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Development features of volcanic rocks of the Yingcheng Formation and their relationship with fault structure in the Xujiaweizi Fault Depression, Songliao Basin, China

Development features of volcanic rocks of the Yingcheng Formation and their relationship with... 436 Pet.Sci.(2012)9:436-443 10.1007/s12182-012-0227-4 Development features of volcanic rocks of the Yingcheng Formation and their relationship with fault structure in the Xujiaweizi Fault Depression, Songliao Basin, China 1, 2, 3 4 1, 2, 3 1, 2, 3 Cai Zhourong , Huang Qiangtai , Xia Bin , Lü Baofeng , Liu 1, 2, 3 1, 2, 3 Weiliang and Wan Zhifeng School of Marine Sciences, Sun Yat-sen University, Guangzhou, Guangdong 510006, China Guangdong University Key Laboratory of Offshore Oil Exploration and Development, Guangzhou, Guangdong 510006, China Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou, Guangdong 510006, China Guangzhou Institute of Geochemistry, CAS, Guangzhou, Guangdong 510640, China © China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2012 Abstract: The Xujiaweizi Fault Depression is located in the northern Songliao Basin, Northeast China. The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formation (K yc). Based on seismic interpretation, drill cores and the results of previous research, we analyzed the distribution of faults and the thickness of volcanic rocks in different periods of K yc, and studied the relationship of volcanic activities and main faults. Volcanic rocks were formed in the Yingcheng period when the magma erupted along pre-existing fault zones. The volcanic activities strongly eroded the faults during the eruption process, which resulted in the structural traces in the seismic section being diffuse and unclear. The tectonic activities weakened in the study area in the depression stage. The analysis of seismic interpretation, thin section microscopy and drill cores revealed that a large number of fractures generated in the volcanic rocks were affected by later continued weak tectonic activities, which greatly improved the physical properties of volcanic reservoirs, and made the volcanic rocks of K yc be favorable natural gas reservoirs. The above conclusions provide the basis to better understand the relationship of the volcanic rock distribution and faults, the mechanism of volcanic eruption and the formation of natural gas reservoirs in volcanic rocks. Key words: Volcanic rock, development features, Yingcheng Formation, Xujiaweizi Fault Depression, Songliao Basin from the Ancient Central Uplift, and the transitional slope 1 Introduction areas in the southeast and northeast of the depression are The Xujiaweizi Fault Depression is located in the separated from the Zhao 59–Wanlong ancient uplift and the northern Songliao Basin, Northeast China, where the largest northeast uplift area, respectively (Fig. 1). The depression oil and gas reservoirs in the Songliao Basin have been found, extends approximately north-south, with a maximum width such as the Daqing Oilfield. It is the most highly explored of 40 km in E-W direction and an N-S length of about 80 part of the basin. The reservoirs are characterized by oil km. The structure pattern is characterized by rifting in the in shallow strata and gas in deep strata. Since the shallow west and overlap in the east, block-faulting in N-S direction. exploration entered the stage of high water-cut production, The deep strata of the depression include the Upper Jurassic and the well Xushen 1 obtained a high-yield natural gas Huoshiling (J hs), the Lower Cretaceous Shahezi (K sh), 3 1 flow from Cretaceous volcanic rocks in 2002, exploration Yingcheng (K yc), Denglouku (K d) and Quantou Formations 1 1 has discovered substantial reserves of deep-layer gas (Zou (K q) from bottom to top. The Huoshiling period was the et al, 2010). The western depression is separated by faults initial rifting stage, dominated by volcanism. The Shahezi period was the strong rifting stage and the strata were mainly *Corresponding author. email: xiabin01@gmail.com lake sediments. The Yingcheng period was the fault-sag Received November 3, 2011 WUDQVLWLRQVWDJHWKH¿UVWDQGWKLUGPHPEHUVZHUHGRPLQDWHG Xuzhong Fault Xudong Fault Zone Xuxi Fault (N) Northeast Uplift Area Xuzhong Fault Xuxi Fault (S) Xuxi Fault Pet.Sci.(2012)9:436-443 437 by volcanism, the second member was absent and the fourth from drilling, more information has been obtained about the member of Yingcheng Formation (K yc ) was river-delta lithology, composition and distribution of volcanic rocks of and shallow lake facies. The Denglouku period and the latest K yc (Ren et al, 2004; Hou et al, 2004; Feng, 2006; Huang et period were the depression stage, and river delta and lake al, 2006). However, research into the mechanisms of volcanic facies sediments were developed (Chen et al, 1999; Chi et eruption is incomplete, and the study of key fundamental al, 2002; Yin et al, 2002). The Xuxi Fault (including north problems in the relationships between the complex fault and south sections), Xuzhong Fault and Xudong Fault belts structure and volcanic development is inadequate, which were developed in the fault depression, and controlled the restricts the understanding of the volcanic gas reservoirs formation and evolution of deposition and structure (Meng and the level of gas exploration in a certain extent. Based on et al, 2006; Zhang et al, 2010) (Fig. 1). Large natural gas 3D seismic exploration and drill core data, we analyze the reservoirs have been discovered in deep strata in volcanic relationship between the volcanic development characteristics 1 3 rocks of K yc and K yc . In recent years, on the basis of and fault structure, and try to reveal the evolution and genetic 1 1 high-resolution 3D seismic and rock geochemistry data mechanism of the volcanic rocks of K yc. Ûƍ Ûƍ Ûƍ Ûƍ Depth,m K yc K sh 0 10km Main fault J hs Slope area ƍ Sag area Uplift area Section location Line1216 Fig. 17KHVWUXFWXUHGLDJUDPDQGVHLVPLFSUR¿OHRIWKH;XMLDZHL]L)DXOW'HSUHVVLRQ rose. Previous studies based on lithology and geochemical 2 Development characteristics of volcanic characteristics of the volcanic rocks show that the volcanic rocks in K yc rocks in Xujiaweizi and the adjacent areas have a dual tectonic setting of continental rift and marginal arc (Wang et The volcanic rocks of K \FPDLQO\GHYHORSHGLQWKH¿UVW al, 2009; Liu et al, 1994). and the third members (Jin et al, 2005; Tang et al, 2008; 1 3 Zhao et al, 2007). The volcanic rocks of K yc and K yc 1 1 1 3 2 2.2 Generalities of volcanic rocks in K yc and K yc 1 1 directly contact due to the absence of K yc . The