1. School of Geophysics and Spatial Information, China Geo University, Wuhan, Hubei 430074; 2. Key Laboratory of Structure and Oil and Gas Resources, Ministry of Education, China Geo University, Wuhan, Hubei 430074.
In order to study the physical properties of various sedimentary environments in the sedimentary system, the sedimentary system of carbonate platform margin and clastic coastal zone in Keping-Bachu outcrop area of Tarim Basin was systematically sampled. The velocity and density of ultrasonic longitudinal wave and shear wave were measured at normal temperature and pressure. The main conclusions are as follows: ① The ultrasonic velocity of rock samples is closely related to the sedimentary environment in which rock samples are located. On the profile of biological reef beach, the velocity from reef bottom, reef core to reef cover increases (equivalent to shallow sea deposition on the platform edge); On the delta sedimentary profile, the velocity of silt increases from the underwater distributary channel and estuary dam to the front edge; ② On the reef profile, the bioclastic content is the main factor affecting the velocity. The higher the bioclastic content, the lower the speed; ③ The size and growth direction of organisms in the reef are one of the main factors to control the velocity anisotropy of rock samples.
Keywords ultrasonic velocity, reef delta, Lower Paleozoic in Tarim Basin
1 Introduction
The physical basis of seismic exploration is the difference of physical parameters, and it is also an important reference for geologists and geophysicists to identify seismic facies and sedimentary facies from seismic data, in which velocity is the most critical physical parameter in seismic data. The more direct method to study rock physical properties is logging or rock physical properties measurement technology. In this chapter, the petrophysical properties of carbonate platform margin sedimentary system and clastic coastal sedimentary system in Keping-Bachu outcrop area of Tarim Basin are measured by indoor physical property measurement method, in order to find the variation law of acoustic velocity and density of each sedimentary facies and provide lithological basis and scientific clues for the identification and prediction of this kind of underground reservoirs.
Rock ultrasonic testing results are widely used in engineering geological exploration and petroleum exploration. Some studies show that rock lithology can be judged by density, P-wave velocity ratio or Poisson ratio (Meng Qingshan, 2005), and some people directly study the acoustic attenuation characteristics of sedimentary rocks themselves (Mou Yongguang, Fang, 2006). Ultrasonic measurement has become an indispensable means in the study of rock physical properties, but few people have conducted in-depth and systematic research on the relationship between sedimentary environment (sedimentary facies) and sedimentary rock physical properties in previous studies.
The outcrop profile records abundant sedimentological information. Detailed ultrasonic research on outcrop sedimentary system, summarizing and comparing the sound velocity characteristics of rocks corresponding to various environments in sedimentary system with potential reservoirs, can accurately guide geological modeling and geophysical forward simulation of sedimentary system, and serve as a constraint of geophysical inversion, which is conducive to improving the interpretation accuracy and prediction accuracy of seismic favorable reservoir facies zones.
Both Ordovician and Silurian are important targets for oil and gas exploration and development in Tarim Basin (Pi, liuchu, Chen Ying, etc. , 2007; Zhang Jun, Pang and Liu Luofu, 2003). Ordovician reservoir lithology in Tarim basin is mainly platform beach limestone and reef (mound) limestone (Luoping, Zhangxingyang, Gujiayu, etc. , 2003). In recent 10 years, Bachu, Hashike, Keping and Lunnan reefs in Tarim Basin have been discovered and confirmed successively (Li Xiangming and Yang Shengu, 2006; Lu, He Gong, 2007). Silurian reservoirs in Tarim Basin are mainly composed of littoral and shallow marine clastic sediments in Keping and Tabei areas, tidal flat sediments in the mouth of Tazhong area and coarse clastic sediments in the continental river-braided river delta in Tadong area (,,,,,2007). In this study, the typical profiles of carbonate platform margin sedimentary system and clastic coastal sedimentary system in Keping-Bachu area of Tarim Basin were systematically sampled, and the ultrasonic velocity of rocks was measured indoors, and the velocity variation characteristics of rocks in each sedimentary system were explored.
