| With the continuous development of new energy field,the enhancement of international environmental protection awareness and the exhaustion of conventional fossil energy,natural gas hydrate as a new type of large reserves of clean energy has been widely concerned.Identification and exploration of natural gas hydrate has become an important part of the development of new energy.As an important symbol of gas hydrate identification,the seafood-like reflection zone(RSR)cannot solve all the problems of gas hydrate identification,that is,there is no exact correspondence between BSR and gas hydrate.The technique ofΔV_P/ΔV_S can effectively solve the problem of hydrate identification without the presence of BSR,but it is not absolutely stable.When the inversion of P-wave velocity and S-wave velocity are not accurate and the error of calculating the P-wave velocity and S-wave velocity in water-saturated formation is large,the smallΔV_S will lead to the occurrence of abnormalΔV_P/ΔV_S,thus reducing the credibility of the identification results.Aiming at the above problems,this paper,based on the petro-physical model of hydrate occurrence,simulates the P-wave velocity and S-wave velocity andΔV_P andΔV_S in the formation under the theoretical hydrate condition.The simulation results show that the relationship betweenΔV_P andΔV_S varies with hydrate saturation is determined under certain geological conditions,and the intersection point ofΔV_P andΔV_S at outliers is clearly distinct from the theoretical relationship.Based on this difference,the method of cross-plot ofΔV_P-ΔV_S is proposed to distinguish the abnormalΔV_P/ΔV_S caused by smallΔV_S,so as to improve the accuracy of identification results ofΔV_P/ΔV_S.In addition,the study also found under the theoretical model,the slope ofΔV_P change with hydrate saturation curve is similar to the P-wave velocity with hydrate saturation curve,the slope ofΔV_S change with hydrate saturation curve is similar to the S-wave velocity with hydrate saturation curves.Therefore,according to the mathematical law,the relationship betweenΔV_P/P-wave velocity andΔV_S/S-wave velocity varies with hydrate saturation is also determined under certain geological conditions.The intersection point ofΔV_P/P-wave velocity andΔV_S/S-wave velocity at outliers is clearly distinguished from the theoretical intersection point.Furthermore,in this paper,the method ofΔV_P/P-wave velocity andΔV_S/S-wave velocity cross-plot is proposed to distinguish the value of abnormalΔV_P/ΔV_S caused by smallΔV_S,so as to improve the accuracy of identification results ofΔV_P/ΔV_S.The influence of mineral composition and depth changes on the theoretical relationship curves and the identification results of theΔV_P/ΔV_S by the two cross-plot methods on the ODP leg 204 show that the two methods proposed in this paper are effective.These methods can effectively solve the problem that theΔV_P/ΔV_S is difficult to identify when the P-wave velocity and S-wave velocity are not accurate and the error of calculating the P-wave velocity and S-wave velocity in water-saturated formation is large. |
