Study On Wide-angle Seismic Exploration Of Deep Tectonic In The South Yellow Sea

Because the requirements of the social and economic development increase and the continental petroleum exploration becomes more and more difficult, the marine oil and gas exploration is becoming a new oil and gas resources strategic field in China. Distribution of the Mesozoic-Palaeozoic marine strata in the South Yellow Sea region is relatively stable. Parts of the strata are affected weakly by the late tectonic evolvement. Among them, the residual thickness of the Mesozoic strata is large in Northern Depression. Also, the residual thickness of the Palaeozoic strata is large in both Central Uplift and Wunansha Uplift. These strata all are good oil and gas prospect district in the South Yellow Sea region. However, the marine residual basin in the South Yellow Sea has undergone complex tectonic evolution. In the shallow Central Uplift and Southern Uplift of the South Yellow Sea, marine carbonate strata with high velocity cover directly on low-velocity layers, which causes velocity inversion. Because of the small depth, the shielding effect of shallow high-velocity layers and the tiny physical differences between deep carbonate strata, it is difficult to identify deep weak reflectors from seismic data. Imaging of deep target layers is hard and cannot meet the requirements of oil and gas exploration. Therefore, the study on the wide-angle seismic exploration of deep tectonic in the South Yellow Sea is of great importance.Based on the complex seismic-geologic conditions in the South Yellow Sea, the effects of the high-velocity shielding layers on the propagation of seismic waves are studied with theoretical research and numerical simulation. According to the previous research achievements, the seismic velocity-depth model of the South Yellow Sea is built to study the feature of the wide-angle seismic waves related to the deep marine weak reflectors using the seismic forward modeling technique. On this basis, the data of multi-channel seismic acquisition test are analyzed to study the optimization of the acquisition parameters in the wide-angle seismic exploration. Because the conventional seismic exploration has limitations for the prospecting depth, the data processing and interpretation methods of ocean bottom seismograph, namely OBS, are researched with the actual OBS data in the South Yellow Sea. Depending on the above studies, the following achievements and innovations have been obtained.(1)On the basis of the theoretical research and numerical simulation of the high-velocity layers shielding effects, the influence of the complex seismic-geologic conditions on the quality of the marine seismic data is discussed in three major areas: energy shielding, absorption attenuation and scattering effect. With the seismic interpretation profile of the South Yellow Sea and the previous research results about the petrophysical characteristics, the seismic velocity-depth model corresponded to the seismic-geologic conditions is established. The seismic forward modeling aiming at the wide-angle seismic exploration of the deep weak reflectors is carried out to analyze the feature of the wide-angle reflections related to the marine sedimentary strata. Based on the previous studies, this thesis proposes an efficient method using the wide-angle seismic exploration technology as an unconventional seismic exploration method, which is proved to be very applicative for the seismic exploration of weak reflectors under the shielded layers of high velocity in the South Yellow Sea region.(2)Based on the seismic wave field research of the wide-angle seismic exploration in the South Yellow Sea, this thesis analyzes the data of the seismic acquisition test about multi-level air-gun array and longer streamers. The influences of the different acquisition parameters on the deep reflections are compared by spectrum signature, single shot records and stacked sections to obtain the optimization scheme of the multi-channel wide-angle seismic acquisition parameters in the South Yellow Sea. Obviously, the multi-level air-gun array is much better than the conventional air-gun array, because it is conducive to suppressing the notch effect, improving the low-frequency seismic energy, expanding the low-frequency seismic waves and broadening the effective frequency band. The longer streamers are beneficial to acquiring the wide-angle reflections with high energy at far offsets, which can improve the quality of the deep tectonic imaging.(3)Based on the study of the OBS imaging, the multi-scale traveltime tomography processing and interpretation research about the South Yellow Sea has been carried out. With the relevant methods proposed, such as correlation stacking, picking and selecting of first arrivals and joint interpretation, the inversion result of the deep velocity structure is obtained with high credibility. The multi-channel seismic section and the velocity structure inverted from the OBS are composited together. As a result, this two sets of data can supplement each other. With the help of the joint interpretation, the deep tectonic can be described more clearly and the deep tectonic imaging precision of the South Yellow Sea can be improved.