Natural Gas Hydrates: Geologic Constituents from the Birth of Planet Earth to the Present
Authors: Lowrie A. , McGee T. , Lerche I. 
Published: October 2008

Natural gas hydrates are crystalline structures that combine natural gases and water under appropriate conditions of temperature and pressure. The components are found throughout the Solar System, even in interplanetary space. Thus, it is nearly certain that they were incorporated into the proto-Earth and that hydrates formed as soon as the appropriate conditions evolved. On Earth today, hydrates of methane and other hydrocarbon gases are found across polar zones and in many oceanic sediments. In highly faulted fracture zones and mid-ocean ridge provinces, heretofore undiscovered hydrocarbon hydrates may well exist in abundance.

Hydrates probably have existed throughout the operation of plate tectonics. As tectonic plates migrate into and out of polar zones and when their margins change from passive to active or passive margins are born, hydrate stability zones form and are destroyed. As new sediments are added to the sea floor, hydrates are buried and dissociate when burial becomes deep enough that they lie below the local limit of hydrate stability. They then reform when their gases migrate upward and reenter the hydrate stability zone. With gradual cooling during the Neogene, the total amount of hydrates may have increased.

Biogeochemical research has revealed that hydrocarbon gases locked in hydrate crystals provide a stable source of nutrition for bacteria. This seems to have been the case for a very long time because some of the most primitive bacterial forms are involved. They supply part of the food chain for chemosynthetic communities and one of their byproducts is the precipitation of authogenic carbonate rock. Some bacteria also provide positive feedback to the hydrate system by secreting chemicals that enhance the formation of more hydrates. Thus, natural gas hydrates seem to be inextricably linked to the evolution of life on Earth.

In Earth systems analyses, hydrates need to be included in explanations as a constant, varying in intensity, but spanning from initial formation to the present. Changes in planetary sedimentation patterns and types, and catastrophic events such as asteroid impacts, must be incorporated into an overall hydratetheory.

Introduction

Gas hydrates are crystalline minerals composed of water and certain gases. They have been described extensively (Kvenvolden, 1993a,b; Max and Lowrie, 1992, 1996; Howell, 1993; Max, 2000) and are known to occur in the Arctic (Max and Lowrie, 1992) and in sediments underlying the world's oceans (Fleischer et al., 2001, Kvenvolden et al., 1993) whether that occurrence is obvious (e.g., Hovland and Judd, 1988) or not (Lowrie et al., 1997).

Hydrates form under a particular range of temperatures and pressures (fig.1) and occur wherever the appropriate conditions of pressure and temperature coincide with the availability of water and suitable gases. Hydrates are often ephemeral (Hovland and Judd, 1988, Rowe and Gettrust, 1993a, b), their previous presence sometimes being marked by features of explosive gas release such as pockmarks and mud volcanoes. Within the world ocean, seawater becomes cooler with depth, especially below the thermocline, and pressure increases by an atmosphere (14.7pounds/in2) for approximately every ten meters of depth.