AbstractsEarth & Environmental Science

Distribution and characteristics of gas hydrates in shallow sediments of pockmarks and seepage sites

by Jiangong Wei




Institution: Universit├Ąt Bremen
Department: FB5
Degree: PhD
Year: 2015
Record ID: 1115768
Full text PDF: http://elib.suub.uni-bremen.de/edocs/00104224-1.pdf


Abstract

Cold seeps are regions where fluid migrates from deep subsurface and escapes to the water column through the sea floor. Such fluids are usually rich in low molecular weight hydrocarbons, mainly methane, with small amount of other gases, such as carbon dioxide and hydrogen sulfide. In areas of water depth greater than 300-600m, gas hydrate could form depending on the bottom water temperature and geothermal gradients. Formation and decomposition of gas hydrate potentially change the properties of marine sediment. Therefore, in order to understand the evolution and morphological changes of marine cold seeps, it is important to know the distribution of gas hydrate in shallow sediment. Gas enclathrated in hydrate also contains important information and is helpful to understand the source of the gas. One aim of the thesis is to understand the distribution of gas hydrate within the sediment of a pockmark field at the Nigerian continental margin by using infrared thermal scanning and pore water chloride concentration. Both methods show similar hydrate distribution with different resolutions. By using coring information, the seismic profiles obtained in 2008 were calibrated to show the overview of free gas accumulation and hydrate distribution in the whole pockmark field. Elevated geothermal gradients were observed in the center of a pockmark where gas flares in the water column were imaged by hydroacoustic method, indicating the pockmark was active. The interaction among the fluid flow, hydrate formation and dissolution, and the thermal regime governs the formation and evolution of the pockmarks in this area. Knowledge of gas composition and crystal structure of natural gas hydrates is important for determining the stability of hydrate, which is a key factor to understand the fate of methane gas bubbles and hydrate bulks in the water column and the size of the hydrate reservoir in marine sediment. Methane is the dominant gas of hydrate samples recovered from pockmarks at the Nigerian continental margin, indicating that the gas is mainly biogenic. Besides methane, hydrogen sulfide and carbon dioxide were also detected using both Raman spectroscopy and gas chromatography. The thickness of gas hydrate stability zone (GHSZ) increases significantly due to the existence of hydrogen sulfide and carbon dioxide although their contents are relatively low. Hydrogen sulfide containing hydrate has only been discovered at Hydrate Ridge and Niger delta. It is a big surprise to observe hydrogen sulfide within the samples from Nigerian continental margin. In order to know whether such type of hydrate is restricted only in specific areas, we did additional Raman spectroscopy analysis on the hydrate samples recovered at cold seeps from different areas, including the Black Sea and the Makran continental margin. Results show that both samples contain hydrogen sulfide. Therefore, we infer that hydrogen sulfide containing hydrate is not restricted to certain area, but widely occurs within seepage sites where intensive upward methane flux exists…