2 Collection and description of rock samples
2. 1 section position
The rock samples used in the test are from four typical sections of Tarim Basin (Figure 1). The first and second sections are located in Tage Mountain, Leya Yili, Bachu, belonging to the Ordovician Yijiafang Formation (O2y), which is a reef-beach symbiotic facies on the platform edge. Rock samples are taken from the reef base, reef core and reef cover (equivalent to shallow sea deposits on the platform edge). The third member, located in Dawangou, Keping area, belongs to the Silurian Tage Formation (S 1t) in tatar, and is a delta front deposit. Rock samples are taken from the genetic facies such as estuary dam, underwater distributary channel and front mud. The fourth member, located in Sishichang, Keping area, belongs to the middle-upper part of Silurian Kepingtage Formation (S 1k) and is tidal flat deposit. The specific field work route is shown in figure 1.
six
2.2 rock sample description
A total of 25 rock samples were selected from 4 sections for ultrasonic testing, and the rock samples were cut into cuboids, and the surface to be tested was polished with sandpaper (Figure 2). Because the original shape of some rock samples is extremely irregular, only one short axis and one long axis can meet the measurement requirements when cutting. The length of the minor axis (a) is 0.05m, and the length of the major axis (b) varies from 0.06m to 0. 12m (table 1).
Fig. 2 is a photograph (5cm×5cm×9.7cm) of No.25 rock sample of Silurian Tage Formation (S1T) in Dawangou, tatar.
3 experimental methods
3. 1 experimental equipment
The instrument used to measure the sound speed is the RSM-SY5 intelligent sound wave detector developed and produced by Wuhan Institute of Geotechnical Mechanics, Chinese Academy of Sciences, with a time resolution of 0.1μ s. Two ultrasonic transducers are used. One is the longitudinal wave transducer, which is developed and produced by Jianghan Logging Research Institute, and the receiving frequency is 50kHz. The second type is shear wave transducer, which is developed and produced by Wuhan University of Technology. The receiving frequency is (90 10) kHz.
Basic principle of ultrasonic sound velocity measurement: The sound velocity measurement system of rock samples is shown in Figure 3. In the process of measurement, the electrical signal sent by the ultrasonic instrument is converted into sound waves by the probe A (transmitting transducer), transmitted to the probe B (receiving transducer) through the rock sample, and then converted into electrical signals and transmitted to the acoustic instrument. Then read out the propagation time t' (the first arrival time of the waveform, as shown in Figure 4) of the sound wave in the rock from the computer, and remove the additional delay time of the sound wave passing through the probe, the coupling material (the coupling agent between the probe and the rock sample) and the instrument circuit-zero-zero t0, and the propagation time of the sound wave in the rock is t=t '-t0. If the length of the rock sample is l, the wave velocity V=L/t can be calculated. The whole measurement process is carried out at normal temperature and pressure.
Table 1 Ultrasonic Velocity Test Results of Rock Samples
Fig. 3 RSM-SY5 ultrasonic measurement and analysis system
3.2 waveform detection method
According to the literature (Wang Rangjia, 1997), in the longitudinal wave velocity test, using liquid or emulsion as coupling agent can achieve good coupling effect. Shear waves are shear vibrations, and only materials that can withstand shear forces can be used as coupling agents for shear wave velocity testing. In this experiment, the coupling agent used for longitudinal wave velocity measurement is dextrin, and the coupling agent used for transverse wave velocity measurement is phenyl salicylate. There is a difference between the velocities of P-wave and S-wave. When the S-wave lags behind the P-wave, it is difficult to pick it up (Wei Jianxing and Wang Chunyong, 2003). However, the S-wave has a certain polarization, and the amplitude of the received S-wave will change regularly when the corresponding angles of the transmitting transducer and the receiving transducer are rotated. Using this characteristic, S wave can be identified and the first break time can be determined. In Figure 4, shear wave (a) is the waveform received by the shear wave transducer when testing the 25th rock sample, and shear wave (b) is the waveform received after the receiving transducer rotates180, and the amplitude of the shear wave head wave is opposite, so the first arrival time of shear wave can be clearly identified in Figure 4.
