Abstract

Since mercury is known to be a naturally occurring component of geologically derived hydrocarbons in the Gulf of Thailand, the Bongkot Operation Marine Environmental Monitoring concentrated on the measurement of mercury content in commercial species of demersal fishes in the vicinity of the Bongkot Operation Platform. The main species studied were groupers Epinephelus areolatus, red snappers Lutjanus sebae, sharptooth jobfish Pristipomoides typus. But in the two most recent surveys, sharptooth jobfish was not available so it had to be replaced with other species e.g. painted sweetlips Diagramma pictum.

Before late in 1997, treated produced water from the production platform was discharged into the sea. This water contained certain amount of mercury. Since then, the treated produced water was re-injected back into the depleted gas wells. And less and less direct discharge was made until in the year 2002, the volume of produced water re-injected was over 99.9%. Hence, the seawater surrounding the Bongkot Platform showed only the background level of mercury in natural seawater and fish should not be contaminated with mercury. From the results of the fish monitoring in 1997, 1998, 2000, 2002, 2004, the mercury content in fishes appear to be lower, within the natural range of values among each individual fish of the same species. The exception is painted sweetlips which showed a slightly higher average in 2004 compared to the concentration found in 2002, although the number of this species used in the analysis was much smaller than other species. Equally large numbers should be used to ascertain this issue in the next monitoring survey.

Background

The Gulf of Thailand lies on the Sunda Shelf between latitude 6° and 14° N and longitude 99° and 105° E with the maximum depth of about 84 m and a sill depth of 67 m. The first extensive oceanographic survey of the Gulf took place during the years 1958 to 1961 by the Scripps Institution of Oceanography in a joint Thai-US-Vietnam survey. After that, the political environment did not foster any cooperative marine survey until the Southeast Asian Development Center (SEAFDEC) conducted a cooperative study on oceanography of the Gulf of Thailand and Peninsula in 1995 to 1996 (1). The study also expanded into the adjacent seas of Sabah, Sarawak and Brunei, of the west coast of the Philippines, then of the coast of Vietnam in 1999 (2). These series of surveys, using modern instrumentation, gave us much detailed knowledge of the water column stratification previously unknown. Exchange of oceanographic data between the riparian countries became possible after cooperative researches were undertaken. However, offshore oceanographic surveys in the Gulf are still too few and there are great lack of data on water circulation, both offshore and near shore.

Mercury is a naturally occurring component of geologically derived hydrocarbons and the concentrations in gas and oil reservoirs varies over several orders of magnitudes depending upon location and geology. In the Gulf, the source of mercury lies in geologic formation and occurs in association with natural gas, condensate, condensate sludge and produced water. During the first few years of gas production in the Gulf, most of the mercury was removed except the very small amount which still remains in the produced water and this was the amount that was formerly released into the sea after some removal treatment. But since that last 7 years, the Bongkot Field has re-injected almost all of this water back into the depleted wells. In 2004, the efficiency had reached 99.98% and on most days there were zero discharge.

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