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.

Shallow Seal Mechanisms

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

1. Max. M.D., Mienert, J., Andreassen, K., and Berndt,C., 2000, Gas hydrates in the Northern Atlantic ocean: In Natural Gas Hydrates (ed: M.D., Max), Kluwer Academic Publishers (printed in the Netherlands), 171-182.

2. Hyndman, R.D., and Davis, E.E, 1992, A mechanism for the formation of methane hydrate and sea floor bottom simulating reflectors by vertical fluid expulsion: J. Geophys.

3. Waseda, A., and Uchida, T., 1998, Generation and accumulation models of natural gas hydrates: Bull. Geological Survey of Japan, V. 49, 527-539.

4. Bily, C., and Dick, J.W.L., 1974, Natural occurring gas hydrates in the Mackenzie Delta, Northwest Territories: Bulletin of Canadian Petroleum Geology, v. 22, 340-352.

5. Katsube, T.J., and Jonnasson, I.R., 2002, Possible seal mechanisms in shallow sediments; Implications for shallow-water flow: Society of Exploration Geophysicists (SEG) Summer Research Workshop (SRW), Galveston TX, May 12-17 2002, Presentations and Abstracts, http//www.kmstechnologies.com/galveston%202002.htm, Session II (May 13), Abstarct, Summary and 12 slides.

6. Katsube, T.J., Dallimore, S.R., Uchida, T., Jenner, K.A., Collett, T.S., and Connell, S., 1999, Petrophysical environment of sediments hosting gas-hydrate, JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well: in Scientific Results from JAPEX/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, North West Territories, Canada: (ed.) S.R. Dallimore, T. Uchida, and T.S. Collett; Geological Survey of Canada, Bulletin 544, 109-124.

7. Katsube, T.J. and Williamson, M.A., 1998, Shale petrophysical characteristics: permeability history of subsiding shales: SHALES AND MUDSTONES II (ed.: J. Schiber, W. Zimmerle, and P.S. Sethi), Stattgard, p.69-91.

8. Issler, D.R., 1992, A new approach to shale compaction and stratigraphic restoration, Beaufort-Mackenzie Basin and Mackenzie Corridor, Northern Canada: AAPG, 76, 1170-1189.

9. Katsube, T. J., Scromeda, N, and Connell, S., 2000, Thicknesses of adsorbed water layers on sediments from the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research Well, Northwest Territories: Geological Survey of Canada Current Research, 2000-E5, 6p.

Figure 1. The pore-structure evolution model for the three-stage compaction process [7]: (1) Stage-I: Sand or silt grains are suspended in fine grained material. (2) Stage-II: Either the sand/silt grains have come into contact with each other forming a framework supported pore-structure (IIa), or the fine grained material has been compacted to form a matrix supported porestructure (IIb). (3) Stage-III: Pore-structure for diagenetically altered shales, with the dominant phases being cementation (IIIa) and dissolution (IIIb).