Gas Hydrates – Geological Perspective and Global Change Keith A. Kvenvolden 大氣所碩一 闕珮羽 R

Introduction Deep ocean and permafrost The total amount of methane hydrate likely exceeds 10 9 g of methane carbon Greenhouse gas Potential energy resource Geological hazard Global climate change

Definition Solid comprised of water molecules forming a rigid lattice of cages, each containing a molecule of natural gas, mainly methane.

Controls Temperature Pressure Composition Enormous methane Assume pure water and pure methane system

Locations

Geophysics Bottom Simulating Reflection High sonic velocity

Geophysics Wire line logs

Geochemistry

Aspects of Gas Hydrates 1.Potential Energy Resource 2.Geologic Hazard 3.Global Climate Change

1. Potential Energy Resource Attractive: (1) enormous amount of methane (2) wide geographical distribution of the gas hydrate Production methods: Thermal stimulation :ΔH dissociation 54.2kJ/mol Inhibitor injection :methanol, low efficiency Depressurization :suitable

2. Geologic Hazard Natural Anthropogenic

3. Global Climate Change During global warming, deep-sea gas hydrates become more stable, but gas hydrates of polar continents and continental shelves are destabilized, leading to methane release over long time scales.

Past climate change Uncontrolled release of methane? The release methane enhances global warming and triggers deglaciation. Limiting the extent of glaciation during a glacial cycle. Nisbet et al. Paull et al.

Present climate change Kvenvolden have suggested that gas hydrate deposits of the polar continental shelves are presently most vulnerable to climate change. The polar shelves has experience a +10°C or more change in temperature over at least the past 10,000 year. Sea level rise about 100m The amount of methane released by this process has been estimated to be about 3*10 12 g/yr of methane carbon.

A test of this idea was conducted on the continental shelf north from Oliktok Point, Alaska [Kvenvolden et al.,1992]. We discovered that methane concentrations in the water under the ice are 6 to 28 times greater than the atmospheric equilibrium concentration. Questions: destabilized gas or methane contributes to the seasonal cycle of atmospheric methane?