Velocity model and time-depth conversion for the northwestern South China Sea deepwater areas
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Velocity model and time-depth conversion for the northwestern South China Sea deepwater areas LIU Aiqun1*, CHEN Dianyuan1, LI Wentuo1, FAN Caiwei1, HE Jianwei1 1 Zhanjiang Branch of China National Offshore Oil Corporation Limited, Zhanjiang 524057, China
Received 25 June 2014; accepted 6 December 2014 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2015
Abstract
There are rich natural gas resources in the northwestern South China Sea deepwater areas, with poor degree of exploration. Because of the unique tectonic, sedimentary background of the region, velocity model building and time-depth conversion have been an important and difficult problem for a long time. Recent researches in this direction have revealed three major problems for deepwater areas, i.e., the way to determine error correction for drilling velocity, the optimization of velocity modeling, and the understanding and analysis of velocity variations in the slope areas. The present contribution proposes technical solutions to the problems: (1) velocity correction version can be established by analyzing the geology, reservoir, water depths and velocity spectrum characteristics; (2) a unified method can be adopted to analyze the velocity variation patterns in drilled pale structural positions; and (3) across-layer velocity is analyzed to establish the velocity model individually for each of the layers. Such a solution is applicable, as shown in an example from the northwestern South China Sea deepwater areas, in which an improved prediction precision is obtained. Key words: time-depth conversion, velocity modeling, deepwater areas, continental slope, South China Sea Citation: Liu Aiqun, Chen Dianyuan, Li Wentuo, Fan Caiwei, He Jianwei. 2015. Velocity model and time-depth conversion for the northwestern South China Sea deepwater areas. Acta Oceanologica Sinica, 34(4): 56–61, doi: 10.1007/s13131-015-0649-0
1 Introduction A vast deepwater area, covering several major sedimentary sags in the Qiongdongnan and the western Pearl River Mouth Basins, lies in the northwestern South China Sea. The water depth varies from 300 m to 3 000 m. It covers an area of 52 000 km2 with rich oil and gas resources and enticing prospect, but its exploration degree is very low. Characterized by rough seafloors, multi-phase slopes, low-velocity layers, and sag-uplift interlacing which causes the large shear velocity variations (Liu et al., 2011; Ren et al., 2011; Yu et al., 2010; Tian et al., 2006), a systematic research of velocity variations in this region is needed for regional time-depth conversion, to provide a technical support for sag evaluation and target optimization. At the present stage, velocity modeling and time-depth conversion method in shallow waters is relatively mature, with a good prediction precision. However, for deepwater areas, because of extremely high exploration cost and a lack of prior knowledge, the methods of velocity modeling and time-depth conversion represent an enormous challenge (Han and Batzle, 2005; Fruehn et al., 20
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