strata and the development of the tectonic (QIU et al, 2006, MA, 2005).Zhuangnan fault zone is located between Changdi fault and Chongdong fault. Fault strike is EW. Zhuangnan fault zone is the accommodation zone with other faults which strike is EW, nearly NE and NW in Zhuanghua sag. Zhuangnan fault zone has the characteristics of longer horizontally stretching distance, large fault throw and long active time whose active lives covered the period from Mesozoic to Neogene, which controls the sedimentation, tectonic evolution and oil-gas accumulation in Zhuanhua sag. Geological evolution and dynamics of Zhuangnan fault zone is closely related to the activity of the three boundary faults (Chengdong fault, Changdi fault and Wuhaozhuang fault). Zhuangnan fault zone reached to Chengdong fault in western Zhanhua sag, and joined Changdi fault in eastern Zhanhua sag. Wuhaozhuang fault developed on the southeast part of Zhuangnan fault zone (figure 3). In the structural pattern of Zhanhua sag, Wuhaozhuang fault ,Guxi fault and Luoxi fault which fault strike is NW are arranged in echelon to the left row from north to south. These features clearly reflect the left-lateral strike-slip character of Tan-Lu fault zone in Early Mesozoic. Kendong fault, Changdi fault and Gudong fault which fault strike is NNE are arranged in echelon to the right row from north to south, reflect the right-lateral strike-slip character of Tan-Lu fault zone in Late Mesozoic.(WU et al,2004.XIANG ,2001) The stress system converted from left-lateral strike-slip to right-lateral strike-slip during the Mesozoic. The changes of stress in different geologic periods created complex superimposed structures and forming the present fracture systems in Zhuangnan fault zone (ZHANG et al, 2005. ZHANG et al, 2003. PENG, 2009)

FIG.3 TECTONIC STRESS FIELD OF ZHUANGNAN FAULT ZONE

4 GEOLOGICAL EVOLUTION AND DYNAMICS OF ZHUANGNAN FAULT ZONE Based on the analysis on the tectonic evolution and regional dynamics background, the Geological evolution and - 80 http://www.j-es.org

dynamics of Zhuangnan fault zone in Zhanhua sag need further discussion and clarification The geometry characteristics of Zhuangnan fault zone was systematic investigated and the results indicated that Zhuangnan fault zone has three-piece planar characterization (eastern part, middle part, western part). The structural feature of eastern part and western part is more complex than the middle part. First, it aims to definitude inheritance relationship of faults in Zhuangnan fault zone by analyzing and calculating the activity of main faults developed in the eastern part and the western part. West Zhuangnan fault zone reached to Chengdong fault which fault strike is NNE (fig 3). Data from moving velocity of both faults show that the activity of Zhuangnan fault was consistent with Chengdong fault (fig 4). There were two peak-activity periods of the two faults. They were the sedimentary period of third member of Shahejie Formation and the sedimentary period of the first member of Shahejie Formation (ZHAO, 2007.ZHANG et al, 2005.ZAHNG Xi, 2006). The first peak-activity period of Zhuangnan fault zone was a bit later than Chengdong fault. The second peak-activity period of the two faults is quite satisfactory (ZHONG, 2010. FENG, 2009). The active time of Chengdong fault is longer than Zhuangnan fault since the Late Cretaceous Period to the sedimentary period of Minghuazhen Formation. The active time of Zhuangnan fault is since the middle sedimentary period of the fourth member of Shahejie Formation to the sedimentary period of Minghuazhen Formation (QU, CHEN, 2009. LIU, 2005).When the active time of Chengdong fault reach the first peak Zhuangnan fault have started to develop. From the sedimentary period of the third member of Shahejie Formation on the moving velocity of both faults is quite satisfactory. These indicate that the activity of Chengdong fault is the most important direct factor in the activity of West Zhuangnan fault zone. Meanwhile, Zhuangnan fault play a great role in the reconstruction of Chengdong fault.

