Geological Features Liwan 3-1 Gas Field In Northern South China Sea Pipeline Routing Area Disasters

With the development of the northern South China Sea offshore oil and gasresources exploration, the development of Liwan3-1gas field is of strategicimportance as China’s first deep water gas field. The submarine pipelines lying on theseabed has played an important role in transportation of the oil and gas. The seabedtopography and geological conditions were complex and unstable. Once the seabedstability was broken, the seabed pipelines would be damaged, leading to economicloss. In order to design the route of submarine pipeline and insure the security ofdeepwater oil and gas field development, it’s necessary to identify the topography,seabed conditions, stratigraphic architecture and potential geological hazards in thestudy area.Study area located in200-1700m zone of the northern part of Baiyun Sag on thedeep water slope of Pearl River Mouth Basin. Results presented in this paper wereessentially based on the analysis of multi-beam bathymetric data, multi-channelseismic profiles and side-scan image acquired in the study area. Morphology andsediment architecture of study area were studied systematically. The morphology,distribution and internal characteristics of sand waves, ancient coral reefs, scarps,landslides and submarine canyons were studied in details. Main controlling factors oflandslides and origin of submarine canyons were discussed in this paper. Potentialhazards of disaster geological factors on pipelines were analyzed.The study showed that from top to bottom of the study area can be divided intothree morphology bands. The upper part is smooth monoclinic slope, the medium partis steep fault terrace slope and the lower part is the turbidite accumulation with gentleslope. Submarine canyons accompanied by erosion landslide, collapse valley and thealluvial fan were developed in the medium part, it’s the most complex topography ofthe study area.Sand waves, ripples and ancient cores reefs developed in the northwest corner ofthe study area, sea water depth of200-420m. Activities of the sand waves not onlycaused the pipeline emptied and spanning, sand and pipe friction can also damage the pile or perforation, resulting in leakage.14submarine canyons were recognized in the range of500~1700m water depth onthe north slope of Baiyun Sag, trending in the direction of NNW-SSE or nearly N-S,and ranging from12~39.2km in length,1.1~3.4km in width, with maximum incisiondepth of234m. The eastern walls of the canyons were steeper than the western ones,with the deepest parts of the canyons close to the eastern walls. There was none or asmall amount of sediments in the upper segments of canyons and the stratigraphicreflectors tilted slightly eastward. The lower segments of canyons were located on theturbidite fan where stratigraphic reflectors were in parallel or sub-parallel mode.Faults were densely developed in the turbidite fan to the west of the C7canyon. Andtwo slides were recognized in the southwestern corner of the study area. There weremany hummocky convex on the slope hillock.Fault scarp, landside, slumps and peristaltic were distributed on the both sides ofcanyons and canyons ridge. They were obstacle for lying pipeline. The movement ofblock may impact on the pipeline, resulting deformation and fracture.