SPE 131813 The Study of Fracture in Volcanic Reservoir Jie Fang, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences; Research Institute of Petroleum Exploration and Development, PetroChina; Chengzao Jia, Qiquan Ran, Xinmao Zhou , Gang Hui , Research Institute of Petroleum Exploration and Development, PetroChina Copyright 2010, Society of Petroleum Engineers This paper was prepared for presentation at the CPS/SPE International Oil & Gas Conference and Exhibition in China held in Beijing, China, 8–10 June 2010. This paper was selected for presentation by a CPS/SPE program committee following review of information contained in an abstr act submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any posi tion of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract mus t contain conspicuous acknowledgment of SPE copyright. Abstract The 1’st members of Yingcheng formation, lower Cretaceous in Xingcheng development area, Songliao Basin mainly have rhyolite gas reservoirs. The 1’st member of Yingcheng is belong to the low-porosity and low-permeable reservoir, the reservoir space are mainly fracture-pore. By the means of field out crops observation, core description, thin section analysis, well logging identification and the seismic forecasting, the fractures of 1’st member of Yingcheng volcanic reservoir was analyzed in three aspects: the genetic, the attitude and the width. The dominate fracture type of 1’st member of Yingcheng are the tectonic related fractures(78.5%), then the diagenetic related fractures(17.0%), corroded fractures(4.2%) are the last. The high angle fractures and the vertical fractures are most common in tectonic related fractures; the oblique fractures are the following. The dominate type of diagenetic related fractures are level fractures, followed by high angle and oblique fractures. The fractures in the reservoir were mainly wide fractures (width>1mm) and narrow fractures (0.1mm<width<1mm), the fractures of middle width are the minority. The distributions of the fractures were mainly controlled both by the regional stress field and the activity of the faults, the fractures are displayed by belts in coincident with the strike of the main faults. Keywords: volcanic gas reservoir, reservoir fracture, structural fractures, diagenetic fractures, high angle fracture The development and exploration of volcanic gas reservoir is now the hot spot in petroleum fields. The volcanic gas reservoirs have been discovered in Songliao basin, Chaidamu basin, Sichuan basin and so on [1-2] . It is quite difficult to study the fracture characteristics of volcanic gas reservoir. This paper takes the volcanic gas reservoir of the Yingcheng Formation of Xushen area in Daqing oilfield for example to study the fracture of volcanic reservoir. The volcanic gas reservoir of the Yingcheng Formation in Xingcheng area is located in Shengping-Xingcheng structural belt of Xujiaweizi Fault-Depression in the north of Songliao basin, mainly developed in the first member of Yingcheng Formation in Lower Cretaceous Series. The buried depth of reservoir is 3000~3500m. The volcanic gas reservoir is mainly the rhyolite. The types of reservoir are mainly fracture-porosity style, which belongs to low porosity and low permeability reservoir. 1. Lithology and lithofacies characteristics The lithology and lithofacies characteristics control the development of fractures and cavity, which is the dominant factor to make the types of reservoir space and reservoir heterogeneity complicate [3-4] . By the chemical analysis of core gathered from 7 wells, the chemical composition of volcanics is mainly the SiO 2 (74.24% on average) and next is Al 2 O 3 (10.4% on average) and K 2 O+Na 2 O(8.42% on average). The figure of alkaloid-SiO 2 is made by the analysis ,which suggests that the lithology in the area is mainly acidic volcanic rock and the next is the intermediate lava. The type of rocks is mainly the rhyolite and others are the mixpah, trachydacite, dacite, andesite and necrolite. 2 SPE 131813 Figure 1 The volcanics in the first member of the Yingcheng Formation alkaloid— SiO 2 󲃕TAS figure󲃖 Most of the volcanics are central vent eruption and others are linear eruption. The volcanic cone mainly consists of lava and volcaniclastic rock. The lava mainly includes acidic rhyolit, which has a high viscosity and plays the role of matrix. The volcanic cone made of rhyolit is called layered volcano. There are five types of lithofacies in the area: volcanic conduit facies, extrusive facies, explosion facies , effusive facies and volcanic sediment. 