--> Progress Toward a Petroleum System Approach to Gas Hydrate Resource Assessment, by Kirk G. Osadetz and Zhuoheng Chen; #90035 (2004)

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PROGRESS TOWARD A PETROLEUM SYSTEM APPROACH TO GAS HYDRATE RESOURCE ASSESSMENT

Kirk G. Osadetz and Zhuoheng Chen
Geological Survey of Canada, 3303-33rd Street, NW, Calgary, AB, Canada, T2L 2A7

We re-examine the Beaufort Sea-Mackenzie Delta Basin (BMB) gas hydrate resource to analyze sources of variation among previous resource estimates with the intent to re-define the volumetric gas hydrate resource as a function of accumulation characteristics within the total petroleum system. The BMB provides an ideal setting to develop and test gas hydrate petroleum assessment methods. The basin hosts an immense conventional petroleum resource co-located with a potentially larger non-conventional (gas hydrate) petroleum resource (Dixon et al., 1994; Majorowicz and Osadetz, 2001). BMB gas hydrate resources were estimated previously using the inferred gas hydrate stability zone volume, discounted for an estimated frequency of gas hydrate occurrence (Smith, 2001; Majorowicz and Osadetz, 2001; Smith and Judge, 1995). However, both an environment favoring gas hydrate stability and a timely petroleum flux of appropriate composition are necessary conditions for gas hydrate accumulation. Accumulation characteristics affect recovery technology and economics making it desirable to classify the resource as a function of accumulation characteristics and setting. The assessment of gas hydrate petroleum potential is complicated additionally by gas hydrate accumulation mode and geoscience data quality and availability.

End-member Gas Hydrate Accumulation Types

Accumulation mode and characteristics suggest three end-member gas hydrate accumulation types:

  1. Type 1: Higher saturation, discrete, sediment/rock pore space accumulations commonly co-located with conventional petroleum accumulations.
  2. Type 2: Lower saturation, possibly continuous, sediment/rock pore space accumulations commonly inferred to be sourced by biogenic petroleum.
  3. Type 3: Discrete, massive seafloor/near seafloor accumulations.

Type 1 accumulations, which predominate in the BMB, are inferred to result from the clathration of migrated petroleum gases within the conventional thermogenic petroleum system. Type 2 deposits are generally marine and are commonly associated with the extensive bottom-simulating seismic reflections (BSRs). Type 3 accumulations are inferred to result from focused fluid expulsion at the seafloor. Reservoir performance studies indicate that the production technology and economics will be affected by accumulation setting and reservoir characteristics, especially the efficiency with which the forces driving dissociation and production can be transmitted to the gas hydrate. The situation is analogous to that of contact with free gas, free water, or lack thereof in coal bed methane reservoirs.

Data Sources:

The data set used to assess gas hydrate accumulations was generally collected in the course of other activities, primarily conventional petroleum exploration. As a result, the data set suffers from numerous deficiencies attributable to the age, location and type available. For example, conventional BMB petroleum exploration during the 1960’s to 1990’s resulted in 4111 logging curves from 263 wells, although the depths pertinent to gas hydrates were either not logged, or the quality of the logs is poor. Only 146 wells contribute data that can be used to infer gas hydrate occurrence and characteristics. This can be augmented by seismic velocity studies from 142 wells, which also indicate gas hydrate occurrences, some of which are not detected by well logs, due to formation damage (Brent et al., in press). Offshore wells show that gas hydrates can occur in the absence of a BSR, while critical parameters, including gas saturation and gas hydrate volume are not easily inferred from available data. The top of the gas hydrate layer is not commonly visible on conventional multi-fold reflection seismic records, whether in the sediments or at the seafloor. In permafrost regions petroleum well location and design has neither tested regions that could determine if gas hydrate occurs “off-structure” nor have engineering practices always preserved evidence for gas hydrates. Therefore gas hydrate occurrence and gas saturation are both obscured and incomplete and the historical data set is biased with respect to both occurrence and richness. More recent gas hydrate specific research provides superior characterization in local regions (Dallimore et al., 1999).

Method:

Using an approximation of the gas hydrate stability equation (Collett, 1994), the geophysical sonic and resistivity log responses of gas hydrate-bearing intervals (Miyairi et al. 1999) and the Archie approach to infer gas saturation we map gas hydrate accumulation characteristics in the BMB. Gas hydrate accumulation and reservoir parameters were evaluated in specific wells, including the calculation of inferred gas saturation, where data are available. Key gas hydrate reservoir parameters were then mapped as regionalized variables, permitting the estimation of gas hydrate pore volume as a function of gas saturation. Gas hydrate accumulations within the ice-bearing permafrost (Dallimore and Collett, 1995) were not included.