Fig. 4 The arrow of acoustic wave waveform displayed during the test of No.25 rock sample points to the first arrival time of P-wave and S-wave.
4 experimental results and analysis methods
See table 1 for the acoustic wave test results of rock samples. The velocity measurement is carried out along the short axis (a) direction and the long axis (b) direction of the rock sample shown in Figure 2. VP(a) and VS(a) represent P-wave and S-wave velocities measured along the short axis (a) respectively; VP(b) and VS(b) represent the longitudinal and shear wave velocities measured along the long axis (b), respectively. In order to estimate the velocity anisotropy of rock samples, P-wave velocity anisotropy index KP and S-wave velocity anisotropy index KS are introduced, which are defined as follows:
Study on outcrop of sedimentary system and reservoir modeling in carbonate platform edge area
Study on outcrop of sedimentary system and reservoir modeling in carbonate platform edge area
5 Discussion of measurement results
5. Velocity characteristics of rock samples in1reef beach profile
In profiles ① and ② of biological reef beach (Figure 1), a single reef is small in scale, but there are many reefs, most of which are connected together. The distribution of reefs is stable, and the lateral extension direction can be traced to other reefs in the corresponding horizon, and the reefs overlap each other in the vertical direction. Reef is generally composed of reef core, reef bottom and reef cover (Hu, Zhu Zhongde, He Ping. , 2002). There are 15 rock samples in the profile of biological reef beach for ultrasonic detection. According to the different positions of the rock sample in the reef, the relationship between the position of the rock sample and its P-wave, S-wave velocity and average velocity is drawn (Figures 5 and 6).
Relationship between P-wave velocity of rock sample and its position in reef.
Relationship between shear wave velocity and rock sample position in reef profile.
As can be seen from Figures 5 and 6, the average values of P-wave velocity and S-wave velocity gradually increase from the bottom of the reef, the core of the reef to the cover of the reef. The increase of longitudinal wave velocity is greater than that of shear wave velocity. The measured velocity values of reef cover rock samples have little change, but the velocity values of reef base and reef core rock samples in two axial directions are obviously different. Fig. 7 is a crossplot made with the previously defined velocity anisotropy indices KS and KP. It can be seen from Figure 7 that the velocity anisotropy index of the rock samples in the reef cover is basically concentrated in the range of 0% ~ 10%, while the rock samples in the reef base and core are mostly distributed in the range of 20% ~ 40%. The velocity anisotropy of reef bottom and core is obviously higher than that of reef cover.
Crossplot of shear wave velocity anisotropy index KS and longitudinal wave velocity anisotropy index KP of biological reef beach profile.
By observing the profiles ① and ② of the reef beach, it can be seen that most of the reefs are gray coarse-grained spark limestone, with high particle content, accounting for more than 80%, and the particle size is about 1 ~ 4 mm, mainly the broken stems of sea lilies, as shown in Figure 8b. The reef core is mainly composed of Calathium (Hu, Zhu Zhongde, He Ping. , 2002; Li Xiangming and Yang Shengu, 2006; Jiao Yangquan, Rong Hui, Wang Rui, etc. (20 1 1) is composed of gray blocky barrier rocks, with dense crochets of reef-building organisms, accounting for more than 80% of the total fossils, and the crochet length can reach 10c m, as shown in Figure 8a. The cover rocks of biological reefs are mostly middle-layer bioclastic micrite limestone with good bedding, often mixed with small pieces of reef limestone, and the structure is compact. Rock samples on biological debris such as reef base and lily stem are arranged in disorder and biological particles are loose; For reef core rock samples, the body cavity of crocidolite is filled or dissolved by calcite, and some rock samples split along the organism to form large fractures. The shape, size, growth direction and cracks of these fossils all affect the propagation speed of sound waves in rock samples.
