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    基于各向异性弹性敏感参数的孔隙−裂缝型致密砂岩储层岩相识别研究以 JY 地区须家河组致密砂岩储层为例

    Lithofacies identification of porous-fractured tight sandstone reservoirs based on anisotropic elastic parameters: A case study from the Xujiahe Formation, JY area

    • 摘要: 致密砂岩储层地质环境复杂、物性差异小、流体变化剧烈,且地震岩石物理响应特征不明确,传统的基于各向同性假设的储层敏感参数难以适用于此类复杂储层的识别与评价。提出了一种基于各向异性岩石物理敏感参数的岩相逐级识别策略与方法。以JY地区须家河组四段的20块岩心样品为研究对象,综合采用成像测井、岩心观察、孔渗测试及铸体薄片观测等多尺度分析手段,对样品类型进行初步识别与划分。根据孔隙与裂缝特征及含流体状态,将样品分为饱气孔隙型、饱和水孔隙型、饱气裂缝−孔隙型及饱和水裂缝−孔隙型4种岩相类型,采用全方位超声波各向异性测试系统对样品进行多方向声波测试。结果表明,饱气裂缝−孔隙型样品在波速与振幅方面表现出显著的各向异性特征。为量化上述特征差异,构建了基于纵波各向异性参数(ε)与振幅各向异性参数(εA)的岩石物理交会模板,研究发现,当εεA均大于20%时,可有效识别饱气裂缝−孔隙型岩相。此外,基于全方位波形的相似度系数谱,定义并推导出了一种新的慢度各向异性参数(εslow),用于刻画波速慢度的方向性变化,实测数据显示,饱和水孔隙型样品的εslow值普遍低于0.01,表明其各向异性程度极低。进一步结合λρ-μρ岩石物理模板,设定λρ<60和μρ<30作为判别阈值,实现了对上述4类岩相的有效区分与识别。研究成果为致密砂岩储层的精细预测与流体识别提供了重要的理论依据。

       

      Abstract: Tight sandstone reservoirs as the product of complex geological settings often exhibit subtle petrophysical property variations, significant fluid heterogeneities, and ambiguous rock physical responses, which pose challenges to reservoir prediction based on conventional isotropic sensitive parameters. To address these limitations, this study proposes a hierarchical identification strategy based on anisotropic rock physical parameters sensitive to lithofacies. The investigation focuses on 20 core samples obtained from the fourth member of the Xujiahe Formation in JY area. A multi-scale analytical approach, incorporating imaging logging, core analysis, porosity and permeability measurements, and cast thin-section observation, is adopted for the identification and classification of four lithofacies types: gas-saturated porous, water-saturated porous, gas-saturated fractured-porous, and water-saturated fractured-porous, in terms of pore and fracture characteristics along with fluid states. As per the multi-directional acoustic tests on the samples using an improved omnidirectional ultrasonic anisotropy test system, gas-saturated fractured-porous samples are remarkably different from additional samples in significant velocity and amplitude anisotropies. A rock physics cross plot of P-wave anisotropy (ε) and amplitude anisotropy (εA) is constructed to quantify these characteristic differences. The values of ε and εA both exceeding 20% tend to indicate gas-saturated fractured-porous lithofacies. Based on the similarity coefficient spectrum of full-azimuth waveform, a new anisotropy parameter: slowness anisotropy (εslow), is formulated to characterize the directional variation of slowness. The measured εslow values of water-saturated porous samples consistently below 0.01 indicate the extremely low anisotropy of this lithofacies type. By further integrating λρ < 60 and μρ < 30 from the λρ-μρ cross plot, we achieve the effective differentiation and identification of all four lithofacies types. These findings offer a robust quantitative framework for tight sandstone reservoir prediction and fluid identification.

       

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