基于单相与双相介质拟海底反射的AVO研究

A study of AVO responses of bottom simulating reflection in single-and two-phase media

中国地质大学暨教育部地下信息探测技术与仪器重点实验室,北京 100083

Geodetection Laboratory, Ministry of Education, China University of Geosciences, Beijing 100083, China

天然气水合物是一种潜在的巨大能源,目前普遍认为它存在的重要标志之一是地震剖面上的拟海底反射(BSR),因此研究天然气水合物层的物性和研究含天然气水合物层顶、底界面的AVO特征就有着重要的现实意义。目前的AVO方法主要是利用单相介质理论,但实际的地层都是含有流体的多孔隙介质,因此利用双相介质理论研究分界面上的AVO特征是非常必要的。Ecker提出了水合物沉积的3种微观模式:①水合物是流体的一部分;②水合物是固体骨架的一部分;③水合物胶结固体颗粒接触面。利用单相介质理论和双相介质理论分别计算了上述3种含天然气水合物地层,以及同样条件下不含水合物地层与含游离气地层分界面上的反射和透射系数,并对单相介质理论和双相介质理论的计算结果进行了对比。对比发现,在地震勘探的低频范围内,第1种和第2种模式的反射和透射系数非常相似,二者的差值随孔隙度的增大而增大,第3种模式的AVO曲线表现出和前2种模式完全不同的特点,可以根据这种特点把它与前2种模式区分开来。

Gas hydrate is a kind of potentially energy The"bottom simulating reflectors", (BSR) on seismic sec-tions are generally accepted as an indicator of the ex-istence of gas hydrate. Understanding of the proper-ties of hydrate-bearing layers and study of the AVOcharacteristics of BSR are of great significance. Sin-gle-phase medium theory is commonly used in AVOanalysis at present, but natural media are actuallyporous and fluid-bearing. Ecker proposed three micromechanical models of hydrate formation. Hydrate isa part of pore fluid, a part of solid matrix, or hydratecements grain contacts. Based on single-and two-phase medium theory, we calculated the reflectioncoefficients and the transmission coefficients on aninterface between two layers, among which one iswithout hydrate and the other is with free gas in thethree models. Calculation shows that in the seismicfrequency range, the reflection coefficients and thetransmission coefficients resulted from the first andsecond models are very similar. The difference be-tween them increases with porosity. The third modelgenerates a completely different AVO curve. There-fore, the latter can be qualitatively distinguishedfrom the former.