Figure 8. Basket (a) is the main reef-building organism of reef core, and stem (b) is the main component of reef-based biological debris.
5.2 Velocity characteristics of rock samples in tidal flat sedimentary profile and delta front sedimentary profile
The Silurian in the Keping-Bachu area consists of the Keping Tage Formation, the tatar Tage Formation and the Yimugantawu Formation (,,Zhang,) from bottom to top. , 2007). Four rock samples used for ultrasonic testing in section ④ are all taken from asphalt sandstone section of Kepingtage Formation (Wu Liqun, Jiao Yangquan, Rong Hui, 20 1 1), belonging to tidal flat system (Table 1). Six rock samples used for ultrasonic testing in Section ③ are taken from Herta Formation S 1t in Tata, and belong to the genetic facies such as delta front mud, estuary dam and underwater distributary channel (Table 1). The two groups of rock samples have a certain chronological relationship and a certain correlation in sedimentation. Put this 10 rock sample together, and draw the relationship between different sedimentary systems and their P-wave and S-wave velocities according to their different sedimentary environments (Figure 9, Figure 10).
Fig. 9 Relationship between P-wave velocities of rock samples in tidal flat system and delta front system.
Figure 10 Relationship between Shear Wave Velocity of Rock Samples in Tidal Flat System and Delta Front System
It can be seen from Figure 9 and Figure 10 that the P-wave velocity and S-wave velocity of four sandstone blocks in tidal flat system are relatively stable, about 4000m/S and 2500m/S respectively, while the velocities of six sandstone samples taken from various genetic facies in the delta front are obviously different, and the velocity of underwater distributary channel sandstone samples is the lowest, and mudstone No.29 is not considered due to fracturing. See figure11(intersection of ks and KP) for the degree of velocity anisotropy in two different sedimentary environments. In the figure 1 1, the anisotropy index of four rock samples of tidal flat system is basically concentrated in the range of 0 ~ 10%, and the two rock samples at the edge of 10% are No.20 and No.21. Rock samples with anisotropy index of delta front system exceeding 10% belong to underwater distributary channel.
Figure 1 1 Crossplot of shear wave velocity anisotropy index KS and longitudinal wave velocity anisotropy index KP of tidal flat system and delta front system.
According to the photos of rock samples (Figure 12), all four rock samples of tidal flat system are immersed in oil. Among them, the No.20 rock sample has traces of oil immersion, but the intergranular pores are not filled with asphalt. Rock sample 2 1 contains a lot of biological debris, and the intergranular pores of No.22 and No.23 are almost completely filled with asphalt, and the rock mass is black. Among the six rock samples in the delta front, the sand body of underwater distributary channel facies has coarse grain size and developed pore structure, while the rock samples near the front mud have fine and dense grain size. Combined with the characteristics of sediments, the grain size of sediments in the delta front gradually decreases from shore to lake, that is, from underwater distributary channel and estuary dam to front mud (table 1), while the pores affecting sound wave propagation become smaller and smaller, and the sound speed gradually increases.
6 Conclusion and discussion
Through the ultrasonic velocity test experiment of rock samples in the above outcrop sedimentary system, the following points can be obtained:
The ultrasonic velocity of 1) rock sample is closely related to the sedimentary environment (sedimentary system or genetic facies) where the rock sample is located, showing a certain change law. On the profile of biological reef beach, the velocity increases from reef bottom, reef core to reef cover; On the sedimentary profile of the delta front, the velocity of mud increases from the underwater distributary channel and estuary dam to the front. It is feasible to use the relationship between rock acoustic velocity measurement results and sedimentary environment and its changing law to guide the modeling of sedimentary system.
Figure 12 Fresh surface photos of tidal flat facies profile and delta profile after cutting.
No.20 ~ 23 belong to tidal flat facies profile, and No.24 ~ 29 belong to delta front sedimentary profile.
