Eco-environmental Impact Of Shengli Chengdao Offshore Oil Exploration And Development

Bohai Sea is an important area for China's offshore oil resource, Shengli Chengdao oil field is one of the earlier developed offshore oil field in China. As the offshore oil exploration and development increased every year, its impact on the marine environment needed more concern. Domestic research about the eco-environmental impact of offshore oil exploration was still rare and there had no report about systematic research on the eco-eivironmental effect of offshore oil exploration of Chengdao offshore oil field.This paper collected the monitoring data from 1989 to 2009, including a number of indexes about water quality,sediment and biology, combining with statistical software such as SPSS, PRIMER and AMBI, using the standard index method, the composite index method, principal component analysis and other methods to analyze the water quality and organic pollution level of adjacent water, sediment quality and potential ecological risk of heavy metals in surface sediment, and also the distribution and interference status of biological community, the distribution of ecological factors and their mutual relations were analyzed as well. The eco-environmental impact of Chengdao offshore oil exploration was discussed by analysis of the annual change of all the factors and comparing the content of contaminants in the study area with that in the external area, in order to provide a reference for environmental protection of offshore oil platforms in the future.The results showed that:Chengdao offshore oil exploration activities caused a high value area of oil content in the vicinity water of platforms, content of nutrient and some heavy metals are mainly influenced by land-based sources; The nutrient content was the main factor that impact the sea water quality of study area, the average content of inorganic nitrogen in the water of study area was 0.296mg/L, the inorganic nitrogen concnetration exceeded standard value at almost all stations, the average content of phosphate was 13.7μg/L with the content exceeded the standard value at part of stations, the water showed tendency to be eutrophic; The average organic water pollution index was 1.78 and the water had begun be polluted by organics, the integrated water quality index was 1.159, the water was slightly polluted. The historical trend showed that the inorganic nitrogen content increased significantly from 0.0565mg/L to 0.296mg/L, Hg content reduced from 0.0872μg/L to 0.0307μg/L, Cu content decreased from 5.73μg/L to 2.84μg/L, the overall content of other factors changed little; the integrated water quality index fell from 1.31 to 1.16, water quality was better than the early stage of development of the offshore oil field.Chengdao offshore oil exploration caused a higher oil content in the surface sediments, the average content was 1295mg/kg, which had exceeded the standard of typeⅡsediment, the maximum value of oil was 2640mg/kg, which had exceeded the standard of typeⅣsediment; The redox properties of surface sediment was weak acid and sedimentary environment had been polluted by organics; Content of heavy metals in surface sediment was conformed to standard of typeⅠsediment, the heavy metal pollution index was 2.017 and the potential ecological risk index was 14.882, both of them were at low pollution levels, among which cadmium had the highest potential ecological risk index of 4.974; Sedimentary environment quality index was 1.944,which was within the scope of light pollution; The historical trend showed that the content of copper, lead, chromium, zinc and oil was significantly increased, the content of copper increased from 4.01mg/kg to 25.94mg/kg, the content of lead increased from 2.64mg/kg to 18.41mg/kg, the content of chromium increased from 4.47mg/kg to 28.69mg/kg, the content of zinc increased from 22.5mg/kg to 37.34mg/kg and the content of oil increased obviously from 52.05mg/kg to 1295.06mg/kg, the content of heavymetals was significantly higher than background values; With the increased amount of Chengdao offshore oil extraction, the integrated sediment quality index increased from 0.476 to 1.868, indicated that the sedimentary quality of Chengdao sea area was gradually decreased.The number of hydrocarbon oxidizing bacteria in study area ranged from 5 to 5000 every milliliter, it was higher than other coastal areas and the Antarctic area, the phosphate concent was significantly possitive correlated with the number of hydrocarbon oxidizing bacteria; The diversity of phytoplankton ranged from 1.299 to 3.16, and the abundance ranged from 1.544 to 2.592, the eveness varied between 0.28 and 0.68, due to the impact of Chengdao offshore oil exploration, the diversity and eveness of stations within 1000m from the platform was lower than those farther away from the platform; The oil discharged from the offshore oil platforms had significant influence on zooplankton community structure, the diversity of zooplankton ranged from 1.302 to 2.217, the abundance ranged from 0.886 to 2.108, and the eveness ranged from 0.542 to 0.739, the habitats of zooplankton had begun to be contaminanted;The benthic diversity ranged from 1 to 2.95, the corresponding benthic environment pollution level was mild to moderate, the AMBI value ranged from 1.05 to 3 and the benthic community of most stations were slightly disturbed, owing to the offshore oil exploration of Chengdao oil field, the disturbance of benthic community within the mixing area of the platform was more serious; Among all the environmental factors, oil content in surface sediment had maximal effect on benthic community, which was in line with the case that oil content in surface sediment exceeded standard value seriously. The historical trend showed that benthic biomass decreased obvioulsy from 12.98 to 2.16, the benthic environment declined with the decrease of sedimentary quality as the increase of offshore oil exploration.The integrated environmental quality of each station in study area was less than 1, thus the comprehensive environmental quality was at a good level.