(Exploration and Development Research Institute of Xinjiang Korla Tarim Oilfield Company 84 1000)
About the author: Tang Yangang, male, engineer, majoring in petroleum geological exploration. Email: tangyg-tlm @ petrochina.com.cn.
Abstract: Tight sandstone gas reservoirs mainly refer to large-scale natural gas reservoirs found in the center of the basin or distributed continuously. The discovery of this gas reservoir shows a huge amount of resources and plays an increasingly important role in the current reserve growth and energy supply. Yinan 2 gas reservoir in Kuqa Depression of Tarim Basin was discovered in the late 1990s. According to the understanding of conventional sandstone gas reservoirs, after years of exploration and evaluation, the drilling deployed in this gas reservoir was unsuccessful. The study of Yinan 2 gas reservoir in Kuqa depression of Tarim basin shows that the gas reservoir has favorable geological conditions for forming tight sandstone gas reservoirs: the structure is located in Yiqixi thrust belt of Kuqa foreland basin, which has typical foreland thrust deformation characteristics, providing a good structural background for the formation of tight sandstone gas reservoirs; At the same time, the lithology of the Jurassic Ahe Formation reservoir is dense, and the physical properties are getting better, which provides storage space for the formation of gas reservoirs. The Mesozoic Jurassic-Triassic coal measures provide rich gas sources for the formation of tight sandstone gas reservoirs. At the same time, the top of the reservoir is a set of thick mudstone sandwiched with coal seam in the middle and lower part of Jurassic Yang Xia Formation, and the bottom is thick mudstone and coal seam in Triassic, which provides an important guarantee for the preservation of tight sandstone gas reservoirs. The test data show that there is a gas-water relationship between deep gas-bearing and shallow water-bearing in the structure, the pressure in the layer where the gas reservoir is located is abnormal, the deep and shallow pressure systems are inconsistent, and the test productivity of oil and gas wells with industrial productivity before reservoir reconstruction is low, all of which show most characteristics of tight sandstone gas reservoirs. By studying the types of Yinan 2 gas reservoir, we can correctly understand the geological characteristics and reservoir-forming mechanism of tight sandstone gas reservoir, and put forward testing measures and suggestions for tight sandstone gas reservoir, which has certain guiding significance for evaluating the resource potential of tight sandstone gas reservoir as a whole and developing this kind of gas reservoir reasonably.
Key words: Tarim basin; Kuqa depression; Yinan 2 gas reservoir; Tight sandstone gas reservoir
Reservoir characteristics and exploration prospect of tight gas sandstone in Yinan 2 reservoir in Tarim basin
Tang Yangang, Lei, Ma Yujie, Yang, Chen, Li Wei
(Institute of Petroleum Exploration and Development, Tarim Oilfield Company, Korla, Xinjiang 84 1000)
Abstract: Tight sandstone gas reservoir refers to the natural gas found in the center of the basin or distributed continuously in a large area, which also indicates huge resources. The most important influence it brings is the increase of reserves and the regulation of energy. Inan 2 gas reservoir in Kuqa depression of Tarim basin was discovered in 1990. After years of exploration, it failed as a conventional sandstone gas reservoir. Through research, Yinan No.2 structure may have favorable geological conditions for forming tight sandstone gas reservoirs: Yinan No.2 structure is located on the slope of Kuqa foreland basin, with typical foreland thrust nappe deformation and good structural background; The resource reservoir of Jurassic Ahe Formation is dense, and its physical properties become better, which provides a good reservoir space for gas reservoirs. Jurassic-Triassic lacustrine environment coal measures strata provide abundant natural gas resources; At the same time, the thick coal and shale at the top and bottom of Ahe Formation provide an important guarantee for it. The test data show that the structure gas and water are inverted, with water in the shallow part and oil and gas in the deep part of the structure; The formation pressure of gas reservoir is abnormal, and the pressure system of deep and shallow layers is inconsistent; The pilot production gas output of industrial production oil and gas wells is low before stimulation treatment. All these phenomena reflect the main characteristics of tight sandstone gas. Based on the investigation of Yinan 2 gas reservoir, the reservoir-forming model and geological characteristics of the gas reservoir are verified, and the test steps and suggestions are put forward according to the understanding of tight sandstone gas. It has important theoretical guiding significance for overall rte and rational development of Inan 2 gas reservoir.