2) On the reef profile, the content of biological debris is the main factor affecting the sound wave velocity. The higher the bioclastic content, the lower the speed; On sandstone profile, porosity is the main factor affecting measurement. The smaller the porosity or the higher the filling degree, the higher the flow rate.
3) On the reef profile, the size and growth direction of organisms are one of the main factors to control the velocity anisotropy of rock samples, while the sandstone profile is considered to be closely related to pores. The anisotropy of reef limestone velocity is greater than that of sandstone.
In the process of collecting rock samples in the field, we got help from other students from Wang Rui, Wang Shihu, Rong Hui and China Geo University. In addition, Cai Lan, a teacher from Wuhan University of Technology, also gave guidance on shear wave measurement, and I would like to express my heartfelt thanks.
refer to
, Mou Yongguang, Fang. 2006. Relationship between velocity and attenuation of sedimentary rocks and physical properties of rocks. Petroleum Geophysical Exploration, 41(2):187 ~192.
Hu, Zhu Zhongde, He Ping, Zhang Jun,. 2002. Ordovician reef reservoir characteristics in Lunnan-Bachu area. Petroleum and natural gas geology, 23 (2): 179 ~ 182.
Jiao Yangquan, Rong Hui, Wang Rui, Wu Liqun, Yan Jiaxin, Zeng Fanping, Gu Yuan and Li Rong. Analysis of sedimentary system of Ordovician platform margin reservoir in an outcrop area in western Tarim Basin. Journal of Petrology, 27 (0 1): 285 ~ 296.
Li Xiangming, Yang deep valley. Biological characteristics of Middle Ordovician reefs in Bachu area in 2006. Journal of Petroleum and Natural Gas, 28 (4): 25 ~ 27
Li Xiangming, Yang deep valley. 2006. Characteristics of Fang Yi Formation Reef in Bachu Area, Tarim Basin. Journal of Petroleum and Natural Gas (Journal of Jianghan Petroleum Institute), 28 (3): 13 ~ 16.
Lu, Gong. 2007. Research status, problems and prospects of reefs in offshore oil and gas areas. Journal of Earth Sciences-China Geo University, 32 (6): 87 1 ~ 878.
Luo Ping, Zhang Xingyang, Gu Jiayu, Wang, Lin Peng. Geophysical characteristics of outcrops of Ordovician reefs in Tarim Basin in 2003. Acta sedimentosa, 2 1 (3): 423 ~ 427.
Meng Qingshan, Wang Min. 2005. Study on acoustic characteristics of carbonate rocks and its application. Karst in China, Volume 24, No.4: 344 ~ 348 pages.
Pi, liuchu, Chen Ying, Sun Zuoyu, Hao Weicheng. 2007. Discussion on chemical stratigraphic characteristics from the top of Ordovician to the bottom of Silurian in Dawangou, Keping, Xinjiang. Journal of Peking University (Natural Science Edition), 43 (2): 183 ~ 189.
Wu Liqun, Jiao Yangquan, Rong Hui. 20 1 1. Sedimentary characteristics of asphalt sandstone of Silurian Kepingtage Formation in Sishichang section of western Tarim Basin. Modern Geology, 25 (1): 48 ~ 54.
, Zhang,,, yang shen. 2007. Division and correlation of Silurian in Tarim Basin and its geological significance. Petroleum Geology of Xinjiang, 28 (2): 185 ~ 188.
Wang Rangjia. 1997. Classification of acoustic rocks and analysis of dynamic elastic mechanical parameters of rocks. Beijing: Geological Publishing House.
Wei Jianxing, Wang Chunyong. 2003. Laboratory research on shear wave testing technology. Petroleum Geophysical Exploration, 38 (6): 630 ~ 635.
Zhang Jun, Pang, Liu Luofu, Jiang, Liu. 2004. Distribution characteristics of Silurian asphalt sandstone in Tarim Basin and its petroleum geological significance. China Science Series D: Earth Science, 34 (Supplement 1): 169 ~ 176.