塔里木盆地缝洞型储层地震采集激发接收因素优化

Optimized seismic acquisition for fractured-vuggy reservoirs inTarim Basin

塔里木盆地碳酸盐岩储层是重要的油气藏储层类型之一,可分为缝洞型、裂缝型和裂缝缝洞型储层,缝洞体的尺度远小于地震波长,其地震波场响应以绕射波为主,因而对绕射波的保护和增强是影响地震资料采集质量的关键因素。随着勘探开发的不断深入,较大尺度缝洞储集体动用程度越来越高,勘探目标向小尺度缝洞储集体转移。通过小尺度溶洞、裂缝储层的模拟波场特征研究发现,缝洞型储层地震响应波场具有能量弱、视速度低、能量强度与激发子波主频正相关等典型特征。基于以上特征开展了激发、接收的理论研究和野外试验。塔河地区激发模拟表明,小药量激发的地震子波主频更高,有利于增强小尺度地质体的绕射能量。4,6,10 kg 3种激发药量的二维试验证明,4 kg和6 kg药量激发能够拓展地震成像的高频成分,提升成像分辨率;模拟研究表明,20 m以上组合基距接收对低视速度绕射波有较大损伤,并影响缝洞体偏移聚焦能量,采用组合基距小于12 m的接收方式对小尺度缝洞型储层成像质量影响较小,为兼顾原始资料信噪比,较理想的接收方式是相同观测系统参数下,采用小组合接收、小药量激发的方式,对于风化壳缝洞储集体的识别能力更强,小串珠数量增加约30%。

Carbonate reservoirs,including fractured-vuggy reservoirs,fractured reservoirs,and fractured and fractured-vuggy reservoirs,are an important reservoir type in the Tarim Basin.Fractured-vuggy units,which are much smaller than seismic wavelength,mainly cause diffracted waves,and how to preserve and enhance diffracted waves is a ticklish problem in seismic acquisition.Since more and more large fractured-vuggy reservoirs have been recovered,small reservoirs become exploration targets.As per the study of reservoir models with small dissolved cavities and fractures,seismic responses feature low energy,low apparent velocity,and positive correlation between energy intensity and the dominant frequency of source wavelet.Theoretical study and field tests of shooting and receiving conditions in Tahe area show that a small quantity of dynamite may excite the source wavelet with high dominant frequency,which is favorable for diffraction enhancement for small geologic units.The quantities of 4 and 6 kg used in 2D tests perform better than 10 kg in high-frequency enhancement to improve imaging resolution.An array length larger than 20 m may lead to a considerable loss of diffracted waves with low apparent velocities and thus impair focusing energy in migration to image fractured-vuggy units.The array length not greater than 12 m is a good choice to balance imaging quality of small reservoirs and signal-to-noise ratio.For equal geometry parameters,the combination of small array length and small dynamite quantity is favorable for detecting weathering-crust fractured-vuggy reservoirs,as indicated by 30%more bead-like reflections identified.