development characteristics of the two members of volcanic rocks in K yc Pyroclastic facies, extrusive facies and a mixture of the will be analyzed based on the seismic data, drilling data and two facies are the main volcanic facies of the first member the results of previous studies. and third member in K yc, accompanied with a small amount of intrusive facies, channel facies and volcanic sedimentary 2.1 Difference of volcanic rocks between K yc and facies. The pyroclastic facies in the seismic profiles are K yc characterized by mound-like clutter, high amplitude and weak The volcanic rocks of K yc are mainly distributed in reflection (Fig. 2), mainly belonging to pyroclastic rocks, the central and southern fault depression, and most are including agglomerate, breccia, and tuff. The extrusive facies intermediate-acid. The base of the volcanic rocks has a small are characterized by intermittent and similarly parallel strong amount of basic extrusives. However, the volcanic rocks of DPSOLWXGHDQGVWURQJUHÀHFWLRQDQGWKHURFNW\SHLVPDLQO\ K yc are mainly distributed in the central and northern fault lava, including rhyolite, dacite, andesite, and basalt (Chen et depression, and are dominated by acid rocks accompanied al, 2000; Liu et al, 2003). Studies on the two members of K yc with basic and intermediate rocks. The difference of spatial show that the multi-phase pyroclastic facies and extrusive distribution between the two members of volcanic rocks is IDFLHVRYHUOD\HDFKRWKHULQWKHYHUWLFDOSUR¿OH7KHWKLFNQHVV mainly affected by regional tectonic evolution (Cai et al, and facies characteristics of volcanic rocks are influenced 2010), and the difference of lithology may be caused by obviously by the distribution of craters. The volcanic rocks different mixing degrees of crust material when the magma are mainly pyroclastic facies with large thickness when Xudong Fault Zone Zhao 59-Wanlong Uplift Ancient Central Uplift 438 Pet.Sci.(2012)9:436-443 close to the crater, and they are mainly extrusive facies with lithology and facies changing rapidly. small thickness when far away from the crater. The volcanic It can be seen that the characteristics of volcanic facies, 1 3 activities were characterized by eruption in high areas and rock type and eruption type of K yc and K yc are similar, 1 1 ¿OOLQJLQORZO\LQJDUHDV )LJ ,QDGGLWLRQWKHGLVWULEXWLRQ but there exist differences in the spatial distribution and of volcanic rocks shows the characteristics of center thick lithology. The similarity of the two members of volcanic rocks and surrounding thin due to weathering and erosion. Some researchers (Jiang et al, 2010; He et al, 2008) relied on drill profile in the middle depression. Previous studies suggested core and seismic data to study the lithology of volcanic rocks that the two members of volcanics of K yc are formed by the of K yc in the Xujiaweizi Fault Depression, and proposed same original magma chamber, and both have multi-stage and that there were multiple stages of volcanic eruption in the intermittent volcanic eruptions (Wang et al, 2009; 2007; Tang et Yingcheng period, and the eruption types included fissure, al, 2001). The above development features of volcanic rocks of central-vent and composite types, which resulted in the K yc show that there is a close relationship between the volcanic volcanic rocks overlying each other in space, as well as rocks and fault structure. Depth, m W E k yc k yc k sh (a) Line800 2350m Wang903 Wangshen1 Danshen6 k d Depth, m 1 S k d k d k d k yc 2500 (b) Trace1470 2350m Extrusive facies Channel facies Pyroclastic facies 1 3 Fig. 2 The lithofacies seismic interpretation sections of volcanic rocks of K yc (a) and K yc (b) (the section location is shown in Fig. 3) 1 1 southern section of the Xuxi Fault. The thickness is largest 3 Relationship between volcanic rocks of in the central depression, and the volcanic rocks gradually K yc and fault structure thin out to the east at the Xudong Slope. The thickness distribution shows that the distribution and eruption of the 3.1 Control of main faults on the volcanic eruption 1 volcanic rocks of K yc are closely related to the activity of and distribution the Xuzhong Fault. The thickness contour of the volcanic The superimposed figures (Fig. 3) of the relationship rocks is consistent with the strike of the Xuzhong Fault. 1 3 between faults and the volcanic rocks of K yc and K yc The volcanic rocks are thick near the Xuzhong Fault, and 1 1 were drawn from drilling data (Table 1), the structure maps thin when far away from the fault, which reveals that the and thickness maps of 3D seismic interpretation data. These volcanic activity along the Xuzhong Fault is characterized reveal the characteristics of distribution and development of by fissure type and partial central-vent type eruption. The volcanic rocks of K yc and the mechanism restricted by the volcanic rocks are absent in the Ancient Central Uplift which main faults of the Xujiaweizi Fault Depression. is located at the rising footwall of the Xuxi Fault, but are still The volcanic rocks of K yc are mainly distributed in the found in the small sag of the western Ancient Central Uplift. central and southern depression, and the west boundary is the Therefore, the Xuxi Fault has played an important role in GLI¿FXOWPDNHVDFFXUDWHO\LWWRGLVWLQJXLVKVHLVPLFWKHRQWKHP 800 do g ul Zon X u n Fa t e Xuxi Fault (N) Xuzhong Fault Zone Xuxi Fault (S) X uxi Fault (N) X u hong Faul t Xuxi Fault (S) Pet.Sci.(2012)9:436-443 439 controlling the distribution of volcanic rocks of K yc . In the LQWHUSUHWDWLRQSUR¿OHVZHGHGXFHWKDWWKHJHVFDOHODUVWULNH north-south direction, volcanic rocks of K yc thin from south slip activity of the Xuzhong Fault occurred from the Late to north and they are not developed in the uplift area of west Shahezi to the Yingcheng period, which was the fault-sag Xuxi Fault (N) and further north (Fig. 3). The Xuzhong Fault transition stage. The faults were strongly active and extended zone is the eruption center of the volcanics. From an analysis to the basement, which could have provided favorable of the relationship between strata and the faults in seismic channels for volcanic eruption. 