横波分裂和纵波方位各向异性在油气勘探中的应用

本文首先回顾了横波分裂和纵波方位各向异性应用于油气勘探的发展历程; 在此基础上重点讨论了二者在上地壳中的分布情况、相互关系及应用条件.横波分裂是地震波传播经过裂缝时所独有的横波双折射现象, 40年的科学研究及生产应用表明这一物理现象在上地壳中广泛存在, 并主要存在于近地表1200 m以浅.纵波方位各向异性从浅到深在上地壳中分布差异较大; 应用于检测裂缝时仅对含气裂缝发育区敏感, 对含其他流体的裂缝区不敏感.针对裂缝发育区的预测, 本文利用理论和实验数据分析论证了横波分裂与纵波方位各向异性的关系; 阐明在油气勘探中横波分裂分析与纵波方位各向异性预测裂缝储层须首先消除近地表的影响, 这在一定程度上限制了二者的应用.同时阐明二者的应用还需要满足近地表相对简单、裂缝储层相对较厚等条件; 且需采集非零偏或环形垂直地震剖面(VSP)资料进行标定.即多分量地震数据品质、近地表地震地质条件及储层方位各向异性发育程度是影响横波分裂及纵波方位各向异性应用效果的3个关键因素.

Shear-wave splitting and P-wave azimuthal anisotropy in oil and gas exploration seismic

This paper first gives a brief review of the historical development of shear-wave splitting and P-wave azimuthal anisotropy in oil and gas seismic exploration, and focuses mainly on their distribution in the upper crust, the link between them, and their application requirements for seismic fracture detection. When a shear-wave enters a rock containing vertically aligned fractures, it splits into two travelling with different speeds, which is referred to as shear-wave splitting, or birefringence. Forty years of researches and applications of shear-wave splitting reveal that shear-wave splitting occurs widely in the upper crust, but most of the splitting is concentrated in the near surface (< 1200 m). While P-wave azimuthal anisotropy in the upper crust varies considerably from weak to strong and shows no consistence. When applied to predict fractures, P-wave azimuthal anisotropy is only sensitive to gas-saturated fractures, and very insensitive to other fluid-saturated fractures. For fracture detection, theoretical analyses and experiment studies of the link between shear-wave splitting and P-wave azimuthal anisotropy verify that, it is essential to correct the near surface effects, which might limit the application to some extents. It is important to select target areas with relatively simple structure and relatively flat surface, as well as relatively thick reservoirs. Non-zero offset vertical seismic profiles (VSPs) or Walkaround P- or S-wave VSPs are also required to correct the near-surface effects and calibrate the reservoir response. In summary, the multi-component seismic data quality, the near-surface and reservoir conditions are three key factors affecting the application of shear-wave splitting and P-wave azimuthal anisotropy for fracture detection in oil and gas exploration.