Key words: Tarim basin; Kuqa basin; Yinan 2 gas reservoir; Tight sandstone gas reservoir
1 tight sandstone gas reservoir characteristics
Tight sandstone gas reservoir refers to the gas reservoir with low porosity (generally less than 10%) and high water saturation (more than 40%) located in deep or deep basin, and the permeability can only barely make natural gas seep out. It is estimated that there are (800 ~1000 )×10/2m3 unconventional natural gas resources in the world at present, among which the tight sandstone gas resources are (75 ~100 )×10. At present, tight sandstone gas reservoirs have been discovered and utilized in the United States, Canada and other countries. From the point of view of tight sandstone gas reservoirs found at present, their geological characteristics are quite different from those of conventional gas reservoirs: (1) porosity and permeability are low, the porosity is generally between 3%- 12%, and the permeability is all below 0.0 1× 10-3μm2. (2) Secondary pores are developed, which are common in tight sandstone gas reservoirs, and there are also a few intergranular pores. There is no obvious linear relationship between porosity and permeability, and the clay content in pores is high. (3) Formation pressure change. (4) Natural gas in tight sandstone is mainly concentrated in stratigraphic traps. More commonly, multi-layer lenticular gas-bearing layers scattered in thicker intervals (such as continental strata in the Rocky Mountain Basin in the United States) are fluvial deposits. (5) accompanied by cracks, especially microcracks. In clastic rocks, with the increase of burial pressure and temperature, the diagenesis such as compaction, cementation and secondary increase of particles is strengthened, which reduces permeability and increases brittleness, thus producing cracks. (6) The maturity of gas source rocks is not high. According to foreign exploration examples, the maturity of organic matter is generally not high when gas is generated in dense layers, and the Ro is generally 1% ~ 2%. (7) The distribution of gas-water relationship is complex. In the rock series with tight gas, the upward part is water, the downward part forms gas reservoir, and there is a gas-water transition zone in the middle.
Yinan 2 gas reservoir in Kuqa Depression of Tarim Basin was discovered in the late 1990s (Figure 1). According to the understanding of conventional sandstone gas reservoirs, after years of exploration and evaluation, the drilling deployed in this gas reservoir has been unsuccessful. Through the analysis of Yinan 2 gas reservoir in Kuqa depression, it is considered that Yinan 2 gas reservoir is an unconventional gas reservoir with the characteristics of tight sandstone gas reservoir, which is located in the gentle slope of structural slope, with thick coal measures, dense reservoir lithology, inverted gas-water relationship and abnormal formation pressure in structure. The analysis of Yinan 2 gas reservoir provides a theoretical basis for evaluating the resource scale and exploration potential of Yinan 2 gas reservoir.
Characteristics of Geological Conditions of Yinan 2 Tight Gas Reservoir
Yinan No.2 structure is located in Yiqixi thrust belt in Kuqa depression, which is influenced by Yanshan and Himalayan tectonic movements and has typical foreland thrust deformation characteristics. Yiqixi fault thrusts to the surface, and a series of east-west linear anticline structures are developed on the upper wall of the fault, such as Yiqixi and Tugeming anticline structures, while a series of fault nose and anticline structures are formed on the lower wall of the fault, such as Yinan 2 fault nose structure.
Yinan 2 gas reservoir is mainly distributed in Jurassic Ahe Formation, followed by Yang Xia Formation. According to the analysis results of mid-measured natural gas, the natural gas in Ahe Formation of Jurassic is characterized by low relative density (0.6283 ~ 0.6335g/cm3) and high methane content (88.6 104% ~ 89.4456%), which is dry gas. The formation temperature is 1 16 ~ 152℃, the formation pressure is 68.59 ~ 8 1.47 MPa, and the pressure coefficient is 1.73 ~ 1.84, which is a dry gas reservoir with normal temperature and high pressure.