2. Fracture characteristics of reservoirs 2.1 Fracture development types (1)Genetic classification According to the analysis of the core and thin section, the fracture of volcanic reservoir can be divided into diagenetic fracture, structural fracture, weathered fracture and solution fracture in terms of genetic. diagenetic fracture The diagenetic fracture is referred as condensate fracture formed by volume contraction of rocks due to the changes of temperature when the magma poured out of the surface and get in course of flowing and consolidation. These fractures have not a large scale and have an irregular shape. Their distribution mostly looks like reticulate, concentric circles, horsetail, broom and diffract, and don’t have fastening trending direction. The width is small and fracture width observed from the thin slice mostly is smaller than 0.1mm (Figure 2). This types of fracture usually developed in rhyolit, breccia lava and tuff, which can connect gas pore and other pore and form the good reservoir space system. Figure2 diagenetic fracture (reticular contracted microfracture in variolitic, Xushen 6 well 3870m,pyromeride) Figure 3 structural fracture (Xushen 8 well 3709.51m, rhyolitic welded Breccia) structural fracture The structural fracture is formed by the tectonic action or tectonic action. In general, structural fracture is generated by the fractures overlying each other which have different tectonic order and varied tectonic movement period. Also the manifestation is quite complicated. The structural fracture developed widely in the region with the characteristics of large fracture aperture, far-extention, large fracture angle and complicate filling materials (Figure 3). SPE 131813 3 weathered fracture The weathered fracture is formed by the surface water activity and atmospheric weathering in the course of volcanics outcropping of surface. It usually developed in the top of volcanic massif and generated by undergoing the weathering in the long term and mechanical breaking of rocks. These fractures have an irregular shape included horsetail, echelon, diffract, venation and so on. The filling materials are usually calcite or mud, which are help to develop structural fracture later and denudation (Figure 4). The weathered fractures nearly don’t exist in the core but can be observed in the thin section. solution fracture The solution fracture belongs to secondary fracture and mainly includes structural-solution fracture and diagenetic-solution fracture. The fracture has uneven width and irregular boundaries. The size of fractures is related to the scale of primary fracture. These fractures are usually partly filled with zeolite and calcite (Figure 5). Figure 4 venation weathered fracture 󲃕Xushen 5 well 3778m󲃙crystal tuff󲃖 Figure 5 the solution fracture filled with calcite󲃕Xushen 1-1 well 3413.12m󲃙rhyolitic crystal welded breccia󲃖 (2)Attitude classification The dip angle of fracture is referred as the angle between fracture and vertical plane of medial axis of core. According to the size of dip angle, it can be divided into 3 types: horizontal fracture, with the dip angle 0 -10 ; oblique fracture, 10 -60 ; high angle fracture, 60 -90 .The high angle fracture takes a large proportion in the region. (3)Fracture size classification Based on the width of the fracture, it can be divided into large fracture, middle fracture, small fracture, micro-fracture. The width of large fracture is greater than or equal to 1mm, while the middle fracture between 0.5mm and 1mm, small fracture between 0.1mm and 0.5mm, micro-fracture less than 0.1mm. 2.2 Development regularity of fracture There are 12 core wells for igneous rocks in this region, and the total length of core reaches 170.43m. 977 fractures are observed in the breccia lava and other primary lithologies. Among them, the number of structural fracture reach 767 and takes 78.5%; the number of diagenetic fracture is 166, and takes the percent of 17.0%; the number of solution fracture is 41, and takes 4.2%. It suggests that fractures in the region are mostly the structural fracture, the next is diagenetic fractures and the solution fractures is less. Among the 767 structural fractures, the number of welded tuff is 250 and takes 32.6%; the number of breccia lava is 167 and takes 21.8%; the number of crystal tuff is 578 and takes 75.4%. In terms of attitude of fracture, volcanic reservoir of No.1 Yingcheng formation mainly developed high angle fracture and oblique fracture, because structural fracture predominated in