AVO技术在孟加拉湾含气性检测中的应用

Application of AVO technology to gas detection in Bengal Bay

孟加拉湾研究区砂体比较发育,主要为海洋深水沉积水道和朵叶体,在地震剖面上表现为强振幅的亮点特征,横向变化快,但并非都含油气,因此开展油气检测在该地区尤为重要。本文利用Zoeppritz方程的Aki&Richards近似式和Shuey近似式反演了一系列叠前AVO属性体,如:P波速度、S波速度、P波阻抗、S波阻抗、截距(P)、梯度(G)、P×G、近道叠加数据和远道叠加数据、流体因子、泊松比等等。结合钻井资料进行正演模拟、流体替换及岩石物理分析,确定了相应的地震振幅响应特征,并计算了相应的AVO属性,为直接利用地震资料进行AVO油气检测奠定了基础。研究表明,如果储层含气,振幅随炮检距的增大而增大,此外,P波速度、P波阻抗、λρ和泊松比变化较大,属于Ⅲ类AVO异常;如果储层含水,这些属性仅有微小的变化。本文优选了P×G、流体因子、泊松比、P波阻抗和λρ进行分析,在研究区发现了多个含气目标。

Sand bodies are rich in our study area of Bengal Bay.Most of them are offshore deepwater sedimentary channels and lobes.They represent high amplitude as bright spots,and lateral changes are fast.But not all of them contain hydrocarbon.Thus hydrocarbon detection is very important in the study area.The Aki & Richards equation and Shuey equation are aplieded in this paper.Series of pre-stack AVO attribute cubes are calculated based on these equations,such as P-wave velocity,S-wave velocity,P-wave impedance,S-wave impedance,intercept(P),gradient(G),P×G,near offset stack amplitude,far offset stack amplitude,fluid factor,Poisson ratio and so on.The forward modeling,fluid substitution and petrophysical analysis with well data have been carried out.The corresponding amplitude characteristics are defined,and relevant AVO attributes are calculated,which serve as the basis for the later seismic AVO attribute analysis.The results of this research indicate that if a reservoir contains gas,the amplitude increases with offset,P-wave velocity,P-wave impedance,λρ and Poisson ratio all change fast.These features belong to AVO anomalies of type Ⅲ.And if a reservoir contains water,the above attributes only show tiny changes.In this paper,P×G,fluid factor,Poisson ratio,P-wave impedance and λρ are used for analysis.Many gas prospects have been detected in the study area based on this method.