2. 1 Yinan 2 structure is located on the structural slope, which provides a good structural background for the formation of tight gas.
Yinan 2 gas reservoir is located in the middle of Yiqixi thrust belt, and faults are developed in this area due to the strong compressive stress in the north-south direction. Yinan No.2 structure is mainly controlled by two thrust nappe faults, Dina North fault in the south and Yinan fault in the north. Yinan fault is the main fault that controls Yinan No.2 fault nose. It is a regional northward thrust fault, with strike near NE, extension length of about 100km, fault distance of 300-400 m, fault plane in plow shape, dip angle of 30-40, disappearing downward in basement and upward in salt rock and gypsum mudstone of Neogene Jidike Formation. A series of thrust faults are developed between Yinan fault and Yiqixi fault, and the strike is basically consistent with Yinan fault, resulting in repeated strata.
Figure 1 Location Map of Yinan 2 Gas Reservoir
The main tectonic activities in Yinan area began in the late Yanshan period, and under the influence of Tianshan uplift, large-scale tectonic activities began in Himalayan period, gradually forming the structural pattern of Yinan area today. The sedimentary stage of Kuqa Formation is the peak of Himalayan orogeny and the main active period of tectonic movement in Kuqa foreland basin. In the northern part of this area, an Yiqixi fault runs through the surface, and the Yinan fault continues to move, and the upper wall of the fault is strongly squeezed. A series of thrust faults were formed, and many faults overlapped with each other, resulting in repeated strata. The fault nose structure in the footwall of Yinan fault continued to develop, and finally Yinan No.2 fault nose structure was formed.
Yinan 2 gas reservoir is located in the footwall of Yinan fault, and its structure is a fault nose inclined to the south. The structural evolution process is stable, and it has been stable during the Kangcun-Kuqa period when the exhaust gas is concentrated, which is consistent with the characteristics that tight sandstone gas reservoirs are mostly distributed in the syncline axis or structural inclination of the basin. In the depression area, sedimentation is active and subsidence speed is fast. Due to the interactive environment of continental and lacustrine facies, coal measures strata are developed, and diagenesis accelerates the formation of tight reservoirs. Later, with the rapid deepening of burial depth, it began to enter a stage of hydrocarbon generation and expulsion, and accumulated in the trap to form oil and gas reservoirs. According to several rounds of structural research, Inan 2 well encountered 264m Ahe Formation, and all logging tests explained that the gas reservoir was obviously not controlled by low-amplitude anticline.
Yinan 2 gas reservoir is located in the south of Yiqixi anticline and at the edge of Baicheng sag, the largest hydrocarbon-generating sag in Kuqa area. Its superior geographical location makes it easier to capture oil and gas during hydrocarbon generation.
2.2 The tight sandstone of Jurassic Ahe Formation in Yinan area provides a reservoir space for the formation of gas reservoirs.
Yinan 2 gas reservoir is mainly located in the sandstone of Ahe Formation in Jurassic, which is mainly braided distributary channel and underwater distributary channel sand body in braided delta, and consists of several positive rhythms from gray coarse medium sandstone or gravelly coarse medium sandstone to gray, light gray medium fine sandstone or silty sandstone. Lithology is mainly lithic sandstone, feldspathic lithic sandstone and lithic feldspathic sandstone, with composition maturity higher than structure maturity and medium roundness. The sand body of this group is thick, with few argillaceous interlayers, vertical superposition and stable lateral distribution. The reservoir is dominated by low porosity and low permeability, with a small amount of mesopore-medium permeability (Figure 2). Well Yinan 2 encountered 264m thick sandstone in Ahe Formation of Jurassic. According to logging interpretation, the gas layer is 45m/9, the poor gas layer is 43.5m/ 1 1, and the dry layer is 46.5m/ 18. The completion test proves that they are all water-free gas reservoirs.