125°00’ 125°15’ 125°30’ 125°45’ 125°00’ 125°15’ 125°30’ 125°45’ 47° 47° 00’ 0 00’ 0 10km N 10km Dashen 1 Dashen 1 46° 46° Dashen 302 45’ Dashen 302 45’ Dashen 2 Dashen 2 Songshen 1 Songshen 1 Shengshen 2 46° 46° Shengshen 2 30’ 30’ Songshen 6 Songshen 6 Fangshen 801 Fangshen 801 Xushen 42 Xushen 42 Shengshen 5 Shengshen 5 Chuangshen 102 Chuangshen 102 Xushen 232 Xushen 232 Xushen 1 Fangshen 5 Xushen 1 Fangshen 5 46° Xushen 21 46° Fangshen 701 Fangshen 701 Xushen 21 15’ 15’ Xushen 211 Xushen 201Xushen 211 Xushen 201 Xushen 25 Xushen 25 Zhaoshen 5 0 Zhaoshen 5 Zhaoshen 9 Zhaoshen 9 Zhaoshen 1 46° Zhaoshen 6 46° Zhaoshen 6 Zhaoshen 1 00’ 00’ Zhaoshen 8 Zhaoshen 8 Zhaoshen 11 Zhaoshen 11 Fault Fault Zhaoshen 2 Zhaoshen 2 45° 45° 45’ 45’ Volcanic isopach, m Volcanic isopach, m Section location Section location (b) (a) 1 3 Fig. 3 The relationship between distribution of volcanic rocks of K yc (a) and K yc (b) and faults in Xujiaweizi Fault Depression 1 1 The volcanic rocks of K yc are distributed in the central Fault shows zonal distribution in the eastern depression. The and northern fault depression, and are superimposed on the deep main fault laid the foundation for the volcanic eruption 1 3 top of the volcanic rocks of K yc . The maximum thickness of K yc 1 1 of volcanic rocks is located along the Xudong Fault zone, for the lateral migration of magma. which shows that the volcanic rocks of K yc erupted along The above analysis indicates that the volcanic rocks of WKHIDXOWZLWK¿VVXUHW\SHDQGSDUWLDOO\FHQWUDOYHQWW\SH7KH K yc erupted from the center to the south of the depression in north section of the Xuxi Fault is the western boundary of the early Yingcheng period, and the eruption and distribution the third member volcanic rocks which did not develop in the were mainly controlled by the Xuzhong Fault. The Xuxi Fault west of the fault. The southern boundary is located along the GH¿QHVWKHZHVWERXQGDU\RIWKHYROFDQLF7KHURFNVYROFDQLF straight line from well Xushen 301 to well Xushen 27. The rocks of K yc erupted from the center to the north of the volcanic rocks of K yc are thick in the middle and thin in the depression in the late Yingcheng period, and the eruption and east and west wings, as well as thick in the south and thin in distribution were mainly controlled by the Xudong Fault zone the north. The thickness is distributed along the fault zone. (Jiang et al, 2010; Cai et al, 2010). The main faults formed The above analysis shows that the distribution and eruption of in the rifting stage (from the Huoshiling to Shahezi periods) volcanic rocks of K yc are controlled by the main fault (Fig. became the major channels and eruption center of rising 3). There is a close relationship between the development of magma, and they also controlled the distribution range of the Xudong Fault zone and the eruption and distribution of the two volcanic rock members. In the three main faults, the volcanic rocks. The Xudong Fault zone consists of a series characteristics of fault cutting strata in the seismic section and of small faults with nearly N-S strike. The main fault in the previous studies reveal that the Xuzhong Fault was formed lower layers was formed in the strong rifting stage (Shahezi at the end of the Shahezi period, and was a strike-slip fault period), and the upper small faults with flower structure with normal and reverse faults alternating. The fault activities mainly developed during the period of K yc . The Xudong occurred strongly in the Yingcheng period and weakened in DQGWKHXSSHUÀRZHUVWUXFWXUHSURYLGHGWKHFKDQQHOV Xuzhong Fault Xuxi Fault 440 Pet.Sci.(2012)9:436-443 Drilling data of volcanic rock thickness in the Xujiaweizi Fault Table 1 the depression period. The Xudong Fault zone was formed in Depression the period of K yc of a main fault dipping southwest and several branch faults. Thickness, m Well depth No. Well It provided channels for the eruption of volcanic rocks of 1 3 K yc K yc 1 1 K yc . The Xuxi Fault played a key role in the process of 1 Chuang102 3466 0 0 the eruption of the volcanic rocks. It was formed in the initial stage of the rifting period or earlier, extended directly 2 Zhaoshen1 3050 306.5 0 to the deep crust, and was a key detachment fault which 3 Zhaoshen 2 3175.2 117 0 controlled the formation and evolution of the Xujiaweizi Fault Depression. The measurement of the fault displacement 4 Zhaoshen 5 3970 367 194 shows that the strong activity of the Xuxi Fault took place in 5 Dashen 1 4650 0 452 the strong rifting stage (i.e. Shahezi period) when the fault 6 Dashen 2 4650 0 452 displacement increased to more than 14 km, and during the Yingcheng period it was only 4 km. Therefore, the Xuxi Fault 7 Dashen 302 3600 0 198 played a leading role in the dynamic mechanism of volcanic 8 Fangshen5 3340 34 0 eruption in the Yingcheng period. It reached the deep magma chamber by connecting with the crustal detachment faults. 9 Fangshen 701 3935 124 0 The Xuzhong Fault and Xudong Fault were the branch faults 10 Fangshen 801 3910 22 0 which developed later than the Xuxi Fault, and became the two main channels after the magma reached the basement of 11 Shengshen2 3816 0 772 the fault depression. The upwelling and eruption of a large 12 Shengshen 5 4175 47.5 0 amount of magma decreased the speed of fault extension, 13 Songshen 1 3935 0 565 and speeded up the conversion of the structure from rifting to depression in the Xujiaweizi area (Fig. 4). 14 Songshen 6 3850 0 256 15 Xushen 1 4548 258 0 3.2 Transformation of volcanic activity on pre- existing faults 16 Xushen 201 4333 305 0 The tensional fractures formed during the strong rifting 17 Xushen 232 4238 423 211 period provided channels for the volcanic eruptions of K yc. 18 Xushen 25 4483 244 190 The volcano had an important transformation effect on the 19 Xushen 42 4546 443 0 faults when it rose along the faults due to the erosion by the heated magma. Some fault zones turned into the areas where 20 Xushen 21 4273 409 167 channel facies of volcanic rocks developed, which showed 21 Xushen 211 4291 180 104 different reflection characteristics compared with strata on both sides of the fault. A large area of volcanic activity could 22 Zhaoshen 11 3415 37 0 make the pre-existing faults blur beyond our recognition 23 Zhaoshen 6 4117 190 0 or even disappear in the seismic profile. For example, the Xuzhong Fault was one of the deep faults within the 24 Zhaoshen 8 3450 209 0 depression with NW-SE strike and high dip angle, and the 25 Zhaoshen 9 3982.3 223 0 fault extended down to the basement and also spreaded out k yc k sh k h Basement Magma Main fault The model graph of volcanic eruption of K yc Fig. 4 Xudong Fault zone DQGZDVDÀRZHUVWUXFWXUH]RQHFRPSRVHG Pet.Sci.