Testing and characteristic analysis of elastic parameters of basement rock in the paleo-central uplift belt in the northern Songliao Basin
DAI Shili1, WANG Lijuan1, XIN Zhaokun2
1. Research Institute of Exploration and Development, Daqing Oilfield Company, PetroChina, Daqing, Heilongjiang 163712, China; 2. PetroChina Daqing Geophysical Research Institute of GRI, BGP, Daqing, Heilongjiang 163453, China
Abstract：The overlying strata of the basement in the paleo-central uplift belt lack sedimentary strata in the fault depression period, and the bedrock has been exposed to the surface for a long time, with the crust of weathering developing widespread. The basement lithology is complex, and three major types of rocks, namely, granites, shallow metamorphic sedimentary rocks, and shallow metamorphic volcanic rocks, are developed. The basement mainly develops granite and metamorphic rock reservoirs, and the fracture development degree of different reservoirs varies. Well drilling reveals that granite is brittle and easy to form fractures. The Granite reservoir has the best physical property and belongs to the pore-fracture reservoir. The reservoir of the crust of weathering of granites in the basement has strong heterogeneity and poor physical property and is dominated by fractures. At present, there are few laboratory studies on the characteristics of petrophysical elastic parameters of granites in the basement and metamorphic rocks. Therefore, the elastic parameter testing methods of ultrasonic and seismic frequency bands are used to systematically analyze the effects of confining pressure and pore pressure on rock's P-wave and S-wave velocities and study the relationship between elastic parameters of the rock and micro-fracture density, as well as pore's aspect ratio under different porosity and saturation conditions. According to the cross-analysis of elastic parameters, the sensitive parameters of the reservoir and fractures of the crust of weathering in the basement are defined, and a petrophysical model conforming to the characteristics of the pore-fracture reservoir in the basement is established. The following understandings are obtained:① The ultrasonic and seismic frequency bands are used to test the 33 lithologic samples from the basement in the paleo-centric uplift belt in the northern Songliao Basin. The measured P-wave velocity (6430 m/s) of the standard aluminum block by ultraso-nic frequency band is basically the same as the reference value (6400 m/s), and the relative error is 0.5%, which indicates a high accuracy. ② With the increase in confining pressure, both P-wave and S-wave velocities increase under water and gas saturation conditions, and S-wave velocity changes slightly. With the increase in pore pressure, the P-wave and S-wave velocities decrease under water and gas saturation conditions. In addition, the variation of P-wave velocity under gas saturation is greater than that under water saturation, and the decreasing trend of S-wave velocity under water and gas saturation conditions is similar. ③ The velocity difference between the ultrasonic frequency band and the seismic frequency band at different pressures indicates that the micro-fissure is the main factor affecting the absolute variation of the dispersion. With the increase in the pressure, the micro-fissure gradually closes, the fluid flow effect between pores weakens, and the injection effect decreases. ④ The optimal sensitive parameters of reservoirs by seismic and ultrasonic frequency bands are the same. In other words, parameters such as the ratio of P-wave and S-wave velocities (vP/vS), Poisson's ratio (ν), Lame coefficient (λ), ratio of Lame coefficient and shear modulus (λ/μ), and product of Lame coefficient and density (λρ) are the most sensitive to the fluid and can be used as sensitive parameters of the reservoir to identify the crust of weathering in the basement. ⑤ A saturation-porosity interpretation chart is established when the fracture density is three fractures per meter, and the fracture dip angle is 50°. In addition, a fracture density-porosity interpretation chart is established when the water saturation is 50%, and the fracture dip angle is 50°, which lays a theoretical foundation for predicting pore-fracture reservoirs of the crust of weathering.
戴世立, 王力娟, 辛朝坤. 松辽盆地北部古中央隆起带基底岩石弹性参数 测试及特征分析[J]. 石油地球物理勘探, 2023, 58(2): 443-453.
DAI Shili, WANG Lijuan, XIN Zhaokun. Testing and characteristic analysis of elastic parameters of basement rock in the paleo-central uplift belt in the northern Songliao Basin. Oil Geophysical Prospecting, 2023, 58(2): 443-453.
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