volcanic reservoir Among 977 fractures of volcanic reservoir, the large fracture reaches 414 and takes 42.4%, mostly filled with calcite, mud, magma; the middle fracture reaches 172 and takes 17.6%; the small fracture reaches 391 and takes 40%, mostly open fracture. Therefore the large fracture and small fracture developed widely in the reservoir, but the small fracture and micro-fracture make great contribution for the reservoir for availability. From the aspect of relative percentage, the lithologies of small fracture and micro-fracture developed eruptive breccia, rhyolite, welded tuff and crystal tuff. Therefore the volcanic reservoir of No.1 Yingcheng formation mainly developed high angle fracture or oblique fracture, and big fracture, small fracture and micro-facture develop widely, but the middle fracture is less. 2.3 Validity evaluation of fracture Effective fracture is referred as the fracture which has a good development, keep open, can connect the pore or gas cavity. This paper studies the filling material and period of development of fracture. 4 SPE 131813 (1)Filling material of fracture There are usually filling material in the fracture, therefore it is very important to judge the period and times of fracture forming and its validity to describe the filler. According to the number of the fillings, it is divided as following: open fracture. It is the fracture that there are not the mineralize material or cement in spaces enclosed by the wall of fracture. packed fracture. It is the fracture which is filled or blocked by cementing material or mineralizes material. The cracks in different deformation periods, the reason of system forming can be recognized by the types, stages, deformation and so on. Then present evidences for determining palaeotectonic stress field in this area. The filling condition of volcanic fracture was showed in table 1. It suggests that there are 977 volcanic fractures, 614 open fractures account for 62.8%, so the open degree of the volcanic fractures is very high. Table 1 The filling condition statistics of volcanic fracture lithology fracture number open filling number Proportion (%) number Proportion (%) Breccia lava 199 171 85.9 28 14.1 Welded breccia 46 8 17.4 38 82.6 welded tuff 325 222 68.3 103 31.7 crystal tuff 220 94 42.7 126 57.3 volcanic breccia 119 76 63.9 43 36.1 rhyolite 68 43 63.2 25 36.8 total 977 614 62.8 363 37.2 (2)The fracture period and its validity The period of fracture forming is related to the later reformation, it could present the basis and reference for the effective evaluation of fracture by the recognition and evaluation. There are many alignment plagioclase phenocryst in which a series of crossing fractures grow. These fractures are only limited in phenocryst, and they are not exist in the surrounding matrix of phenocryst. This indicates that these fractures referred as early diagenetic fracture are formed in the diagenetic process of volcanic material. Many reticular cracks are discovered in the volcanic rocks, some of them in the matrix, some cutting the phenocryst, put the matrix and phenocryst into diamond or irregular shapes. It suggests that these fractures called middle structural fracture were formed after the micro-fracture in the phenocryst. In the volcanic rock and sandstone rock, there are a group of fractures that extend very long and cut other micro-fractures. The shapes of fractures are irregular, large width, sometimes bifurcate, sometimes merging. They are results of late stress fields and lots of them are late structural fracture. Inclination angle of these fractures are large and some nearly vertical. (3)The evaluation of fracture period and its validity In general, Fractures in later structures with interface open are effective for the improvement of reservoir property and the development of reservoir; however, lots of the early fractures are filled with mineral, and maybe the invalid fracture in early development. But they maybe become the potential valid fracture of affecting the development as the reservoir developed. 