FIG.4 ACTIVITY OF ZHUANGNAN FAULT,CHENGNAN FAULT AND WUHAOZHUANG FAULT

East Zhuangnan fault zone reached to two faults, Changdi fault which fault strike is NNE and Wuhaozhuang fault which fault strike is NW respectively (figure1, figure3). The activity characteristic of Wuhaozhuang fault that has long-term activity controls the development of the tectonic framework of Zhuangnan fault zone (QU, CHEN, 2009). From the fourth member of Shahejie Formation to the third member of Shahejie Formation, The activity of Zhuangnan fault was really just started and gradually increasing. However, activities of Wuhaozhuang fault has come to the last stage and large scale fault activities stop. The structure activity of East Zhuangnan fault zone has been less affected by Wuhaozhuang fault (figure4). - 81 http://www.j-es.org

Our knowledge for the fault properties of Changdi fault has some disagreements among the experts (QU, CHEN, 2009. LIU, 2005). At present, the unified understanding of Changdi faults have reached is primarily the regional structure evolution. Changdi fault is located in the west Tanlu strike-slip fault zone. Affected and controlled by the strike-slip fault zone, faults are developed. The formation of Changdi fault experienced a complex evolutionary process. Changdi fault still prominently featured in the characters of strike-slip and extrusion structure despite being reconstructed by NS extension in Cenozoic. The eastern segment of Changdi fault exhibits a complex structure composed of several branches, shapes the “入 ” Chinese character form fault array with Changdi fault (LIU, 2005).Structures differ greatly in features between the two sides of Changdi fault. The western segment of Changdi fault was cut into a series of complex fault noses and fault blocks which fractures are faulted steps on the section by faults of EW fault strike. The differences in trend and margin condition are the main reasons for the different actions in structural belts. Zhuangnan fault zone play a vital role in balancing the differences of margin condition as main EW strike-slip accommodation structure. In turn, Changdi fault make important effects on the fault evolution of East Zhuangnan fault zone.

5 CONCLUSIONS Zhuangnan fault zone has two functions as the main E-W strike-slip accommodation structure. That is, on the one hand, Zhuangnan fault zone inherited the rules of development of regional tectonic. On the other hand, the present structures framework of Zhanhua sag was reconstructed by Zhuangnan fault zone. Zhuangnan fault zone plays an important role on hydrocarbon migration and accumulation in oil geology research field. Systematic research is made on the internal structure of Zhuangnan fault zone has great significance for finding out the law of oil and gas migration and guiding exploration in the future. In conclusion, below is a summary of what we learned through this study. First, Zhuangnan fault zone has its special regularity and characteristic which has three-piece planar characterization (eastern part, middle part, western part) and has arches bending in middle part. The structure characteristic of the east and west parts of Zhuangnan fault zone is more complex than the structure characteristic of the middle part of Zhuangnan fault zone. Second, the activity of the two main boundary faults of Zhanhua Sag (Changdi fault, Chengdong fault respectively) influenced the faulting evolution of the eastern and western parts of Zhuangnan fault zone. Zhuangnan fault zone takes on east-west structural framework under the right-lateral strike slip and the NS extension stress field in Cenozoic had developed on the tectonic background of Meso-Paleozoic structures. There were two peak-activity periods of the two faults. They were the sedimentary period of third member of Shahejie Formation and the sedimentary period of the first member of Shahejie Formation. Its structural system inherited the paleostructure, such as complex faulted blocks and rolling anticline. Third, through the tectonic evolution and dynamics research of regional and Zhuangnan fault zone, it was found that Zhuangnan fault zone is manipulated by the activity of the two main boundary faults of Zhanhua Sag (Changdi fault, Chengdong fault) .The offsets of the two boundary faults gradually decrease into the accommodation zone (KONG Dong-yan, 2005), so the tectonic framework of Middle Zhuangnan fault zone has been less affected by boundary faults. Zhuangnan fault is the only fault in dipping S which cut the overlying formations and basement strata. The arches bending in middle part of Zhuangnan fault zone is to balance the significant differences between the activity intensity of the two main boundary faults of Zhanhua Sag.

ACKNOWLEDGMENT The research was supported by the National Natural Science Foundation of China (Grant No. 41202126) and major national science and technology projects (Grant No. 2008ZX05042-001).

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AUTHORS Jing WANG, she is Mongolian nationality, born in Chengde in August 1986, Master of geology, lectorate of Chengde Petroleum College. She currently mainly engaged in the tectonic characteristics of basin and analysis of distribution of oil and gas reservoir. Email: jxinwang@sina.com

- 84 http://www.j-es.org