| With the impressive improvement of seismic exploration technology and the development of shallow reservoir exploration, the prospecting area becomes smaller also reservoir prediction accuracy increases greatly. Thus, we need a more effective method of fluid description. Post-stack seismic data can show the characteristics of seismic wave amplitude. But it is difficult to describe complex reservoirs when there is no obvious characteristic of the impedance. Whereas, pre-stack seismic data completely retains the seismic reflection amplitude varied with offsets and incident angles.This paper mainly researches on frequency-dependent AVO method according to the characteristics of seismic reflection amplitude related to offsets or incident angles and frequency. Also, a cross analysis method is proposed to get characteristic frequencies of the frequency-dependent AVO inversion technology. In addition, this article applies frequency-dependent AVO method to the research of dim spot reservoirs. Finally, as to the practical application of frequency-dependent AVO inversion method, a dispersion magnitude attribute combined factor is introduced for fluid identification. The work and progress of research include the contents as follows:1. This paper puts forward a new numerical modeling method to get frequency-dependent AVO seismic records. Based on dynamic equivalent medium theory proposed by Chapman, frequency-dependent velocity dispersion is introduced to calculate reflection coefficients. Then, through phase-shift wave equation forward modeling method, we can get frequency-dependent AVO pre-stack angle gathers. So, we can find the characteristic of feature parameter of seismic reflection such as amplitude, dispersion and attenuation varied with frequency and incident angles to the greatest extent. As a result, the seismic dispersion and attenuation abnormality related to reservoir fluid can be detected.2. As to frequency-dependent AVO inversion method, this paper proposes a cross analysis method to get characteristic frequency or frequency band. This method can search the most sensitive characteristic frequency as the reference frequency of pre-stack dispersion attribute inversion. Thus, it highlights the dispersion abnormality contrast between different kinds of reservoirs, realizing the distinction and identification of different reservoir fluid type.3. Research on the formation mechanism and identification method of dim spot reservoirs. Based on Chapman’s equivalent medium theory and frequency-dependent AVO numerical modeling method, we can obtain frequency-dependent pre-stack angle gathers of dim spot reservoirs with the introduction of frequency-dependent velocity dispersion effect. Model the characteristic of reflecting interface amplitude between sandstone and its overlying elastic medium being weaker when the fluid type of sandstone changes from water to gas. Also the characteristic of seismic reflection related to frequency and incident angles can be found when fluid substitution of sandstone occurs. Meanwhile, dispersion magnitude attribute of geological model when sandstone is saturated with different fluid can be calculated through frequency-dependent AVO inversion method. The results indicate that when fluid of sandstone changes from water to gas, the seismic amplitude of its top interface becomes weaker, but dispersion features are more obvious. In that case, the value of dispersion magnitude attributes are great in both sandstone’s top and bottom interface especially S-wave. It shows that frequency-dependent AVO method can overcome the problem of the amplitude of dim spot reservoirs being weak and highlight dispersion abnormality related to fluid information. In a word, it provides an effective method for the identification of dim spot reservoirs.4. It comes to a good result when the frequency-dependent AVO inversion method applies to real seismic data. On account of fluid identification indicator of high-sensitivity impedance combined, we combine two kinds of dispersion magnitude attributes aI and bI to build a new fluid identification indicator. As to the practical application of this indicator, dispersion magnitude contrast between different layers with different fluid type becomes bigger. Thus it is beneficial to magnify dispersion abnormality caused by porosity fluid, and decrease background interference independently of fluid type. Besides, it can describe the boundary of hydrocarbon reservoirs more clearly. All in all, frequency-dependent AVO method provides a good result for fluid identification. |
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