(2012)9:436-443 441 LQWKHXSSHUIRUPDWLRQZLWKDÀRZHUVWUXFWXUH2QERWKVLGHV Therefore, although the Xuzhong Fault is a large-scale strike- of the Xuzhong Fault in the southern depression, the Shahezi slip fault, it has not been recognized until recently. The Formation was deformed and displaced (Zhang et al, 2010). reasons are not only the poor quality of previous seismic data, It is easy to identify and track the fault in seismic section. but also the indistinct trace of the Xuzhong Fault which has However, in the northern depression, the Xuzhong Fault went been affected by the transformation of volcanic activities. through the Xingcheng Uplift, and the Shahezi Formation Moreover, some other faults (such as the Xudong Fault was absent on both sides of the fault, which led to the direct zone) have the similar transformation effects of volcanic contact of volcanic rocks of J hs and K yc. In the fault activities. The seismic section in the central Xujiaweizi Fault 3 1 zone and nearby areas volcanic channel facies developed. Depression shows that the channel of magma upwelling cut In seismic profiles, the volcanic rocks along the fault zone through the K sh, while the strata on both sides of the channel have similar reflection characteristics with the surrounding have no obvious distortion. Moreover, the later activities volcanics, and the structural differences are not obvious, so of faults disrupted the strata on the top of K yc, which was LWLVGLI¿FXOWWRLGHQWLI\ZKHWKHURUQRWWKHIDXOWZHQWWKURXJK important evidence to infer that the pre-existing faults were WKHYROFDQLFURFNVIURPWKHLQWHUSUHWDWLRQRIVHLVPLFSUR¿OHV transformed by volcanic activities (Fig. 5(a)). 0 5cm Depth, m (a) (c) (b) 0 1km Pre-existing fault Well Wangshen905, depth of 2997m, k yc Line1400 Well Xushen231, depth of 3760m, k yc Fig. 5 The relationship diagram of faults (fractures) and volcanic rock (the data was obtained from the Exploration and Development 5HVHDUFK,QVWLWXWHRI'DTLQJ2LO¿HOG&RPSDQ\/WG (a) The faults were eroded by magmatic activity, the shaded part is the upwelling magma channel, the red dashed line shows the faults ZKLFKKDYHEHHQPHOWHGDQGDUHGLI¿FXOWWRLGHQWLI\ E )UDFWXUHVLQH[WUXVLYHIDFLHVSHUOLWH7KHWKLQVHFWLRQRSWLFDOPLFUR JUDSKVKRZV that the perlite was cut by many groups of fractures; (c) The fractures showing in drill core Volcanic activity is an important tectonism during the DFWLYLWLHVVLJQL¿FDQWO\GHFUHDVHG =KDRHWDO=KDQJHW formation and evolution of the Xujiaweizi Fault Depression, al, 2010; Chen et al, 2012). Although the fault activities were and has significant impact on the deep strata and structure. weak during the depression period, they lasted for a long time. The volcanic activities of K yc took place in the critical 7KHVHLVPLFLQWHUSUHWDWLRQSUR¿OH )LJ UHYHDOVWKDWZHDN period of fault-sag transition stage, which was just after a fault activities have occurred since the Denglouku period. strong extension in the Shahezi rifting period. The deep-seated However, the fault activities had little impact on continuous faults communicated with the basement magma chamber and development of the strata due to their small scale. These became the main channels for magma upwelling. The rising small faults were not related to the faults formed in the rifting magma not only transformed a large number of pre-existing period, and had no relation with early tectonic movements, fractures, but also occupied the accommodation space, which EXWZHUHQÀXHQFHGLE\WKHQHZVWUHVV¿HOGVRIODWHUWHFWRQLF led to the formation of a series of compressional structure movements (Feng, 2008; Zhang et al, 2009; Zhao et al, 2009). styles (such as compression anticlines), and the change from Even so, post-fault activities played an important role in DQH[WHQVLRQDOVWUHVV¿HOGWRDFRPSUHVVLRQVWUHVV¿HOG7KH improving the physical properties of K yc. Many fractures 1 3 transformation made the deep structure of the Xujiaweizi were formed in the volcanic rocks of K yc and K yc because 1 1 Fault Depression more complex. of continuous fault activities, and they were found in the The transformation of volcanic activity on the pre- thin sections and drill cores of volcanic rocks of erupted existing faults has a great effect on deep gas exploration, and extrusive facies (Figs. 5(b) and 5(c)). A large number of because the faults are the main channels which communicate fractures distributed in the volcanic rocks greatly increased with the deep source rocks and gas reservoirs. The damaged the porosity and permeability of volcanic rocks. Taking the IDXOWFKDQQHOVPDNHLWGLI¿FXOWWRXQGHUVWDQGWKHQDWXUDOJDV volcanic rocks of K yc in the Shengping area as an example, migration and accumulation. the analysis of drill core showed that the porosity of volcanic rocks where fractures developed could reach 20%, and the -3 2 ,QÀXHQFHRIHVSRVWIUDFWXURQSK\VLFDORSHUWLHVSU maximum horizontal permeability was 52.7×10 ȝP and the -3 2 of volcanic rocks maximum vertical permeability was 73.1×10 ȝP . In the The volcanic rocks of K yc were formed in the fault- volcanic rocks where fractures did not develop, the porosity sag transition stage, but the volcanic activities weakened was generally less than 10%, the horizontal permeability was -3 2 gradually after the period of depression, and the fault less than 20×10 ȝP , and the vertical permeability was less 442 Pet.Sci.(2012)9:436-443 -3 2 than 30×10 ȝP . Therefore, fractures have been an important volcanic rocks of the Yingcheng Formation are closely related IDFWRUZKLFKJUHDWO\LQÀXHQFHGWKHQDWXUDOJDVVWRUDJHVSDFH to the faults from the magma upwelling, the volcanic eruption of volcanic rocks. and the diagenetic transformation of rock properties. 4 Conclusions Acknowledgements 1) The differences of spatial distribution and lithology This study was supported by the Major State Basic EHWZHHQWKH¿UVWDQGWKHWKLUGPHPEHUYROFDQLFURFNVRIWKH Research Development Program of China (973 Program Yingcheng Formation in the Xujiaweizi Fault Depression (No.2012CB214705)) and the National Natural Science have been studied. The differences of spatial distribution Foundation of China (No. 41206035). The authors would like are controlled by the tectonic evolution, and the differences to thank the Exploration and Development Research Institute RIOLWKRORJ\UHÀHFWGLIIHUHQWGHJUHHVRIPL[LQJRIWKHULVLQJ RI'DTLQJ2LO¿HOG&RPSDQ\/WGIRUSURYLGLQJWKHRULJLQDO magma and the crustal material during the process of volcanic data. activities. The lithofacies, rock types, and eruption types of References the two volcanic members are similar, which shows that the volcanic activities are characterized by the same original Cai Z R, Xia B, Guo F, et al. Controlling mechanism on volcanic rocks magma chamber, multi-stage and intermittent eruptions, of the Yingcheng Formation of the Xujiaweizi rift depression in the and the main faults play an important role in controlling the northern Songliao Basin. Acta Petrolei Sinica. 2010. 31(6): 941-945 development of volcanic rocks. (in Chinese) 2) Close relationships between the development of Che n J L, Cai X Y, Lin C H, et al. 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Geologic characteristics of volcanic Xujiaweizi depression in the Songliao Basin. Acta Petrolei Sinica. hydrocarbon reservoirs and exploration directions in China. Acta 2002. 23(2): 26-29 (in Chinese) Geologica Sinica. 2010. 84(1): 194-205 Zha ng E H, Jiang C J, Zhang Y G, et al. Study on the formation and (Edited by Hao Jie) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Petroleum Science Springer Journals

