煤系地层致密砂岩气甜点区地震逐级预测——以鄂尔多斯盆地东南缘下二叠统山西组2~3亚段为例

鄂尔多斯盆地东南部下二叠统山西组2~3亚段(以下简称山2~3亚段)为该盆地重要的天然气勘探目的层,但该亚段储层薄、厚度变化快、非均质性强,储层预测和勘探目标优选难度大。为了准确预测该亚段煤系地层致密砂岩气甜点区、提高天然气勘探成功率,针对该套储层的特征和预测难点,提出了90°相移技术识别河道外形、模型约束波阻抗反演刻画砂体厚度和子波衰减梯度属性识别含气砂体的地震逐级预测技术。研究结果表明:①山2~3亚段上覆5号煤地震强反射层,下伏储层地震反射能量弱,加之为稀疏二维地震测网、井控程度低,致使致密砂岩气甜点区预测难度大;②所提出的技术方法通过地震逐级预测约束,可以有效地刻画河道砂体分布并识别有效含气储层,提高了对勘探开发目标预测的精度;③基于该技术方法指导部署的勘探开发目标实钻效果好,地震预测结果横向分辨率高,真实地反映了河道及河道砂体的变化特征。结论认为,采用该方法可以有效地解决二维地震勘探区煤系地层强非均质性、薄储层致密砂岩气甜点区预测的地质难题。

Seismic stepped prediction technology for tight sandstone gas sweet spot in coal measure strata: A case study of the Submember 23 of the Lower Permian Shanxi Formation along the southeastern margin of the Ordos Basin

The Submember 23 of the Lower Permian Shanxi Formation (hereinafter, "Shan 23 Submember" for short) in the southeastern Ordos Basin is an important target of natural gas exploration in this basin. However, the reservoir of Shan 23 Submember is characterized by small thickness, fast thickness variation and strong heterogeneity, and its reservoir prediction and exploration target selection is difficult. In order to accurately predict the sweet spots of tight sandstone gas in the coal measure strata and improve its success rate of exploration, this paper proposed a seismic stepped prediction technology based on the characteristics and prediction difficulties of this reservoir, including the 90° phase-shifting technology to determine channel outline, the model constrained wave impedance inversion to characterize sandbody thickness,and the wavelet attenuation gradient attribute to identify gas-bearing sand bodies. And the following research results were obtained. First, Shan 23 Submember is overlain by No.5 coal bed of strong seismic reflection and its underlying formation has weak seismic reflection energy. In addition, there is a sparse 2D seismic grid and a low well control degree. Therefore, tight sandstone gas sweet spot prediction is high difficulty. Second, under the constraint of seismic stepped prediction, the proposed technology can be used to efficiently characterize the distribution of channel sand bodies and identify effective gas-bearing reservoirs, so as to increase the prediction accuracy of exploration and development targets. Third, the exploration and development targets deployed on the basis of this technology present a good drilling effect, the seismic prediction result has high lateral resolution, and the variation characteristics of channels and channel sand bodies are reflected truly. In conclusion, the application of this method can solve the geological difficulties of predicting tight sandstone gas sweet spots in strong-heterogeneity thin reservoirs of coal measure strata in a 2D seismic exploration area.