2.4 The distribution law of fracture On the basis of observation of fracture of core, the study of plane distribution regulation of reservoir fracture is processed in using seismic data. Coherent body slices are made on different levels using seismic coherency data, the development degree and distribution law of fracture are predicted according to the stand or fall of coherency. The density area of fault and fracture developing shows density coherency band of bad band in coherent body slices, and, the area of non-development fault and incomplete development fracture shows uniformity area of good coherency in coherent body slices. Figure 6 is the coherent body slices; figure 7 and 8 is the map of plane distribution of total fracture density and density of open fracture in core well. It suggests that fault activities give certain effect on development of fractures. SPE 131813 5 Figure 6 the coherent body slices of No.1 Yingcheng formation Figure 7 the plane distribution of open fracture density of No.1 Yingcheng formation Figure 8 the plane distribution of total fracture density of No.1 Yingcheng formation Figure 9 Fracture density and azimuth of first phase of No.1 Yingcheng formation (from Southwest Branch Company) Seismic scattering theory and study of fractured reservoir suggested a link between attenuation of seismic wave and changes of fracture intensity in reservoir space. The attenuation of seismic wave is slower when the wave propagate along the strike of fracture than cross strike direction. Vertical fracture, diagonal fracture and network fracture could result the attenuation of seismic wave and heterogeneous distribution of seismic energy, especially when reservoir accumulate hydrocarbon. Physical property of the reservoir and the fluid cause the attenuation characteristic of anisotropic seismic wave becoming obvious, then enhanced the variety of anisotropy. Therefore, we could know the degree and azimuth of fracture development by the analysis of azimuth frequency. Figure 9 is the first phase of volcanic rock No.1 Yingcheng formation by seismic prediction(from Southwest Branch Company), the red zone indicates the high value of fracture density, and the blue one indicates the direction.It suggests that the fractures of volcanic reservoir developed along the major fault and local fault, and the direction of fractures is mainly north north west and east west. 3.Conclusions: 1.The reservoir of No.1 Yingcheng formation mainly develops the acid volcanic rocks, and the next is the intermediate lava; The type of lithology is mainly rhyolit. Most of the volcanics are central vent eruption. 2.The type of fractures are mainly structural fracture (78.5%), diagenetic fracture (17.0%) and solution fracture (4.2%) the second; The high angle fracture and vertical one predominate the structural fracture and oblique fracture next; Among the diagenetic fracture, the horizontal fracture ranks the first while the high angle fracture and oblique one the second. The fractures of reservoir mainly include large fractures (>1mm) and small fractures(1~0.1mm). The number of middle fractures is small. 3.Fractures in later structures with interface open are effective for the improvement of reservoir property and the development 6 SPE 131813 of reservoir; however, lots of the early fractures are filled with mineral, and maybe the invalid fracture in early development. But they maybe become the potential valid fracture of affecting the development as the reservoir developed. 4.The distribution of fractures is controlled by area stress field and fault activity, which looks like stripes and nearly accorded with the spreading of major fault. References 1.Zhao Wenzhi, Zou Caineng, Feng Zhiqiang, eta. Geological features and evaluation techniques of deep-seated volcanic gas reservoirs, Songliao Basin[J]. Petroleum exploration and development. 2008, 35(2):129-142. 2.Zhuang Ru-xin ,Li Feng.The Characteristics And Formations And Formation Environment Of Tanjianshan Group Volcanic Rock of North Margin of Chaidamu Basin[J]. YunNan Geology.2006,25(2):209-217 3.Hou Yingzi. Characteristics and controlling factors of volcanic reservoir spaces in Xingshan Yingshan area of Songliao Basin[J].Special Oil and Gas Reservoirs,2003,10(1):99-102. 4.Pang Yanming, Zhang Fengqi, Qiu Hongfeng, eta. Characteristics of microscopic pore structure and physical property parameter in acidic volcanic reservoir[J].Acta Petrolei Sinica.2007,28(6):72-77. . of reservoir is 3000~3500m. The volcanic gas reservoir is mainly the rhyolite. The types of reservoir are mainly fracture-porosity style, which belongs to low porosity and low permeability reservoir. . Keywords: volcanic gas reservoir, reservoir fracture, structural fractures, diagenetic fractures, high angle fracture The development and exploration of volcanic gas reservoir is now the hot. takes the volcanic gas reservoir of the Yingcheng Formation of Xushen area in Daqing oilfield for example to study the fracture of volcanic reservoir. The volcanic gas reservoir of the Yingcheng