利用地震品质因子Q比例的分布特征预测南川页岩气藏保存条件

根据地震数据、地质资料和地震属性,开展了南川地区五峰组-龙马溪组下部页岩的地震品质因子Q平面上分布与页岩气保存条件评价研究。针对南川地区页岩气开发中的断裂破碎带,提出了一种获取稳定品质因子Q比例及界限的工作流程和相应的迭代算法。研究表明,地震品质因子Q比例的分布可以快速、直观识别出断裂破碎带的内幕特征与不同类型页岩分布区。其中,断裂破碎带页岩区的品质因子Q比例总体上偏小,大多低于0.2,局部超过0.5,平面上分布杂乱、碎斑状;页岩呈北东向展布的条带,被断裂所撕裂、破碎,保存条件较差。稳定分布的页岩区的品质因子Q比例是较高的,超过0.7,平面变化稳定,没有突变现象,保存条件好。微裂缝发育的页岩区的品质因子Q比例具有渐变的趋势性,品质因子Q比例的分布从该页岩区的边部0.7向中部有规律地减小至0.1,保存条件最佳。实践证明,高产气井大多位于稳定分布的页岩区或内部微裂隙发育的稳定页岩区,地震品质因子Q比例研究方法对页岩气保存评价与开发井优化部署是一个实用工具。

Prediction of preservation conditions for Nanchuan shale gas reservoirs based on distribution of seismic quality factor Q

The distribution of seismic quality factor Q and conditions for shale gas preservation of the lower shale interval of Wufeng-Longmaxi Formations were studied in Nanchuan area based on seismic and geologic data as well as seismic attributes.In regard to the faulted fracture zones in the Nanchuan shale gas block, the workflow and its iterative algorithm were proposed to work out the stable seismic quality factor Q and its limits.Results show that the internal features of the faulted fracture zones and distribution of different types of shales can be recognized rapidly and directly from the distribution of seismic quality factor Q.Thereinto, the seismic quality factor Q of the faulted fracture shale zones is generally low, with its value mostly below 0.2 while locally over 0.5, and shows chaotic or porphyritic distribution; the shales are distributed in the shape of stripes, trending north-east, but ripped and crushed by the fault zones, resulting in poor preservation conditions.The seismic quality factor Q of the stably-distributed shale zones is higher, with its value being more than 0.7, and shows stable distribution in plane without any abrupt changes, leading to good preservation conditions.The seismic quality factor Q of the shale zones with well-developed micro-fractures tends to change gradually, with its value diminishing regularly from 0.7 at the margin to 0.1 at the centre of the shale zones, implying the best preservation conditions.Practices have shown that most high-yield shale gas wells are located within the stably distributed shale zones or stable shale zones with well-developed micro-fractures, and the method of evaluating shale with seismic quality factor Q is a practical tool for assessing preservation conditions for shale gas and optimizing the deployment of development wells.