四川盆地西北部超深层碳酸盐岩孔缝带地震检测

近年来,四川盆地西北部地区(以下简称川西北地区)在超深层海相碳酸盐岩气藏勘探方面取得了突破,多个构造在中二叠统茅口组三段(以下简称茅三段)获得高产气流并证实裂缝是天然气高产的主要控制因素,但是常规地震处理对裂缝识别能力差、预测效果不佳。为此,以川西北地区九龙山构造为例,在归纳总结茅三段储层特征的基础上,分析已钻井茅三段储层的地震响应特征;再通过对原始叠前道集优化处理和频谱恢复高分辨处理提高地震资料分辨率,同时保持裂缝孔洞发育的地震微弱信号;进而采用照明对称体、分频混沌体、导向似然体等叠后地震属性处理技术开展裂缝检测,最后通过三色融合技术预测茅三段缝洞体的平面展布特征。研究结果表明:①川西北地区茅三段超深层碳酸盐岩储层主要为岩溶裂缝型储层,构造裂缝及溶蚀孔缝发育,易形成较大规模的裂缝发育带;②该区超深层原始地震资料主频介于25～30 Hz,对裂缝带响应不明显,采用叠前道集优化处理技术和频谱恢复技术组合,可以保护裂缝带的微弱信号,使地震资料主频达到45 Hz,有效地提升了地震资料的频谱,地质现象表现明显;③采用照明对称体、导向似然体、分频混沌体可以从不同角度反映地震资料表达的地质信息,三色融合技术能较好地反映茅三段碳酸盐岩断裂发育带、岩溶孔缝发育带。结论认为,地震预测裂缝成果与钻井、测井、测试等成果匹配良好,为该区天然气开发井的部署与调整提供了依据。

Seismic detection of the pore and fracture zones in ultradeep carbonate reservoirs in the northwestern Sichuan Basin

In recent years, great breakthroughs have been realized in the exploration of ultradeep marine carbonate gas reservoirs in the northwestern Sichuan Basin, high-yield gas flow has been produced from the third Member of Maokou Formation, Middle Permian (Mao 3 Member for short) in many structures, and fracture has been proved to be the main factor controlling the high yield of natural gas. However, conventional seismic processing technologies are poor in fracture identification and prediction effect. This paper took the Jiulongshan structure in the northwestern Sichuan Basin as the example. After the characteristics of Mao 3 Member reservoir were summarized, the seismic response characteristics of Mao 3 Member reservoir in drilled wells were analyzed. Then, the optimization processing of original pre-stack gather and the high-resolution treatment of spectrum restoration were carried out to increase the resolution of seismic data while the weak seismic signals of developed fractures and vugs were maintained. Based on this, fracture detection was conducted using post-stack seismic attribute processing technologies, e.g. I3D symmetry, frequency division chaotic body and likelihood. Finally, the areal distribution characteristics of fractures and vugs in Mao 3 Member were predicted by virtue of three-color fusion technique. And following research results were obtained. First, the ultradeep carbonate reservoirs of Mao 3 Member in the northwestern Sichuan Basin are mostly karst fractured reservoirs with developed structural fractures and dissolved pores and fractures, which tend to form larger fracture development zones easily. Second, the original seismic data of the ultradeep reservoirs in this area has a primary frequency of 25–30 Hz, which cannot respond to fracture belts strongly. The combination of pre-stack gather optimization processing technique and spectrum restoration technique can protect the weak signals of fracture belts and increase the primary frequency of seismic data to 45 Hz, so that the spectrum of seismic data is improved effectively and the geological phenomena are embodied clearly. Third, I3D symmetry, frequency division chaotic body and likelihood can reflect the geological information expressed by the seismic data from different viewpoints, and three-color fusion technique can better reflect fault development belts and karst pores and fracture development belts of Mao 3 Member carbonate reservoirs. In conclusion, the seismic based fracture prediction result is in accordant with drilling, logging and testing results, so as to provide the basis for the deployment and adjustment of natural gas development wells.