Development features of volcanic rocks of the Yingcheng Formation and their relationship with fault structure in the Xujiaweizi Fault Depression, Songliao Basin, China

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Springer Journals
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Copyright © 2012 by China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Economics
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1672-5107
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1995-8226
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10.1007/s12182-012-0227-4
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Abstract

436 Pet.Sci.(2012)9:436-443 10.1007/s12182-012-0227-4 Development features of volcanic rocks of the Yingcheng Formation and their relationship with fault structure in the Xujiaweizi Fault Depression, Songliao Basin, China 1, 2, 3 4 1, 2, 3 1, 2, 3 Cai Zhourong , Huang Qiangtai , Xia Bin , Lü Baofeng , Liu 1, 2, 3 1, 2, 3 Weiliang and Wan Zhifeng School of Marine Sciences, Sun Yat-sen University, Guangzhou, Guangdong 510006, China Guangdong University Key Laboratory of Offshore Oil Exploration and Development, Guangzhou, Guangdong 510006, China Guangdong Provincial Key Laboratory of Marine Resources and Coastal Engineering, Guangzhou, Guangdong 510006, China Guangzhou Institute of Geochemistry, CAS, Guangzhou, Guangdong 510640, China © China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2012 Abstract: The Xujiaweizi Fault Depression is located in the northern Songliao Basin, Northeast China. The exploration results show that the most favorable natural gas reservoirs are in the volcanic rocks of the Yingcheng Formation (K yc). Based on seismic interpretation, drill cores and the results of previous research, we analyzed the distribution of faults and the thickness of volcanic rocks in different periods of K yc, and studied the relationship of volcanic activities and main faults. Volcanic rocks were formed in the Yingcheng period when the magma erupted along pre-existing fault zones. The volcanic activities strongly eroded the faults during the eruption process, which resulted in the structural traces in the seismic section being diffuse and unclear. The tectonic activities weakened in the study area in the depression stage. The analysis of seismic interpretation, thin section microscopy and drill cores revealed that a large number of fractures generated in the volcanic rocks were affected by later continued weak tectonic activities, which greatly improved the physical properties of volcanic reservoirs, and made the volcanic rocks of K yc be favorable natural gas reservoirs. The above conclusions provide the basis to better understand the relationship of the volcanic rock distribution and faults, the mechanism of volcanic eruption and the formation of natural gas reservoirs in volcanic rocks. Key words: Volcanic rock, development features, Yingcheng Formation, Xujiaweizi Fault Depression, Songliao Basin from the Ancient Central Uplift, and the transitional slope 1 Introduction areas in the southeast and northeast of the depression are The Xujiaweizi Fault Depression is located in the separated from the Zhao 59–Wanlong ancient uplift and the northern Songliao Basin, Northeast China, where the largest northeast uplift area, respectively (Fig. 1). The depression oil and gas reservoirs in the Songliao Basin have been found, extends approximately north-south, with a maximum width such as the Daqing Oilfield. It is the most highly explored of 40 km in E-W direction and an N-S length of about 80 part of the basin. The reservoirs are characterized by oil km. The structure pattern is characterized by rifting in the in shallow strata and gas in deep strata. Since the shallow west and overlap in the east, block-faulting in N-S direction. exploration entered the stage of high water-cut production, The deep strata of the depression include the Upper Jurassic and the well Xushen 1 obtained a high-yield natural gas Huoshiling (J hs), the Lower Cretaceous Shahezi (K sh), 3 1 flow from Cretaceous volcanic rocks in 2002, exploration Yingcheng (K yc), Denglouku (K d) and Quantou Formations 1 1 has discovered substantial reserves of deep-layer gas (Zou (K q) from bottom to top. The Huoshiling period was the et al, 2010). The western depression is separated by faults initial rifting stage, dominated by volcanism. The Shahezi period was the strong rifting stage and the strata were mainly *Corresponding author. email: xiabin01@gmail.com lake sediments. The Yingcheng period was the fault-sag Received November 3, 2011 WUDQVLWLRQVWDJHWKH¿UVWDQGWKLUGPHPEHUVZHUHGRPLQDWHG Xuzhong Fault Xudong Fault Zone Xuxi Fault (N) Northeast Uplift Area Xuzhong Fault Xuxi Fault (S) Xuxi Fault Pet.Sci.(2012)9:436-443 437 by volcanism, the second member was absent and the fourth from drilling, more information has been obtained about the member of Yingcheng Formation (K yc ) was river-delta lithology, composition and distribution of volcanic rocks of and shallow lake facies. The Denglouku period and the latest K yc (Ren et al, 2004; Hou et al, 2004; Feng, 2006; Huang et period were the depression stage, and river delta and lake al, 2006). However, research into the mechanisms of volcanic facies sediments were developed (Chen et al, 1999; Chi et eruption is incomplete, and the study of key fundamental al, 2002; Yin et al, 2002). The Xuxi Fault (including north problems in the relationships between the complex fault and south sections), Xuzhong Fault and Xudong Fault belts structure and volcanic development is inadequate, which were developed in the fault depression, and controlled the restricts the understanding of the volcanic gas reservoirs formation and evolution of deposition and structure (Meng and the level of gas exploration in a certain extent. Based on et al, 2006; Zhang et al, 2010) (Fig. 1). Large natural gas 3D seismic exploration and drill core data, we analyze the reservoirs have been discovered in deep strata in volcanic relationship between the volcanic development characteristics 1 3 rocks of K yc and K yc . In recent years, on the basis of and fault structure, and try to reveal the evolution and genetic 1 1 high-resolution 3D