The Jurassic Ahe Formation has dense reservoirs and developed fractures. Taking Yi 'nan Well 2 as an example, the porosity distribution range of Jurassic Ahe Formation is mainly 0.3% ~ 12.3%, with an average of 5.2%, and the permeability is mainly (0.01~ 41.2) ×10-3. The reservoir space is mostly secondary pores, and diagenesis makes the reservoir physical properties in sand body or sand body form serious heterogeneity. At the same time, the formation of secondary pores makes the sand body of Ahe Formation a low permeability layer, which can not only store oil and gas, but also serve as a shielding layer for oil and gas dissipation. Due to the existence of dense layers, the downward generated oil and gas in coal-bearing strata are captured. Moreover, due to the strong diagenetic compaction of the middle and lower Jurassic reservoirs in Yinan area, the particles are often embedded, and the residual pores are often isolated and lack connectivity, so the reservoir physical properties, especially the permeability, are very poor (often less than 1× 10-3μm2), making it a dense and ultra-low permeability reservoir. The development of fractures can greatly improve the performance of these tight reservoirs. Although the porosity increased by fractures (fractures) is limited (the absolute amount is generally less than 1%), the isolated pores are interconnected, so the reservoir permeability is greatly improved, and the permeability value is increased by orders of magnitude. At the same time, the study shows that the physical properties of the Jurassic Ahe Formation in Yinan 4 and Yishen 4 structures gradually improve from Yinan 2 structure to the north, which provides favorable storage conditions for the formation of tight sandstone gas reservoirs with water above and gas below.
Distribution of regional caprock. The top of the Jurassic Ahe Formation reservoir is a set of coal-bearing mudstone in the middle and lower part of Yang Xia Formation, with the thickness of 1 10 ~ 120 m, and the bottom is Triassic thick mudstone and coal seam, which provides an important guarantee for the preservation of tight sandstone gas reservoirs.
Fig. 2 Correlation map of Jurassic Ahe Formation in Yinan area
2.3 Mesozoic Jurassic-Triassic coal measures strata provided abundant gas sources for the formation of tight sandstone gas reservoirs.
The bottom of Yinan 2 gas reservoir is a set of Triassic lacustrine coal measures strata with large distribution area, which can be used as sufficient gas source. The distribution characteristics of steroidal terpenoids in oil sands of Yang Xia Formation and crude oil of Ahe Formation in Well Yinan 2 are more similar to those of Triassic source rocks.
The effective source rocks in the eastern part of Kuqa Depression include dark mudstone and coal measures of Karamay Formation (T2-3k), Tariq Formation (T3t) and Kakemake Formation (J2q), with a maximum cumulative thickness of about 600m and high organic matter abundance, which generally meets the standard of medium to good source rocks. TOC of Triassic dark mudstone in Well Yinan 2 is 0.45% ~ 24.43%, with an average of 2.95%. The TOC of Jurassic dark mudstone is 0.28% ~ 16.35%, with an average of 4.3 1%. The type of organic matter dominated by terrestrial higher plants is mainly type ⅲ, followed by type ⅱ 2. Since Neogene, the source rocks of Upper Triassic and Middle-Lower Jurassic have gradually matured to generate hydrocarbons (Ro value is 0.6% ~ 1.88%), and the evolution degree of organic matter is relatively high, with the Triassic Ro of Well Yinan 2 being 1.32% ~ 1.43% and the Jurassic Ro being 0.78. According to the study on the evolution history of source rocks in the east of Kuqa, the oil displacement peak of Triassic source rocks is about 23 ~ 12~5Ma, and that of Jurassic source rocks is about 12~5Ma. The hydrocarbon expulsion peak of Triassic and Jurassic source rocks is in Kangcun-Kuqa period, so the early traps mainly capture Triassic crude oil, and the late traps are filled with natural gas, mainly in Yinan 2 gas reservoir.
2.4 According to the characteristics of deep gas-bearing and shallow water-bearing in the southern region, the whole structure reflects the phenomenon of gas-water inversion.
