Could Gas Hydrate in Fine Grained Sediments be a Precursor for Some Shale Gas Deposits?

Shale gas is a major energy resource with significant commercial potential. Understanding the paragenesis of the gas is a key to exploration because gas is unequally distributed in the shale host. Presently, the gas content is regarded as being directly related to the amount of local organic carbon that may constitute up to 8 to10% of the shale. This assumption implies that gas generation took place essentially after the shale became impermeable. However, when high local gas content is not directly related to organic carbon content, it is likely that gas generation and concentration may have a more complex history.

A means of sequestering gas produced early in the diagenetic history of the shale involves the formation of solid, mechanically strong gas hydrate grains, nodules, or veins in the still highly porous, muddy shales. Hydrate concentrates gas by compressing it by a factor of about 164 into its crystalline lattice. A great deal of natural gas is sequestered in gas hydrate. Modern gas hydrate concentrations of 5 to 10% are common in gas hydrate stability zones in fine grained marine sediment sections as thick as 800 feet in continental slopes. Muddy sediments on modern continental margins are estimated to currently hold more gas worldwide than has been identified in all other hydrocarbon deposits. Without doubt, natural gas hydrates also formed in ancient sediments under suitable pressure and temperature conditions. In addition to gas that is locally produced, subjacent gas can also migrate from deeper source beds, which allows the hydrate-enriched muddy sediments to hold more gas than could be generated locally. If pressure - temperature conditions persisted during lithification of shale gas precursor, at least until packing of the clay minerals reduced permeability to a point that the gas released from hydrate by increasing temperature or decreasing pressure could not migrate easily, then a very large part of this gas would remain trapped in the shales as their further compaction proceeded. An implication for exploration is that high gas concentrations may be the result of multiple gas concentration mechanisms acting in the shales. Gas hydrate tends to form preferentially is more siliceous beds and partings, such as lower organic content grey shales and more siliceous shales, whose economic potential is enhanced because these respond well to fraccing.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California