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常规测井资料主要反映岩石的宏观物理信息。低渗透储层孔隙结构复杂，做孔和做缝作用明显，应力敏感性强，利用常规测井资料和地面分析资料精确评价储层的难度校大。核磁共振测井资料为分析地层上覆应力下储层的孔隙结构和渗透性提供了良好的手段，但由于低信噪比和T2截止值问题，其在低渗透储层评价中的适应性值得进一步研究。文章通过岩心调试，建立了长庆上古生界低渗透砂岩储层的核磁共振潮井解释方法和优化采集参数，分析了影响丁z截止值变化的主要因素。结果表明，研究区实潮可动流体截止值变化范围较大，影响其变化的主要因素是储层的岩性，纯石英砂岩相对岩屑砂岩和含泥砂岩具有较高的T2截止值。文章给出了核磁共振测井在提供高精度储层参数、进行储层孔隙结构及可动流体分析、以及低电阻率气层识别方面的实例和使用效果。

Fine Evaluation of Low-permeability Sandstone Gas Reservoir by use of Nuclear Magnetic Resonance Logging

Macropetrophysical information can be mainly revealed by conventional log data and it is difficult to evaluate finely reservoir by use of the conventional log data and surface analysis data because of these characteristics of low-permeability reservoir,as complex pore structure,obvious micropore and microfissure functions and strong stress sensitivity,etc.A good means is provided for analyzing the pore structure and permeability of the reservoir under overburden stress by use of nuclear magnetic resonance log data,owing to the problems of signal-to-noise ratio and T 2 cutoff,however,its adaptability in evaluating low-permeability reservoir is worthy to be further studied.Through core testing,the interpretation and optimizing acquisition parameter methods of nuclear magnetic resonance logging for Upper Palaeozoic low-permeability sandstone reservoirs are proposed and the major factors influencing T 2 cutoff are analyzed in the paper.It is shown that the change range of T 2 cutoffs of the measured movable fluids is relatively wide in research region;the principle factor influencing its change is the lithology of reservoir and clean quartzose sandstone relative to lithic sandstone and argilliferous sandstone is of relatively high T 2 cutoff.Some examples and results of applying nuclear magnetic resonance logging to the following aspects as providing highly accurate reservoir parameters,carrying out the analyses of reservoir pore structure and movable fluids,and identifying low-resistivity gas reservoir,etc.,are put forward in the paper.