Semi-quantitative analysis of gas bearing sand by AVO forward modeling
Pan Renfang1, Chen Silu2, Zhang Liping1, Ai Jianfeng3
1. Key Laboratory of Exploration Techniques of Oil and Gas of Ministry of Education,Yangtze University,Wuhan,Hubei 430100,China;
2. General Research Institute,CNOOC,Beijing 100027,China;
3. Research Institute of Shenzhen Branch,CNOOC,Guangzhou,Guangdong 510245,China
Abstract：Porosity and gas saturation play a significantrole on AVO responses. We use in this paper AVOforward modeling to create the seismic reflectionfor various assemblages of seal and reservoir withvarious porosity and gas saturation. Then we per-form a quantitative analysis of gas saturation bythe variations of AVO intercept and gradient de-rived from Shuey's two-term approximation andCastagna's classification.It is demonstrated thatboth sand porosity and its gas saturation impact onAVO responses:①AVO intercept decreases withporosity increasing，and gradient decreases withgas saturation increasing.②AVO response variesweakly while gas saturation is bigger than 20%.AVO responses of gas bearing sand transits fromclass Ⅱ to class Ⅲ when the acoustic impedance ofseal is lower than the reservoir matrix impedance，and from class Ⅲ to class Ⅳwhen the acoustic im-pedance of seal and the reservoir matrix impedanceare equivalent，class Ⅳ occupies a dominant posi-tion when the acoustic impedance of seal is higherthan the reservoir matrix impedance.
