Massive Gas Deposits in the Deep Sea
Authors: Lowrie A. 
Published: October 2008

Natural gas clathrates occur widely in the world's oceans. Methane and other natural gases are concentrated within the Clathrate Stability Zone (CSZ) that extends from the sediment surface downward to a depth determined by local thermodynamic pressuretemperature conditions. CSZ thickness is known to be about 600 m in about 3 km Water depth at average heat flows. Because the clathrate can occur as an impermeable blanket that has considerable mechanical strength, it can seal off porosity and permeability and form gas traps in the deep oceans in the upper 1 km of marine sediments. Traps can be formed from clathrate alone with closure of the CSZ following bathymetric highs, or from an interaction of clathrate and geological trap. In both cases, exploration and drilling strategies will be somewhat different from those normally applied to classical geological gas and petroleum deposits.

Introduction

Reserves of combustible energy on the Earth are being drawn down at ever increasing rates. The present oil-based economy will be replaced by a natural gas-based economy as early as the first or second decade of the 21st century, or earlier in the event of unfortunate economic Or political circumstances. Coal has its place in energy generation, but it suffers from strongly negative environmental attributes as does nuclear energy. Next to hydrogen fuel, natural gas is the least environmentally negative combustible fuel because it releases fewer and more chemically simple exhaust gases. Unlike many non-combustable energy sources, such as wind, solar, or tidal power generation, gas can be easily stored and provides a high energy source on demand. Vast quantities of natural gas exist in .the deep oceans. The extent of its presence is only beginning to be appreciated. This gas is held in the form of a water-gas crystalline solid in a zone of thermodynamic equilibrium in the upper most marine sediments, and in associated free trapped gas. Shallow gas deposits in the deep oceans have not been explored or evaluated seriously to date by anyone. These shallow gas deposits in the world's oceans are probably the next major combustible energy reserve (Kvenvolden and Cooper, 1987) to be exploited. It is known that recovery of gas from beneath a clathrate trap results in the recovery of some of the gas held in clathrate, through the depressurization technique (Sloan, 1990). This paper will treat only recovery of gas from existing gas deposits trapped at least in part by solid clathrate. The problems of maintenance of reservoir stability with respect to maximum draw-down rates, etc., are considered to be new petroleum engineering topics that we do not deal with here. Our concern is to highlight the nature of clathrate traps and discuss exploration and drilling strategies that will be necessary to exploitation of these deep-sea energy reserves.

Natural Gas Clathrates

Clathrates are compound crystalline substances formed from natural gas (mainly methane) and water, which are stable at the moderate to high pressures commonly found in oceans below about 400 m water depth (pressure increases at an atmosphere per z10 m depth).