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POSSIBLE SEAL MECHANISMS
IN SHALLOW SEDIMENTS: AND THEIR IMPLICATION FOR GAS-HYDRATE ACCUMULATION
T. J. Katsube*, I. R. Jonasson*, T. Uchida**and S. Connell-Madore*
* Geological Survey of Canada, 601 Booth Street, Ottawa, ON, Canada K1A 0E8
** JAPEX Research Center, 1-2-1 Hamada, Mihama, Chiba, 2610025 Japan
Introduction
Hydrocarbon reservoirs usually exist below good seals, such as tight shale formations with permeabilities of <10-20 m2. For mudstones to become tight shales, as such, they normally need to experience maximum burial depths of at least 2000 m’s (Figure 1). Many marine gas hydrate formations in the world [1] exist at depths less than 400 m’s below the sea floor [2,3]. Gas hydrates are stable only within a certain pressure-temperature regime [4], which suggests depths greater than 250 m’s for methane gas [4]. Does this not contradict the actual facts?
A recent study [5] proposed several possible shallow seal mechanisms
, suggesting the
possible existence of moderate seals (<10-19
m2) just below the sea floor, and that their
permeabilities may be sufficient to cause an increased pressure gradient for up-flowing gases to
bring them into the gas hydrate stability zone. If true, this could be the trigger for gas hydrate
formation growth even at shallow depths below the sea floor. Following an introduction to
several shallow seal
mechanisms
, this paper discusses their impact on gas hydrate formation,
texture of gas hydrate formations, gas productivity from gas hydrates, and whether evidence of
shallow seals and their impact can be seen.
Recent petrophysical studies [7] provide a basis for a number of possible shallow seal
mechanisms
. This does not exclude the possibility of a low permeability formation formed at
great depth (>2000 m) and then uplifted to a shallow depth forming a conventional good shallow
seal. A recent study [5] suggests that a good seal (<10-20
m2) consisting of thin laminae (1-3 mm) of sulphide and carbonate cement seen in the Mallik 2L-38 Well in northern Canada, at a depth
of 800 to 900 m depths, [6] is evidence of a seal originally formed at shallow depths below the
sea floor, due to sulfate reduction of sea water and oxidation of methane gas by bacterial activity
(Figure 2). Another study [7] has shown that permeabilities of unconsolidated clay-rich
sediments can be reduced to values of less than 10-19
m2 at effective pressures of less than 5 MPa, implying that moderately good seals could be formed under relatively small overburden pressures
(<500 m depth). Overpressured fluids, due to rapid sedimentation [8], bleeding into lower
formations could retard the flow of upward flowing gases and act as a shallow seal. A recent
study on pore surface adsorbed water [9] shows decreased temperature resulting in increased
adsorbed water layer thickness, causing permeability reduction and becoming another source for
shallow seals.
Gas Hydrate Accumulation and Gas Productivity
One scenario for gas hydrate reservoir
formation is the cooling (such as from under ice
caps) of conventional gas reservoirs [6] which were, originally, formed under conventional good
seals at considerable depth (>2000 m) and then uplifted to a shallow depth (<1000 m). Another
and likely scenario for gas hydrate
reservoir
formation is the increased pore pressure of free
upward flowing gas due to decreased permeability of the poor to moderate shallow seals in the
overlying sediments. This could move the temperature-pressure regime [4] of the upward flowing
gas into the gas-hydrate stability zone which would result in growth of gas hydrates.
Subsequently, these gas-hydrates could be buried to greater depth, such as those seen today in the
Mallik Wells (800-1400 m depth), a sub-arctic regime [6].
Stable textures of framework support or matrix support are reached at depth of >2000 m (Figure 1) under normal burial conditions [7]. Unstable texture could be a mixture of various grain-sizes (sand, silt and clay). Gas hydrates forming conventional gas reservoirs are likely to be pore-space gas-hydrates (Figures 3a and 3c). Little expansion of the frozen texture would be expected if gas hydrates were formed at reasonable to considerable depth. On the other hand, gas-hydrates formed under shallow seals could grow and expand since they would not have to overcome excessive overburden stress conditions. Subsequently when buried to greater depth, ice or gas-hydrate supported texture would develop (Figure 3b), in this case. Gas from gas hydrates within framework supported textures is likely to be produced without any changes in texture, implying stable gas productivity. On the other hand, production from gas hydrates within an ice or gas hydrate supported texture (Figure 3b) would be unstable, due to possible changes occurring in the texture with production. That is, the connecting pores of the gas hydrate or ice supported texture could collapse and reduce the permeability during gas extraction from the formation.
Conclusions
Contrary to the conventional belief that hydrocarbon reservoirs require good seals, this
study suggests that poor to moderate seals may act as a trigger to the formation of good seals
when associated with the temperature-pressure regime of the gas hydrate stability zone. This
implies that a number of shallow seal mechanisms
exist that could allow gas hydrate
accumulation to occur at shallow depths below the sea floor. This does not exclude the
possibility of gas hydrate reservoirs existing under uplifted conventional good seals. A point of
vital importance in relation to shallow seals is that stable texture of the underlying formations,
such as framework supported textures, could be replaced by unstable textures of ice or gas
hydrate supported textures (Figure 3b). These could collapse during gas production, resulting in
unstable and poor gas production characteristics. In the Mallik Research Wells (northern Canada
[6]), some general association is seen between increased gas-hydrate content and increased
effective (
E) and storage porosities (
S), as would be expected. However, there are intervals
where connecting porosity (
C) is more closely associated with the increased gas hydrate
content. This is interpreted to be possible evidence of the existence of
gas-hydrate supported texture which would be unstable during gas hydrate production.
References
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