seismic and rock geochemistry data mechanism of the volcanic rocks of K yc. Ûƍ Ûƍ Ûƍ Ûƍ Depth,m K yc K sh 0 10km Main fault J hs Slope area ƍ Sag area Uplift area Section location Line1216 Fig. 17KHVWUXFWXUHGLDJUDPDQGVHLVPLFSUR¿OHRIWKH;XMLDZHL]L)DXOW'HSUHVVLRQ rose. Previous studies based on lithology and geochemical 2 Development characteristics of volcanic characteristics of the volcanic rocks show that the volcanic rocks in K yc rocks in Xujiaweizi and the adjacent areas have a dual tectonic setting of continental rift and marginal arc (Wang et The volcanic rocks of K \FPDLQO\GHYHORSHGLQWKH¿UVW al, 2009; Liu et al, 1994). and the third members (Jin et al, 2005; Tang et al, 2008; 1 3 Zhao et al, 2007). The volcanic rocks of K yc and K yc 1 1 1 3 2 2.2 Generalities of volcanic rocks in K yc and K yc 1 1 directly contact due to the absence of K yc . The development characteristics of the two members of volcanic rocks in K yc Pyroclastic facies, extrusive facies and a mixture of the will be analyzed based on the seismic data, drilling data and two facies are the main volcanic facies of the first member the results of previous studies. and third member in K yc, accompanied with a small amount of intrusive facies, channel facies and volcanic sedimentary 2.1 Difference of volcanic rocks between K yc and facies. The pyroclastic facies in the seismic profiles are K yc characterized by mound-like clutter, high amplitude and weak The volcanic rocks of K yc are mainly distributed in reflection (Fig. 2), mainly belonging to pyroclastic rocks, the central and southern fault depression, and most are including agglomerate, breccia, and tuff. The extrusive facies intermediate-acid. The base of the volcanic rocks has a small are characterized by intermittent and similarly parallel strong amount of basic extrusives. However, the volcanic rocks of DPSOLWXGHDQGVWURQJUHÀHFWLRQDQGWKHURFNW\SHLVPDLQO\ K yc are mainly distributed in the central and northern fault lava, including rhyolite, dacite, andesite, and basalt (Chen et depression, and are dominated by acid rocks accompanied al, 2000; Liu et al, 2003). Studies on the two members of K yc with basic and intermediate rocks. The difference of spatial show that the multi-phase pyroclastic facies and extrusive distribution between the two members of volcanic rocks is IDFLHVRYHUOD\HDFKRWKHULQWKHYHUWLFDOSUR¿OH7KHWKLFNQHVV mainly affected by regional tectonic evolution (Cai et al, and facies characteristics of volcanic rocks are influenced 2010), and the difference of lithology may be caused by obviously by the distribution of craters. The volcanic rocks different mixing degrees of crust material when the magma are mainly pyroclastic facies with large thickness when Xudong Fault Zone Zhao 59-Wanlong Uplift Ancient Central Uplift 438 Pet.Sci.(2012)9:436-443 close to the crater, and they are mainly extrusive facies with lithology and facies changing rapidly. small thickness when far away from the crater. The volcanic It can be seen that the characteristics of volcanic facies, 1 3 activities were characterized by eruption in high areas and rock type and eruption type of K yc and K yc are similar, 1 1 ¿OOLQJLQORZO\LQJDUHDV )LJ ,QDGGLWLRQWKHGLVWULEXWLRQ but there exist differences in the spatial distribution and of volcanic rocks shows the characteristics of center thick lithology. The similarity of the two members of volcanic rocks and surrounding thin due to weathering and erosion. Some researchers (Jiang et al, 2010; He et al, 2008) relied on drill profile in the middle depression. Previous studies suggested core and seismic data to study the lithology of volcanic rocks that the two members of volcanics of K yc are formed by the of K yc in the Xujiaweizi Fault Depression, and proposed same original magma chamber, and both have multi-stage and that there were multiple stages of volcanic eruption in the intermittent volcanic eruptions (Wang et al, 2009; 2007; Tang et Yingcheng period, and the eruption types included fissure, al, 2001). The above development features of volcanic rocks of central-vent and composite types, which resulted in the K yc show that there is a close relationship between the volcanic volcanic rocks overlying each other in space, as well as rocks and fault structure. Depth, m W E k yc k yc k sh (a) Line800 2350m Wang903 Wangshen1 Danshen6 k d Depth, m 1 S k d k d k d k yc 2500 (b) Trace1470 2350m Extrusive facies Channel facies Pyroclastic facies 1 3 Fig. 2 The lithofacies seismic interpretation sections of volcanic rocks of K yc (a) and K yc (b) (the section location is shown in Fig. 3) 1 1 southern section of the Xuxi Fault. The thickness is largest 3 Relationship between volcanic rocks of in the central depression, and the volcanic rocks gradually K yc and fault structure thin out to the east at the Xudong Slope. The thickness distribution shows that the distribution and eruption of the 3.1 Control of main faults on the volcanic eruption 1 volcanic rocks of K yc are closely related to the activity of and distribution the Xuzhong Fault. The thickness contour of the volcanic The superimposed figures (Fig. 3) of the relationship rocks is consistent with the strike of the Xuzhong Fault. 1 3 between faults and the volcanic rocks of K yc and K yc The volcanic rocks are thick near the Xuzhong Fault, and 1 1 were drawn from drilling data (Table 1), the structure maps thin when far away from the fault, which reveals that the and thickness maps of 3D seismic interpretation data. These volcanic activity along the Xuzhong Fault is characterized reveal the characteristics of distribution and development of by fissure type and partial central-vent type eruption. The volcanic rocks of K yc and the mechanism restricted by the volcanic rocks are absent in the Ancient Central Uplift which main faults of the Xujiaweizi Fault Depression. is located at the rising footwall of the Xuxi Fault, but are still The volcanic rocks of K yc are mainly distributed in the found in the small sag of the western Ancient Central Uplift. central and southern depression, and the west boundary is the Therefore, the Xuxi Fault has played an important role in GLI¿FXOWPDNHVDFFXUDWHO\LWWRGLVWLQJXLVKVHLVPLFWKHRQWKHP 800 do g ul Zon X u n Fa t e Xuxi Fault (N) Xuzhong Fault Zone Xuxi Fault (S) X uxi Fault (N) X u hong Faul t Xuxi Fault (S) Pet.Sci.