Significances And Applications On Geologic Times Of Seismic Velocity

Time is one of eternal subjects in geoscience. The geologic dating methods, which include the paleontology method, archeomagnetism method, and isotopic method, present absolute ages or relative ages of geologic events. The methods face some difficulties, such as few geologic samples, limited experiment methods and so on. Specifically, the number of collected samples is limited because outcrops in the field are relatively fewer; testing periods and advanced instruments are needed; and the experimental analysis to samples also needs much manpower and material resource. If instruments or methods are different, the age data they get will be also different, so that the data need to be checked out. One of the most difficult problems is that some geology bodies can not be obtained because of their locations and geographic conditions. Consequently, other dating methods are needed for the requests of work and studies. In this thesis the relationship between velocity and age is used to dating research, called geophysical dating.Based on above points, this research chooses China continent sedimentary basins as study regions to analyze the relations between velocities, depths and ages of basins. And then the relationship between seismic wave velocity and geologic time is gained and three applications are proposed. In view of the complexity of the problem, this study is based on statistics methods and practical applications making use of the VSP well seismic logging-data of sedimentary basins. Through the research, several important results have been obtained, which yield a new method to determine absolute ages.1. Basic theories of geologic dating and geophysical datingFor many years, the study of geologic dating has been dominated by isotopic methods, which are based on improving the experimental and analysis techniques of high-resolution. However, geologic dating methods cannot work in the regions where samples are difficult to get. Therefore, other methods, known as geophysical dating methods should be developed to solve the problem.Geophysical dating methods were studied early overseas, which considered stabilization sedimentary basins as research regions and established the formula of velocity, depth and age. But less work was done about these since 1963. And, in China, no one has investigated this subject so far. One of the possible reasons is that a simple formula is difficult to obtain to fit all research regions, considering the complexity of geological structures. Another is that the isotopic methods can present more precise absolute ages, so that researchers pay more attention to isotopic dating methods.2. Relationship of velocity, depth and age in continental sedimentary basinsThis thesis introduces the characteristics of China continental sedimentary basins, investigates the velocity, depth and age in these basins. It analyzes the features of kinds of sedimentary basins and relationship between velocity and depth in drilling age from individual sedimentary basins. Finally, the statistical and fitting methods are used to get the relationships between velocity, depth and age in China continental sedimentary basins. The formula is different from Faust's formula because basin features and tectonics are also different between China and America.3. Computation of basement age in several sedimentary basinsBasement ages of several sedimentary basins are computed by using the formula, according to parameter well data in stable sedimentary basins. The formula also can be used to determine the range of sedimentary basement ages with comparing the geologic time determined by isotopic methods. For the Shengli oilfield, the age distribution of oil area basement is computed and the distribution features are analysed. Errors may be brought because China continental sedimentary basins are thought as one tectonic unit.4. Inversion of geoscience transects agesThe geoscience transect ages are inversed by the formula obtained above. The data of velocities and depths are attained from the data collection of present results of geoscience transects including artificial seismic section data by digitalization. The velocity structures are endued with meanings about age. Some characteristics of the geological section ages are also discussed. It is considered that the factors which influence crustal velocities are mostly temperature and pressure. The inversion results on geoscience transects may provide some evidences for geodynamics.5. Applications of age to oil-gas reserve estimationThe porosity is an important parameter in oil-gas reserve computation. The age should be considered because porosity is related to age. For the Dongying depression case, sand porosity distribution is computed for which age is considered. The results are compared with the result that did not consider the influence of age.6. Conclusions(1) The relation formula of velocity, depth and geological age is obtained that fits China continent sedimentary basins, which are thought as one tectonic unit.(2) Making use of the formula, three applications are proposed as computation of basement ages, inversion of geoscience transect ages and oil-gas reserves estimation by ages. It can be educed from this study that the formula is able to compute the range of basement ages, inversion of ages can provide some evidence for geodynamics, and ages should be considered in reserves estimation.