Through reviewing the old wells in Yinan area, especially analyzing the reasons for water testing and reinterpreting logging, it is clear that the same reservoir in Yinan area and the Jurassic Ahe Formation are full of gas reservoirs in the structural downtilt position (Yinan 2 well), but in the structural updraft direction (Yinan 4 well and Yishen 4 well), it gradually becomes a gas-water transition zone, and the gas-water inversion is obvious. From the bottom up, the gas-bearing layer gradually becomes a gas-water transition zone until the water layer (Figure 3).
Fig. 3 Model diagram of Yinan 2 gas reservoir
During the test, a large number of water sample analysis data were obtained from Well Yinan 2, Well Yinan 2C, Well Yinan 5, Well Yinan 4 and Well Yishen 4 in Akuo or Yang Xia Formation of Jurassic. The logging interpretation and water sample data analysis results show that:
The production of Well Yinan-2 in the water analysis layer of Jurassic Ahe Formation is low, in which the water produced in the 4776~4785m test section (2 1.4m3/d) and the 4905 ~ 49 13m test section (22. 14m3/d) are all Triassic water. The color is black, yellow and brown (the formation water is colorless or the color polluted by drilling fluid is light), the chloride ion value is low, ranging from 2158.95438+0 ~18062.84 mg/L, and the total salinity is 6067.11.
The water quality analysis layer of Well Yinan 4 in Ahe Formation of Jurassic is gas-bearing water layer and dry layer, with chloride of 9227.2 1 ~ 32742.04 mg/L and total salinity of16860.89 ~ 61579.26 mg/L. The water quality analysis is all NaHCO3. Well Yishen-4 tested water in 4072 ~ 4093 m and 4 147 ~ 4 169 m sections of Jurassic Ahe Formation, respectively, and the production was not high. The chlorine content in the water samples of Jurassic Yang Xia Formation-Ahe Formation is 6393.83 ~12673.52 mg/L, and the total salinity is14015.13 ~ 25410.83 mg/L.
2.5 The abnormal formation pressure of Jurassic Ahe Formation in Yinan area also shows the characteristics of tight sandstone gas reservoir.
The gas pressure of tight sandstone gas reservoir is always lower than or higher than the regional hydrostatic pressure, showing an abnormal pressure state, and its position and range are consistent with that of Jurassic Ahe Formation gas accumulation, which also reflects the characteristics of oil and gas migration and accumulation in the later period of Inan 2 gas reservoir and still in the process of dynamic adjustment.
Based on the study and analysis of the pressure data of Jurassic test interval in Yinan 2 gas reservoir, it is considered that Yinan 2, 2C and 5 wells belong to the same pressure system affected by buried depth, reflecting the abnormal high pressure environment. Well Yishen-4 and Well Yinan-4 are in a high-pressure environment, and the pressure coefficient of underground Jurassic Yang Xia Formation-Ahe Formation gradually increases from north to south, showing the regular characteristics of low in the north and high in the south (Figures 4 and 5).
Fig. 4 Jurassic pressure distribution map in Yinan area
Fig. 5 Distribution map of Jurassic pressure coefficient in Yinan area
3 Conclusions and suggestions
Through the comprehensive analysis and study of deep tight sandstone gas reservoirs in Yinan, the following understandings are obtained:
Inan 2 structure (1) in Kuqa depression is located in the structural slope area, with high hydrocarbon generation intensity and dense Jurassic reservoir, accompanied by gas-water inversion and abnormal gas reservoir pressure, which shows obvious characteristics of tight sandstone gas reservoir.
(2) If the boundary of tight sandstone gas reservoir is not controlled by structure and other factors, and all parts with downward dip of the structure contain gas, the southern part of the gas reservoir can reach the vicinity of Dina North fault, and the conservatively estimated resources can reach 4200× 108m3, which has great exploration potential.
(3) According to the understanding of tight sandstone gas reservoir, it is suggested that the exploration well Xidi 1 which has evaluation significance for this gas reservoir should be mainly fracturing, using water-based fracturing fluid, adding soil stabilizer and nonionic surfactant, and combining with super-large hydraulic fracturing; At the same time, combined with the development characteristics of tight sandstone gas reservoirs abroad, it is suggested to deploy and collect three-dimensional earthquakes in this well area to accurately understand the heterogeneity and fracture distribution of Yinan 2 structure and its reservoirs, so as to lay a solid foundation for effectively improving the success rate of development wells.
refer to
[1] masters J.A. Deep basin gas reservoir in western Canada [J].AAPG,1979,63 (2):152-181.
