利用地球物理综合预测方法识别页岩气储层甜点——以四川盆地长宁区块下志留统龙马溪组为例

四川盆地长宁区块是中国最早进行页岩气勘探开发的地区,但以往地震工作量投入相对较少,尚未精细刻画出页岩气商业开采的"甜点区"。为此,基于测井及三维地震解释成果等资料,通过地震岩石物理分析和建模,确定页岩气储层甜点地球物理响应特征,并以此为依据,应用全道集叠前反演技术预测了总有机碳含量(TOC)、优质储层厚度、地层脆性及地层压力等关键评价参数的平面分布情况;应用模糊优化综合评价方法确定了甜点的空间展布。研究结果表明:(1)该区页岩气储层甜点具有明显的低密度、低纵波速度、低泊松比、低纵横波速度比特征,且TOC越高,上述特征越明显;(2)优质页岩储层(Ⅰ、Ⅱ类区)在纵向上主要分布于下志留统龙马溪组底界上部约30 m范围内,平面上主要分布于三维地震区块的中部,建议优先开发;(3)Ⅲ类储层区各项关键评价指标均较低,建议选择性开发。

Comprehensive prediction of shale gas sweet spots based on geophysical properties: A case study of the Lower Silurian Longmaxi Fm in Changning block,Sichuan Basin

Shale gas exploration and development in China were firstly conducted in the Changning block of the Sichuan Basin. Due to insufficient seismic surveys in the past years, shale gas sweet spots for commercial production have not been pinpointed. In this paper, based on well logging data and 3D seismic data interpretation results, geophysical responses of sweet spots in shale gas reservoir were determined through seismic rock physical analysis and modeling. Then, the planar distribution of some key reservoir parameters, including TOC, high-quality reservoir thickness, brittleness and formation pressure, were predicted through prestack inversion of all gathers. Spatial distribution of sweet spots was assessed using fuzzy optimization. Results show that shale gas sweet spots feature low density, low P-velocity, low Poisson's ratio, and low P-to-S velocity ratio, especially in the case of higher TOC; that high-quality shale reservoirs (ClassⅠandⅡ) concentrate vertically within an interval of 30 m above the bottom of the Lower Silurian Longmaxi Fm and laterally within the central part of the 3D seismic survey, so these reservoirs should be preferred for recovery; and that Class Ⅲreservoirs should be recovered selectively, for their key parameters are relatively low.