(2012)9:436-443 439 controlling the distribution of volcanic rocks of K yc . In the LQWHUSUHWDWLRQSUR¿OHVZHGHGXFHWKDWWKHJHVFDOHODUVWULNH north-south direction, volcanic rocks of K yc thin from south slip activity of the Xuzhong Fault occurred from the Late to north and they are not developed in the uplift area of west Shahezi to the Yingcheng period, which was the fault-sag Xuxi Fault (N) and further north (Fig. 3). The Xuzhong Fault transition stage. The faults were strongly active and extended zone is the eruption center of the volcanics. From an analysis to the basement, which could have provided favorable of the relationship between strata and the faults in seismic channels for volcanic eruption. 125°00’ 125°15’ 125°30’ 125°45’ 125°00’ 125°15’ 125°30’ 125°45’ 47° 47° 00’ 0 00’ 0 10km N 10km Dashen 1 Dashen 1 46° 46° Dashen 302 45’ Dashen 302 45’ Dashen 2 Dashen 2 Songshen 1 Songshen 1 Shengshen 2 46° 46° Shengshen 2 30’ 30’ Songshen 6 Songshen 6 Fangshen 801 Fangshen 801 Xushen 42 Xushen 42 Shengshen 5 Shengshen 5 Chuangshen 102 Chuangshen 102 Xushen 232 Xushen 232 Xushen 1 Fangshen 5 Xushen 1 Fangshen 5 46° Xushen 21 46° Fangshen 701 Fangshen 701 Xushen 21 15’ 15’ Xushen 211 Xushen 201Xushen 211 Xushen 201 Xushen 25 Xushen 25 Zhaoshen 5 0 Zhaoshen 5 Zhaoshen 9 Zhaoshen 9 Zhaoshen 1 46° Zhaoshen 6 46° Zhaoshen 6 Zhaoshen 1 00’ 00’ Zhaoshen 8 Zhaoshen 8 Zhaoshen 11 Zhaoshen 11 Fault Fault Zhaoshen 2 Zhaoshen 2 45° 45° 45’ 45’ Volcanic isopach, m Volcanic isopach, m Section location Section location (b) (a) 1 3 Fig. 3 The relationship between distribution of volcanic rocks of K yc (a) and K yc (b) and faults in Xujiaweizi Fault Depression 1 1 The volcanic rocks of K yc are distributed in the central Fault shows zonal distribution in the eastern depression. The and northern fault depression, and are superimposed on the deep main fault laid the foundation for the volcanic eruption 1 3 top of the volcanic rocks of K yc . The maximum thickness of K yc 1 1 of volcanic rocks is located along the Xudong Fault zone, for the lateral migration of magma. which shows that the volcanic rocks of K yc erupted along The above analysis indicates that the volcanic rocks of WKHIDXOWZLWK¿VVXUHW\SHDQGSDUWLDOO\FHQWUDOYHQWW\SH7KH K yc erupted from the center to the south of the depression in north section of the Xuxi Fault is the western boundary of the early Yingcheng period, and the eruption and distribution the third member volcanic rocks which did not develop in the were mainly controlled by the Xuzhong Fault. The Xuxi Fault west of the fault. The southern boundary is located along the GH¿QHVWKHZHVWERXQGDU\RIWKHYROFDQLF7KHURFNVYROFDQLF straight line from well Xushen 301 to well Xushen 27. The rocks of K yc erupted from the center to the north of the volcanic rocks of K yc are thick in the middle and thin in the depression in the late Yingcheng period, and the eruption and east and west wings, as well as thick in the south and thin in distribution were mainly controlled by the Xudong Fault zone the north. The thickness is distributed along the fault zone. (Jiang et al, 2010; Cai et al, 2010). The main faults formed The above analysis shows that the distribution and eruption of in the rifting stage (from the Huoshiling to Shahezi periods) volcanic rocks of K yc are controlled by the main fault (Fig. became the major channels and eruption center of rising 3). There is a close relationship between the development of magma, and they also controlled the distribution range of the Xudong Fault zone and the eruption and distribution of the two volcanic rock members. In the three main faults, the volcanic rocks. The Xudong Fault zone consists of a series characteristics of fault cutting strata in the seismic section and of small faults with nearly N-S strike. The main fault in the previous studies reveal that the Xuzhong Fault was formed lower layers was formed in the strong rifting stage (Shahezi at the end of the Shahezi period, and was a strike-slip fault period), and the upper small faults with flower structure with normal and reverse faults alternating. The fault activities mainly developed during the period of K yc . The Xudong occurred strongly in the Yingcheng period and weakened in DQGWKHXSSHUÀRZHUVWUXFWXUHSURYLGHGWKHFKDQQHOV Xuzhong Fault Xuxi Fault 440 Pet.Sci.(2012)9:436-443 Drilling data of volcanic rock thickness in the Xujiaweizi Fault Table 1 the depression period. The Xudong Fault zone was formed in Depression the period of K yc of a main fault dipping southwest and several branch faults. Thickness, m Well depth No. Well It provided channels for the eruption of volcanic rocks of 1 3 K yc K yc 1 1 K yc . The Xuxi Fault played a key role in the process of 1 Chuang102 3466 0 0 the eruption of the volcanic rocks. It was formed in the initial stage of the rifting period or earlier, extended directly 2 Zhaoshen1 3050 306.5 0 to the deep crust, and was a key detachment fault which 3 Zhaoshen 2 3175.2 117 0 controlled the formation and evolution of the Xujiaweizi Fault Depression. The measurement of the fault displacement 4 Zhaoshen 5 3970 367 194 shows that the strong activity of the Xuxi Fault took place in 5 Dashen 1 4650 0 452 the strong rifting stage (i.e. Shahezi period) when the fault 6 Dashen 2 4650 0 452 displacement increased to more than 14 km, and during the Yingcheng period it was only 4 km. Therefore, the Xuxi Fault 7 Dashen 302 3600 0 198 played a leading role in the dynamic mechanism of volcanic 8 Fangshen5 3340 34 0 eruption in the Yingcheng period. It reached the deep magma chamber by connecting with the crustal detachment faults. 9 Fangshen 701 3935 124 0 The Xuzhong Fault and Xudong Fault were the branch faults 10 Fangshen 801 3910 22 0 which developed later than the Xuxi Fault, and became the two main channels after the magma reached the basement of 11 Shengshen2 3816 0 772 the fault depression. The upwelling and eruption of a large 12 Shengshen 5 4175 47.5 0 amount of magma decreased the speed of fault extension, 13 Songshen 1 3935 0 565 and speeded up the conversion of the structure from rifting to depression in the Xujiaweizi area (Fig. 4). 14 Songshen 6 3850 0 256 15 Xushen 1 4548 258 0 3.2 Transformation of volcanic activity on pre- existing faults 16 Xushen 201 4333 305 0 The tensional fractures formed during the strong rifting 17 Xushen 232 4238 423 211 period provided channels for the volcanic eruptions of K yc. 18 Xushen 25 4483 244 190 The volcano had an important transformation effect on the 19 Xushen 42 4546 443 0 faults when it rose along the faults due to the erosion by the heated magma. Some fault zones turned into the areas where 20 Xushen 21 4273 409 167 channel facies of volcanic rocks developed, which showed 21 Xushen 211 4291 180 104 different reflection characteristics compared with strata on both sides of the fault. A large area of volcanic activity could 22 Zhaoshen 11 3415 37 0 make the pre-existing faults blur beyond our recognition 23 Zhaoshen 6 4117 190 0 or even disappear in the seismic profile. For example, the Xuzhong Fault was one of the deep faults within the 24 Zhaoshen 8 3450 209 0 depression with NW-SE strike and high dip angle, and the 25 Zhaoshen 9 3982.3 223 0 fault extended down to the basement and also spreaded out k yc k sh k h Basement Magma Main fault The model graph of volcanic eruption of K yc Fig. 4 Xudong Fault zone DQGZDVDÀRZHUVWUXFWXUH]RQHFRPSRVHG Pet.Sci.