尼日尔三角洲天然气水合物体系地震特征研究

Study on the seismic characteristics of the gas hydrate system in the Niger Delta

天然气水合物体系一般由地震上的似海底反射层BSR和下伏强振幅带(解释为游离气FGZs)所指示,但并非所有BSR及下伏强振幅带都与水合物和游离气有关.本文通过提取多种属性剖面,优选出视极性属性来辅助判断与水合物有关的BSR和下伏FGZs.—般来说,BSR和FGZs的顶部反射层表现为负视极性、高振幅的强反射特征.研究发现,尼日尔三角洲南部的水合物主要分布在与重力作用有关的生长断层及伴生的滚动背斜地区,广泛发育的断层、气烟囱、不整合面,以及砂岩层都可作为流体运移通道.除为水合物体系提供气源,这些流体运移通道还在水合物分解或FGZs超压时发挥作用,使游离气沿通道在FGZs和含水合物地层间循环,部分游离气可运移至海底进入海水甚至大气中·总之,视极性是判断水合物相关地震指示标志的有效属性,流体运移通道对尼日尔三角洲的水合物成藏具有重要作用.

Gas hydrate system is generally indicated by seismic indicators like bottom simulating reflection （BSR） and underlying high amplitudes （caused by Free Gas Zones：FGZs）, but not all BSRs and underlying high amplitudes are caused by hydrates or free gas. In this study, we choose apparent polarity as the optimum attribute to help identify hydrate-related BSR and FGZs, both of which are characterized by negative apparent polarity and high amplitude. In the study area, the gas hydrate system is mainly distributed in areas where developed gravitational structures including growth faults and related rollover anticlines. Faults, chimneys, unconformity and sandstone beds can act as fluid migration conduits, supplying gas to the shallower gas hydrate system. When gas hydrates dissociate or the FGZs are critically over pressured, these conduits can also play a role in gas recycling between the FGZs and hydrate-bearing strata with some free gas escaping to the seabed and even to the atmosphere. Generally, apparent polarity is an effective attribute to identify gas hydrate system, and fluid migration pathways are of great importance for the formation of gas hydrate system in the Niger.