叠前振幅分析直接指示气田的前提条件

PREREQUISITE OF DIRECT OIL/GAS INDICATION BY AMPLITUDE ANALYSIS BEFORE STACK

同济大学

W.J.Ostrander等人利用砂岩含气泊松比降低到0.1左右时,反射系数随入射角增大而增加的性质,提出了叠前振幅分析直接指示气田的方法。当含气砂岩层的厚度比波长小时,由于层内多次反射的影响,反射系数与入射角的关系呈现复杂的特性。本文系统地计算了不同泊松比和层厚时的反射系数谱,并针对特定的地震子波计算相应的反射波。由反射波极大振幅所作的大量曲线表明,不同的入射角范围、不同的层厚,含气砂岩与非含气砂岩的反射波及大振幅的差别是不同的。W.J.Ostrander等人提出的叠前振幅分析技术指示气田的准则的使用是有一定限制的,要成功地使用叠前振幅分析技术,必须根据具体的地质条件设计相应的模型进行正演计算,以获得叠前振幅资料的解释准则。

By use of the property that when the sandstone is gas bearing, its Poison ratio decends to the value of about 0.1 and the reflection coefficient become greater with the increase of incident angle.In accordance with this, W. J. Ostrander et. al. presented a technique of directly indicating the presence of oil and gas by amplitude analysis before stack. When the thickness of gas sand is less then the wavelength, owing to the effects caused by the peg-leg multiples, the relationship between reflection coefficients and incident angles was rather complicated.In this paper, the reflectivity spectrums for different Poison ratio and layer thickness were calculated systimatically. Besides, reflections corresponding to specialized seismic wavelets were also calculated.Lots of curves constructed by the maximum reflection amplitudes show that for different incident angle and different layer thickness, the differece of the maximum reflection amplitude for gas sand and non-gas sand is different. The use of the criterion for indicating the presence of gas field with the technigue of amplitude an.alysis before stack presented by W. J. Ostrander et.al., is confined within some limits. It can't be applied everywhere. To make use of this technique correctly and successfully, it is necessary to design the modul in accordance with the real geological conditions and make the forward calculation so as to draw the criterion for the pre-stack amplitude explanation.