Q值正演模拟约束下的零偏移距VSP资料Q值估算

随着油气勘探目的层逐步走向深部,球面扩散、透射损失、环境噪声等使得VSP资料的初至子波发生改变,导致常规的Q值估算方法质量难以得到保证。为此,利用塔里木盆地托甫台地区TP327井零偏移距VSP资料,综合考虑透射损失和球面扩散的影响,提出一种Q值正演模拟约束下的零偏移距VSP资料Q值估算方法。该方法通过研究区的纵波速度、横波速度和密度建立层状地质模型,利用射线追踪的方法,用大地吸收介质模型的Q值正演模拟结果,对频谱比法和经验公式估算的初始Q值进行修正,使得Q值正演模拟结果与实际地震波的衰减记录在频谱特征上吻合,从而得到更为精确的Q模型。TP327井Q值估算结果表明,该方法与李庆忠经验公式得到的结果在浅层比较接近,但在中层和深层,两者差异较大;Q值正演模拟结果约束的子波与实际VSP资料子波的频谱比较吻合,证实用Q值正演模拟约束得到的层间Q值更为可靠。利用Q值正演模拟约束方法得到的Q模型对研究区地震资料进行反Q滤波能显著提高地震资料分辨率,有助于提高碳酸盐岩地层中缝洞的识别精度。

Q Estimation From Zero-Offset VSP Data Under the Constraint of Q Forward Modeling

As the deepening of target zones for oil and gas exploration, spherical diffusion, transmission loss and environmental noise may lead to the change of VSP first arrival wavelet and then the Q value obtained from conventional estimation method may not be accurate. Therefore, based on the zero-offset VSP data of Well TP327 in Tuofutai area of Tarim basin and considering the effects of transmission loss and spherical diffusion, this paper proposes a method to estimate Q-value by using zero-offset VSP data under the constraint of Q forward modelling. By establishing a layered geological model based on P- and S-wave velocities and densities in the work area, the method uses ray tracing method and Q forward modeling results from an earth absorption model to modify the initial Q values estimated from spectral ratio method and empirical formula method, allowing Q forward modeling results matching with the actual seismic wave attenuation record on frequency spectrum. Then a more accurate Q model has been obtained. The Q estimation result in Well TP327 show that in shallow strata the result is close to that obtained from Liqingzhong’s empirical formula, but in middle and deep strata, the results obtained from the two methods are greatly different. The spectrum of the wavelet constrained by Q forward modeling result well fits with that of the actual wavelet of VSP data, proving that the Q value obtained from the method constrained by Q forward modeling is more reliable. The Q model obtained from the method can significantly improve the resolution of seismic data through performing inverse Q filtering for the seismic data in the study area, which helps to improve the identification accuracy for fractures and vugs in the carbonate strata.