(2012)9:436-443 441 LQWKHXSSHUIRUPDWLRQZLWKDÀRZHUVWUXFWXUH2QERWKVLGHV Therefore, although the Xuzhong Fault is a large-scale strike- of the Xuzhong Fault in the southern depression, the Shahezi slip fault, it has not been recognized until recently. The Formation was deformed and displaced (Zhang et al, 2010). reasons are not only the poor quality of previous seismic data, It is easy to identify and track the fault in seismic section. but also the indistinct trace of the Xuzhong Fault which has However, in the northern depression, the Xuzhong Fault went been affected by the transformation of volcanic activities. through the Xingcheng Uplift, and the Shahezi Formation Moreover, some other faults (such as the Xudong Fault was absent on both sides of the fault, which led to the direct zone) have the similar transformation effects of volcanic contact of volcanic rocks of J hs and K yc. In the fault activities. The seismic section in the central Xujiaweizi Fault 3 1 zone and nearby areas volcanic channel facies developed. Depression shows that the channel of magma upwelling cut In seismic profiles, the volcanic rocks along the fault zone through the K sh, while the strata on both sides of the channel have similar reflection characteristics with the surrounding have no obvious distortion. Moreover, the later activities volcanics, and the structural differences are not obvious, so of faults disrupted the strata on the top of K yc, which was LWLVGLI¿FXOWWRLGHQWLI\ZKHWKHURUQRWWKHIDXOWZHQWWKURXJK important evidence to infer that the pre-existing faults were WKHYROFDQLFURFNVIURPWKHLQWHUSUHWDWLRQRIVHLVPLFSUR¿OHV transformed by volcanic activities (Fig. 5(a)). 0 5cm Depth, m (a) (c) (b) 0 1km Pre-existing fault Well Wangshen905, depth of 2997m, k yc Line1400 Well Xushen231, depth of 3760m, k yc Fig. 5 The relationship diagram of faults (fractures) and volcanic rock (the data was obtained from the Exploration and Development 5HVHDUFK,QVWLWXWHRI'DTLQJ2LO¿HOG&RPSDQ\/WG (a) The faults were eroded by magmatic activity, the shaded part is the upwelling magma channel, the red dashed line shows the faults ZKLFKKDYHEHHQPHOWHGDQGDUHGLI¿FXOWWRLGHQWLI\ E )UDFWXUHVLQH[WUXVLYHIDFLHVSHUOLWH7KHWKLQVHFWLRQRSWLFDOPLFUR JUDSKVKRZV that the perlite was cut by many groups of fractures; (c) The fractures showing in drill core Volcanic activity is an important tectonism during the DFWLYLWLHVVLJQL¿FDQWO\GHFUHDVHG =KDRHWDO=KDQJHW formation and evolution of the Xujiaweizi Fault Depression, al, 2010; Chen et al, 2012). Although the fault activities were and has significant impact on the deep strata and structure. weak during the depression period, they lasted for a long time. The volcanic activities of K yc took place in the critical 7KHVHLVPLFLQWHUSUHWDWLRQSUR¿OH )LJ UHYHDOVWKDWZHDN period of fault-sag transition stage, which was just after a fault activities have occurred since the Denglouku period. strong extension in the Shahezi rifting period. The deep-seated However, the fault activities had little impact on continuous faults communicated with the basement magma chamber and development of the strata due to their small scale. These became the main channels for magma upwelling. The rising small faults were not related to the faults formed in the rifting magma not only transformed a large number of pre-existing period, and had no relation with early tectonic movements, fractures, but also occupied the accommodation space, which EXWZHUHQÀXHQFHGLE\WKHQHZVWUHVV¿HOGVRIODWHUWHFWRQLF led to the formation of a series of compressional structure movements (Feng, 2008; Zhang et al, 2009; Zhao et al, 2009). styles (such as compression anticlines), and the change from Even so, post-fault activities played an important role in DQH[WHQVLRQDOVWUHVV¿HOGWRDFRPSUHVVLRQVWUHVV¿HOG7KH improving the physical properties of K yc. Many fractures 1 3 transformation made the deep structure of the Xujiaweizi were formed in the volcanic rocks of K yc and K yc because 1 1 Fault Depression more complex. of continuous fault activities, and they were found in the The transformation of volcanic activity on the pre- thin sections and drill cores of volcanic rocks of erupted existing faults has a great effect on deep gas exploration, and extrusive facies (Figs. 5(b) and 5(c)). A large number of because the faults are the main channels which communicate fractures distributed in the volcanic rocks greatly increased with the deep source rocks and gas reservoirs. The damaged the porosity and permeability of volcanic rocks. Taking the IDXOWFKDQQHOVPDNHLWGLI¿FXOWWRXQGHUVWDQGWKHQDWXUDOJDV volcanic rocks of K yc in the Shengping area as an example, migration and accumulation. the analysis of drill core showed that the porosity of volcanic rocks where fractures developed could reach 20%, and the -3 2 ,QÀXHQFHRIHVSRVWIUDFWXURQSK\VLFDORSHUWLHVSU maximum horizontal permeability was 52.7×10 ȝP and the -3 2 of volcanic rocks maximum vertical permeability was 73.1×10 ȝP . In the The volcanic rocks of K yc were formed in the fault- volcanic rocks where fractures did not develop, the porosity sag transition stage, but the volcanic activities weakened was generally less than 10%, the horizontal permeability was -3 2 gradually after the period of depression, and the fault less than 20×10 ȝP , and the vertical permeability was less 442 Pet.Sci.(2012)9:436-443 -3 2 than 30×10 ȝP . Therefore, fractures have been an important volcanic rocks of the Yingcheng Formation are closely related IDFWRUZKLFKJUHDWO\LQÀXHQFHGWKHQDWXUDOJDVVWRUDJHVSDFH to the faults from the magma upwelling, the volcanic eruption of volcanic rocks. and the diagenetic transformation of rock properties. 4 Conclusions Acknowledgements 1) The differences of spatial distribution and lithology This study was supported by the Major State Basic EHWZHHQWKH¿UVWDQGWKHWKLUGPHPEHUYROFDQLFURFNVRIWKH Research Development Program of China (973 Program Yingcheng Formation in the Xujiaweizi Fault Depression (No.2012CB214705)) and the National Natural Science have been studied. The differences of spatial distribution Foundation of China (No. 41206035). 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Published: Nov 21, 2012

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