川西致密砂岩气藏“暗点”型河道砂岩识别方法

川西拗陷东部斜坡带沙溪庙组气藏为致密河道砂岩气藏，大多数河道在地震剖面上表现为强波谷振幅、低阻抗的“亮点”反射特征。随着该气藏评价、开发的深入，一些井在非目的层钻遇的河道砂体也有较好的油气显示。但这些河道的在叠后地震资料上具有“中弱波谷振幅”特征，导致地震振幅平面属性上河道外形不清、连续性差、难以识别，称之为“暗点”型河道砂体。首先，通过从“暗点”型河道砂岩地震响应形成机理研究出发，开展正演模型分析，确定了物性差是“暗点”河道形成的关键因素；然后，利用叠前AVO响应特征分析，明确了“暗点”型河道在叠前道集上具有I类或II$_{\rm a}$类AVO特征；最后，利用优势入射角部分叠加技术解决了“暗点”型河道砂体识别问题。实钻证实了该方法的可行性和有效性，有力地推动了该气藏的高效评价建产。

Dim Spot Channel Sandstone Identification for Tight Sandstone Gas Reservoir in Western Sichuan

The Shaximiao Formation gas reservoir in the eastern slope belt of Western Sichuan Depression is a low-permeability tight channel sandstone gas reservoir. Most channels show "bright spot" reflection characteristics of strong trough amplitude and low impedance on seismic profile. With the development and evaluation of this gas reservoir, it is found that some wells drilled in the channel sand body in the non-target zone also have good hydrocarbon display. However, these channels have the feature of "medium and weak trough amplitude" in the post-stack seismic data, resulting in unclear shape, poor continuity and difficulty in identifying the channels on the plane attribute of seismic amplitude, which is called "dim spot" channel sandstone. Based on the formation mechanism of "dim spot" channel sandstone, the forward modelling analysis is carried out, and it is confirmed that the poor physical property is the key factor for the formation of "dim spot" channel sandstone; then, by analyzing the AVO characteristics of gather, it is clear that the "dim spot" channel has the characteristics of Class I or Class II$_{\rm a}$ AVO characteristics in the prestack gather; finally, the problem of "dim spot" channel sandstone identification is solved by using partial stack technique of dominant incident angle. The feasibility and effectiveness of this method are verified by actual drilling, which strongly promotes the efficient evaluation and production of this gas reservoir.