Permafrost

86
Relict Gas Hydrate as Possible Form of Shallow Intrapermafrost Gas
Existence
E.M.Chuvilin, S.Yu.Petrakova, O.M.Gureva
(Dept. of Geocryology, Faculty of Geology, Moscow State University, Russia)
Abstract: Investigation of gas releases from frozen sediments, carried out for different regions
of the North of Siberia, particularly of Yamal and Tar peninsulas, let talk about high gas
saturation of some horizons of permafrost within depth of 40-130 meters (Сhuvilin et al.1998,
Yakushev, Chuvilin, 2000). Accumulations of intrapermafrost gas at shallow depth are mainly
presented by methane of biogenic genesis with small contents of the other gases (carbon
dioxide, nitrogen, rarer others).
Detailed analysis of gas releases from these horizons allows suggest that evidently a part
of these accumulations are in relict gas hydrate state. It is pointed at several indirect evidences
of presence of gas hydrates in the frozen sediments above hydrate stability zone, as well as
available data of anomalous behavior of gas hydrates at negative temperatures.
The opportunity of existence of gas hydrate accumulation on shallow depth represents
special interest as they are at non-equilibrium conditions and can be serious geological hazard
at development of these areas, and also at global warming a climate due to emission of
greenhouse gases.
For research of existence conditions of relict gas hydrate formations in frozen sediments it
is important to carry out experimental investigation.
For experimental study of gas hydrate metastability (self-preservation) in frozen sediments
at negative temperature we used core samples taken from horizons of gas releases as well as
model sediment samples. These samples were placed in special pressure chamber, where they
were artificially saturated with methane hydrate and were cooled to negative temperatures close
to natural. Then non-equilibrium conditions in frozen artificially gas hydrate saturated samples
were set up by means of pressure release below equilibrium and kinetics of dissociation of
porous methane hydrate was studied at negative temperatures.
On the base of study of hydrate dissociation in time quantitative parameters of
self-preservation effect of gas hydrates in frozen samples at non-equilibrium conditions were
obtained. It was experimentally received that the increase of ice content of frozen hydrate
containing samples reduces dissociation intensity of porous hydrate and favours its
self-preservation. The increase of content of fine clay part of samples causes the increase of
decomposition intensity of gas hydrate formations and the decrease of its self-preservation
coefficient.
Generally experimental investigation of self-preservation effect of gas hydrate in frozen
sediments at non-equilibrium conditions make possible to substantiate relict gas hydrate form
existence within permafrost.
These investigations were supported by grants INTAS Nr03-51-4259 and RFBR
Nr04-05-64757.
Key words: Intrapermafrost gas, methane, gas hydrate, frozen sediment, hydrate dissociation,
self-preservation effect