Risk Assesment And Analysis Of Geological Oil Spills Related To The Neotectonic Movements In Bohai Sea

Bohai Sea is the sediment and subsidence center of the Bohai Bay Basin since Oligocene.It is prosperous in oil and gas reserviors.The Neotectonics in this area is severe since Neogene,and this tectonic movement controls the final distribution of the reservoirs and oil fields.But on the other side,it may also bring some danger of oil spills,such as the PL19-3 accident.In this thesis,on the basis of fully digested predecessors' research achievements and collected materials,the author assesed the possibility of geological oil spills related to the Neotectonic movements in Bohai Sea after researching the natural earthquake events,interpretating the high-resolution shallow seismic profile in Bohai Sea,combined with the distribution of oil and gas reservoirs in the study area.The natural earthquake events happened in Bohai Sea and its adjacent regions show us that the distribution of earthquakes is controlled by the fault zones,especially the cross areas.And the earthquakes mainly concentrated in two periods: 1540-1640 and after 1870.According to the repeatability principle and the tectonic analogy principle,five potential earthquake areas are figured out: Bonan Low Uplift-Bodong Low Uplift,Shanan Sag-Bonan Sag,Shaleitian Uplift,Tanggu District,northeast of Laizhou Bay,which are also the areas that most affected by the earthquake accidents.The tectonic interpretations of high-resolution shallow seismic profile in Bohai Sea shows that abuandant high-angle normal faults developed in Pleistocene,which means neotectonic movements are very active at that period.These faults are mainly inherited the Neogene fault development,especially at the borders of the sags or uplifts.Pliocene faults are almost double of the Miocene faults,half of them are inherited,half are new born because of the active neotectonics.After calculating the densities of Quaternary faults developed in different lines,we can get the faults distribution map.From the map,three high-frequency faults-developed zones are figured out: the border between Bonan Low Uplift and Bozhong Depression,Bodong Sag-Miaoxi Uplift,the border between Nanbao Sag and Shaleitian Uplift,which means that the neotectonics are very active in these areas.Compared to these areas,Shijiutuo Uplift has a lower faults density,thus can be regarded as relative high-frequency faults-developed zone.After obtaining the potential earthquake areas and high-frequency faults-developed zones,the author found three potential high-risk geological oil spills areas combined with the distribution of oil and gas reservoirs in Bohai Sea.They are Bonan Low Uplift,Shaleitian Uplift and its south area,Bodong Low Uplift-Miaoxi Uplift.One possible high-risk geological oil spill area is Shijiutuo Uplift and Qinnan Sag.To study the fracture activity in the high-risk geological oil spill areas related to the neotectonic movements,the author chose two seismic lines to calculate the activity rate,which is about 0.025 mm/a on average.Based on this rate,all the Quaternary faults can be regarded as active faults.And the vertical activity rate of Zhangjiakou-Penglai fault zone is bigger than that in Tanlu fault zone.The opening pressure of the faults is 12.9 MPa and the breaking pressure of the layers is 15.9MPa,which means that the flowing pressure after water injection should be no more than 15.9 MPa for the safety of the production.