Discussion:

Observed and inferred gas hydrate bearing wells are geographically restricted to areas where ice-bearing permafrost is generally >200 m thick. Where ice-bearing permafrost is thinner, or absent, free gas occurrences are commonly interpreted at shallow depths. Within regions where gas hydrates are stable indicated and inferred gas hydrate accumulations are strongly associated with conventional petroleum accumulations, while lean accumulations or a lack of gas hydrates are associated with “dry” conventional petroleum prospects. The association is particularly clear when the conventional petroleum discoveries are compared to richer gas hydrate accumulations, where gas saturation are greater than 30% and 50%. Rich gas hydrate accumulations are also spatially associated with significant extensional structures. In general, rich BMB gas hydrate accumulations occur in three regions bounded by major fault zones that control Quaternary depositional environments, affecting ice-bearing permafrost distribution. Extensional faulting also provides a migration mechanism that allows thermogenic gases to enter the gas hydrate stability zone. Assuming a regionalized distribution of gas hydrate occurrence and accumulation parameter variations, and considering what is inferred to be a conservatively biased data-set, the estimated total gas hydrate resource in the study region is approximately 1.1 x1013 m3. If the resource is classified as function of gas saturation, the volume of natural gas in gas hydrate where average gas saturation is greater than 30% is 7.2 x1012 m3. The natural gas resource is approximately 5.0x1012 m3 in those regions where average gas saturations in gas hydrate are inferred to be greater than 50%.

Commonly gas hydrate accumulations are inferred continuous-type resources, based partly on the continuity of the BSR. If rich accumulations are sourced by migrating thermogenic gases, then it is likely that the accumulations are discrete and associated with conventional petroleum accumulations. If accumulations are, or can be characterized as discrete -- by the selection of thickness or richness thresholds, then they are amenable to play-based petroleum assessment techniques. Play-based assessments are the probabilistic analysis of geological characteristics or exploration history data that can provide undiscovered accumulation size information amenable to economic analysis. Assessments can then be formulated geographically and conditioned against both necessary accumulation conditions. Biased, incomplete data suggest that stochastic assessment methods will be valuable.

We propose to develop a stochastic, map-based, “petroleum system” approach that will recognize the roles of:

  1. Environmental controls on gas hydrate formation and stability,
  2. Petroleum system function,
  3. Extensive and intensive in situ gas hydrates characteristics, and
  4. Extensive and intensive in situ sediment/rock reservoir characteristics, all of which will control the economic and technological classification of potential petroleum supply from gas hydrate resources. 

Acknowledgements

IHS Energy Ltd graciously donated 4054 digital well log curves for analysis by this project. The check shot velocity data were kindly provided by Shell Canada Ltd. A Geological Survey of Canada, Northern Resources Development project led by Dr. Dale Issler provided a revised permafrost map as well as the pressure and thermal gradient data for this study. Ms. Kezhen Hu helped with data gathering.

References

Brent, T. A., Riedel, M., Caddel, M., Clement, M., Collett, T. S., and Dallimore, S. R., in press, 2004: Initial geophysical and geological assessment of an industry 3D seismic survey covering the JAPE X /JNOC/GSC MALLIK 5L-38 Gas hydrate research well; in: Scientific Results from Mallik 2000 Gas Hydrate Production Research Well Program, Mackenzie Delta, Northwest Territories, Canada. eds. Dallimore, S. R., et al. Geological Survey of Canada, Bulletin XXX. p. ZZZ.

Collett, T.S., 1994, Permafrost-associated gas hydrate accumulations: in International Conference on Natural Gas Hydrates, (ed.) E. D. Sloan, J. Happel, and M. A. Hnatow; Annals of the New York Academy of Sciences, v.715, p. 247-269.

Dallimore, S. R., and Collett, T. S., 1995: Intrapermafrost gas hydrates from a deep corehole in the Mackenzie Delta, Northwest Territories, Canada. Geology, v. 23 p. 237-530.

Dallimore, S.R., T. Uchida, and T.S. Collett, 1999: JAPE X /JNOC/GSC Mallik 2L-38 gas hydrate research well: Overview of Science Program; in, S.R Dallimore, T. Uchida, and T.S Collett; Scientific Results from JAPE X /JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada, Geological Survey of Canada Bulletin 544, p. 11-18.

Dixon, J., Morrell, G. R., Dietrich, J. R., Taylor, G. C., Procter, R. M., Conn, R. F., Dallaire, S. M. and Christie, J. A., 1994: Petroleum Resources of the Mackenzie Delta and Beaufort Sea. Geological Survey of Canada, Bulletin 474, 52 p.

Majorowicz, J. A., and Osadetz, K. G., 2001: Basic geological and geophysical controls bearing on gas hydrate distribution and volume in Canada. American Association of Petroleum Geologists, Bulletin, 85/7: 1211-1230.

Miyairi, M. Akihisa, K. Uchida, T., Collett, T. S., and Dallimore, S. R., 1999, Well-log interpretation of gas hydrate-bearing formations in the JAPEX/JNOC/GSC Mallik 2L-38 gas hydrate research well, in Scientific Results from JAPEC/JNOC/GSC Mallik 2L-38 Gas Hydrate Research Well, Mackenzie Delta, Northwest Territories, Canada; (ed.), S. R. Dallimore, T. Uchida and T. S. Collett; Geological Survey of Canada, Bulletin 544, p.281-293.

Smith, S. L., 2001: Natural gas hydrates; in A Synthesis of Geological Hazards in Canada, (ed.) G. R. Brooks, Geological Survey of Canada, Bulletin 548, p. 265-280.

Smith, S.L. and A.S. Judge, 1995: Estimate of methane hydrate volumes in the Beaufort Mackenzie region, Northwest Territories; in, Current Research 1995, Part B, Geological Survey of Canada, p. 81-88.