Reeves J. Promoting 3D seismic interpretation method to find the best location of tight gas reservoirs [J]. Journal of the Association of Exploration Geophysicists, 2006,25.
[3] Michael mccracken, Dale Fitz, Tom Ryan. Tight gas monitoring and characterization: the impact of production logging. Keystone, Colorado, USA, 2008(2).
[4]M. M. Abu-Shanab, G. M. Hamada et al., improved porosity estimation of tight gas reservoirs according to nuclear magnetic resonance and density logging. SCA International Symposium, TE53/30/ 2005.
Li Jian, Zeng Dagan, Chen, et al. Exploration and development technology of deep tight sandstone gas reservoir. Beijing: Petroleum Industry Press, 12 ~ 37.
He zhiliang. Oil and gas exploration field of continental non-structural traps in China [J]. Petroleum Experimental Geology.2004,26 (2):194 ~199.
Xue, Pang Xiongqi, and Wang Dehua. Review on the research status of deep basin gas. Progress in Earth Science, 2000, 15 (3): 289 ~ 292.
Niu Baorong, Li Runmei, Yu. Characteristics and key technology prospect of tight sandstone gas reservoir. Tuha oil and gas, 20 10, 15 (2): 300 ~ 307.
Zhang Yundong, Xue, Zhu Rukai, et al. Exploration status of subtle oil and gas reservoirs at home and abroad [J]. China Petroleum Exploration, 2005, 10 (3): 64 ~ 67.
[10] Zhang Shaonan. Genesis and discussion of tight sandstone gas reservoirs [J]. Petroleum and Natural Gas Geology, 2008,29 (1):1~10.
Jeikiy. Study on the Controlling Effect of Diagenesis on the Reservoirability of Tight Sandstone Reservoirs —— Taking the gas reservoir of Shangshaximiao Formation in Xinchang Gas Field in western Sichuan as an example [J]. Journal of Chengdu University of Technology,1999,26 (2):157 ~160.
Tang Haifa, Ren Peng, Zhao Yanchao, et al. Analysis of main controlling factors of tight sandstone reservoir properties [J]. Journal of Youshi University, 2007,22 (10): 59 ~ 63.
Shou Jianfeng, Si, Wang, Pi, et al. Controlling factors of physical properties of lower Jurassic sandstone reservoirs in Kuche Depression, Tarim Basin. Geological review, 200 1, 47 (3): 272 ~ 277.
Chen Zikai, Shou Jianfeng, Zhang, Shen Anjiang, et al. Microscopic heterogeneity of Lower Jurassic reservoirs in Kuqa Depression. Petroleum Geology of Xinjiang, 200 1, 22 (5): 405 ~ 407.
[15] Ginger, Lin, Pang,. Comparison of two tight sandstone gas reservoirs. Petroleum experimental geology, 2006,28 (3): 219 ~ 221.
[16] Gong translated tight gas reservoir geology [M]. Harbin: Heilongjiang Science and Technology Press, 199 1, 57 ~ 93.
[17] horsepower. Study on real productivity characteristics of gas wells in ultra-low permeability sandstone gas reservoirs. Science and technology consultation.2010,26:119
[18], Tang Hongming, Xie, Ye Yunan, Damage evaluation of water-based underbalanced drilling in tight sandstone gas reservoir. Drilling engineering. 28, 2008 (12): 71~ 73.
[19] Fu Yongqiang, He. Application of reservoir reconstruction technology in Xujiahe tight gas reservoir in Jie Bao area. Natural gas technology. 2007, 1 (6): 17 ~ 20.
Yang Jian, Kang Yili, Li Gangui, Zhang Hao. Microstructure and seepage characteristics of tight sandstone gas reservoir. Mechanical progress. 2008,38 (2): 229 ~ 236.