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

古中央隆起带基底上覆地层缺失断陷期沉积地层，基岩暴露地表时间长，风化壳普遍发育。基底岩性复杂，主要发育花岗岩、浅变质沉积岩及浅变质火山岩3大类岩石；基底主要发育花岗岩、变质岩储层，不同类型储层的裂缝发育程度不同。钻井揭示：花岗岩的脆性大，易形成裂缝，花岗岩储层物性最好，属于孔隙—裂缝型储层；基底花岗岩风化壳储层非均质性强，储层物性较差，以裂缝为主。目前有关基底花岗岩和变质岩的岩石物理弹性参数特征的实验室研究较少。为此，采用超声频段和地震频段弹性参数测试方法，系统分析了围压及孔隙压力对岩石纵、横波速度的影响，研究了不同孔隙度和饱和状态下岩石弹性参数与微裂缝密度及孔隙纵横比的关系；应用弹性参数交会分析，明确了基底风化壳储层及裂缝敏感参数，建立了符合基底孔隙—裂缝型储层特征的岩石物理模型，获得以下成果和认识：(1)对松辽盆地北部古中央隆起带基底33块岩性样品，采用超声频段和地震频段实验测试，测得的标准铝块的超声频段纵波速度(6430 m/s)与参考值(6400 m/s)基本相同，相对误差为0.5%,精度较高。(2)随着围压增加，在水饱和和气饱和状态下纵、横波速度均增大，且横波速度变化不大。随...

Testing and characteristic analysis of elastic parameters of basement rock in the paleo-central uplift belt in the northern Songliao Basin

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 (v_P/v_S), 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.