Method Study Of Difference Between VSP Velocity And Seismic Velocity At BD19-2Region,Ying-Qiong Basin

The medium speed of seismic wave propagation is one of the significant parameters in the geophysical exploration. For geophysicists, the objectives they’ve been always striving to pursue are to acquire a precise velocity of stratum and to transform the expected research findings from time-domain to depth-domain. At the same time, the acquisition of a precise velocity plays a critical role in geophysical exploration and oil-gas field development, which is excessively instrumental for us to figure out the real depth of stratum, contact relations between faults, diffracted wave’s homing, the features of multiple wave attenuation, rocks’ lithology and porosity, fluid saturation, and the predicted pressure. However, the relatively vast area for exploration and inadequate drillings in the offshore oil-gas field development exert an influence on the in-depth study of the seismic velocity, thus further affecting the research evaluation of the exploration and the follow-up production. At present, the research methodology of velocity in offshore exploration is:well data will be matched in regions with drillings; the imaging of seismic data will be utilized to deal with the velocity data to achieve the transformation from time-domain to depth-domain in regions with inadequate or even no drillings. But the huge inherent errors of these approaches and the weak effects in transformation from time-domain to depth-domain make it impossible to make an accurate and elaborate assessment of forming a special pattern. It is thus clear that it is worth studying to establish a precise velocity switch from time-domain to depth-domain for implementation of space geometry mentality as well as the follow-up development and production of oil-gas exploration. There exists a speed difference, called "difference between VSP velocity and seismic velocity", between the velocities of stratum obtained by the way of offshore seismic exploration and of Vertical Seismic Profiling exploration in the deep-water area with drillings in QiongDongnan basin, the west of South China Sea. With the BD district, YC district, HK district being the target research center, analyses are made on different depths and lithologies, and varied seismic velocity and VSP change rules in different geological structures and phase belts, and conclusions are made towards the differentiation of VSP and its correction, finally acquiring an approach of velocity correction which is feasible in regions with inadequate or no drillings in the west of the South China Sea. This approach lays a firm foundation for acquiring and studying the change rule of velocity of stratum in Qiongdongnan basin, transforming velocity from time-domain to depth-domain and guiding oil-gas exploration and development in new areas. Besides, it is also of positive significance to deal with the pressure prediction and hydrocarbon detection in deep-water areas.The precise interval velocity can be obtained, on the basis of the acquired accurate seismic interval velocity and relevant logging velocity, through the correction of the well-seismic-joint velocity field under the control of stratum and the establishment of the VSP correction approach for VSP relation research. The VSP correction approach should be established by remedying the defects in traditional approach:(1) Substantially, in the traditional correction approach, the fit t-v curves of a few drillings are to be considered as the correction objective of velocity spectrum t-v curves.(2) The strong subjectivity of positions far away from drillings makes it difficult to judge whether the results are accurate.(3) It can only be applied to regions with small Stratigraphic dip and relatively slight changes in lateral velocities.This paper makes the following improvements:(1) The author analyzed the causes of the VSP difference and acquired precise interval velocity by various methods, such as methods of well-seismic-based structure formation and of Bayesian collocated CoKriging interpolation.The author adopts the model difference technique and utilizes the restriction of stratum to eliminate the VSP difference through the well-seismic-joint modeling, taking the influence of large faults into consideration.(2) Various technologies, such as correction of depth of water, probability statistics, stratum profiles, are applied to make synthetic analysis on stratum velocity change rule, and to summarize qualitatively and quantitatively the relations of stratum velocity change with depth of water, with burial depth, and with lithology change. (3)The author will finally acquire an approach of velocity correction which is feasible in deep-water areas of special geographical conditions with inadequate or no drillings in Qiongdongnan basin in the west of the South China Sea.