流花碳酸盐岩储层“低频阴影”检测分析

“Low frequency shadow” detection and analysis of the carbonate reservoir in Pearl River Mouth Basin

南海北部东沙隆起区广泛发育新生代碳酸盐岩台地,台地上原地生长的生物礁由于孔隙、裂缝的发育加之上覆泥岩盖层的阻挡,形成了非常好的油气储层。文章结合测井资料,利用谱分解方法分析了流花碳酸盐岩台地高孔隙度储层的地震时频特征,发现在孔隙极为发育的储层顶部地震波能量出现明显衰减,时频谱上表现为中心频率显著降低;但在分频剖面上,除了垮塌溶蚀层段可见低频强能量区外,未能在储层位置下方检测到表征能量衰减的"低频阴影"现象。研究结果认为:储层厚度较薄、碳酸盐岩平均速度较高、储层非含气高密度流体(油和水)的存在,是导致未检测到"低频阴影"的3个主要原因。其中,较薄的储层和速度较高的碳酸盐岩会引起衰减不足,而非含气高密度流体会导致顶底阻抗的差异不明显。另外,薄层的调谐效应使得地层在调谐频率处出现强振幅响应,这一峰值频率可以用来估计储层厚度。

Carbonate platforms were widely distributed throughout the Dongsha uplift in Cenozoic. The autochthonous reefs grew above the platform have turned into excellent reservoirs due to holes and cracks developed inside and the blocking of overlying mudstone. In this paper, we analyze the time-frequency characteristics of Liuhua carbonate reservoir with high porosity using spectral decomposition, combining log data. The seismic waves encountered significant attenuation passing through the top of the reservoir, where the porosity was good, as the seismic centroid frequency on the time-frequency spectrum dropped rapidly there. However, the expected Low Frequency Shadows below the reservoir are missing except for some energy speckles shown on low frequency profiles. It is inferred that the relatively thin bed of the reservoir, the high interval velocity and the existence of the dense fluid (oil and water) inside the reservoir account for this phenomenon. The first two of the factors mentioned above give rise to inadequate attenuation of seismic waves and energy absorption. And the last one does not bring about a distinct impedance reflector as gas does. Besides, thin bed tuning effect leads to strong reflections of corresponding subsurface structures on the common-frequency section as it is at the tuning frequency, and this peak frequency could be applied to the estimate the reservoir thickness.