南海北部神狐海域天然气水合物成藏特征及主控因素新认识

New understandings on the characteristics and controlling factors of gas hydrate reservoirs in the Shenhu area on the northern slope of the South China Sea

南海北部陆坡神狐海域烃源岩生烃潜力巨大且烃类运移条件良好,可为水合物成藏提供充足的气源和通畅的疏导通道,而且神狐海域海底沉积层的温度、压力条件满足水合物形成的要求,具备了水合物聚集成藏的地质条件,成为我国水合物勘探开发的重点靶区。为了明确神狐地区水合物的成藏特征和主控因素,首次将随钻成像测井引入对水合物赋存状态、成藏序列、成藏模式和主控因素的研究中,从而为今后水合物的开采和钻井方案的设计提供强有力的理论依据。通过随钻成像测井、电阻率频谱及相对饱和度分析发现神狐海域共发育厚层状、分散状、斑块状、断层附近和薄层状5种赋存状态的水合物,其中厚层状和分散状水合物相对饱和度高且厚度大,开采价值较大,是研究区的主力水合物层,厚层状水合物常分布于水合物层顶部而分散状水合物常分布于水合物层底部;斑块状、断层附近和薄层状水合物相对饱和度较低且分布不规律,开采价值较小。厚层状、分散状、斑块状和断层附近水合物主要为深部热解气通过断层运移至水合物稳定区聚集成藏,为构造渗漏型水合物;薄层状水合物主要为浅部生物气横向运移聚集至水合物稳定域,为地层扩散型水合物。神狐海域发育开启型、填充型和界面型3种类型的断层,断层作为气体和流体的运移通道,沟通了气源和水合物稳定带,控制了水合物在纵向上和横向上的展布范围,为神狐地区水合物的主控因素。

Great potential exists for gas generation in the Shenhu area on the northern slope of the South China Sea.Effective pathways for hydrocarbon migration are favorable in this area for gas hydrate accumulation,for which geological conditions,temperature and pressure in particular,also meet the requirements.As a result,the Shenhu area proved to be the target for exploration and exploitation of natural gas hydrates.To understand the characteristics of controlling factors of the gas hydrate reservoir in this area,and establish a powerful theoretical basis for gas hydrate exploitation and drilling design,logging while drilling（LWD）borehole image log was integrated for the first time to analyze and summarize occurrence state,accumulation sequence,formation model and controlling factors of gas hydrate.Based on LWD borehole image,resistivity spectrum and Sand Counting analysis,five types of gas hydrate are identified as the thick-bedded,disseminated,plaque,near fault and thin-bedded types.The characteristics of high saturation,great thickness and high exploitation value suggest that the thick-bedded and disseminated gas hydrates are the main gas hydrate reservoirs.The thick-bedded gas hydrates are located on the top of the gas hydrate reservoir,whereas the disseminated gas hydrates are at the bottom.In contrast,low saturation and irregular distribution of the near fault and thinbedded gas hydrates resulted in low exploration value of plaque.Thermogenic gas migrate vertically to form deep strata through faults and eventually accumulate in a steady area,leading to the formation of the thickbedded,disseminated,plaque and near fault gas hydrates or the so called structural-seepage hydrates.Biogenic gas migrate laterally to the steady area,resulted in the formation of thin-bedded gas hydrate,which was named the stratigraphic-diffusive hydrate.Three types of faults are identified in the Shenhu area,they are the open fault,filled fault and interface fault.Faults are migration pathways for gas and fluid communicating between the gas source and hydrate steady area,and controlling the range of distribution vertically and horizontally.Faults are the main controlling factor influencing the development of gas hydrate.