QUANTIFICATION OF GAS HYDRATE ABUNDANCE FROM INFRARED IMAGING OF SUB-SEAFLOOR CORES

Philip E. Long¹, William Ussler, III², Jill L. Weinberger³, Michael Riedel⁴, and Anne M. Tréhu^5
1 Pacific Northwest National Laboratory, Richland, WA
² Monterey Bay Aquarium Research Institute, Moss Landing, CA
³ Scripps Institution of Oceanography, San Diego, CA
⁴ Geological Survey of Canada, Dartmouth, NS, Canada
⁵ Oregon State University, Corvallis, OR

Dissociation of gas hydrate is an endothermic reaction that cools sediment cores containing gas hydrates. Direct temperature measurements (thermistors) have been used to detect such cooling as a proxy for gas hydrate occurrence. On Leg 204 of the Ocean Drilling Program (ODP), infrared (IR) imaging cameras (FLIR SC-2000, 320X240 pixels) were used to make continuous thermal images of cores immediately after being brought on deck, but while they were still in plastic liners. Resulting images facilitated on-catwalk identification and sampling of core sections likely to contain hydrate. Temperature data extracted from the images were used to map hydrate occurrence as a function of depth at each of nine sites on or near southern Hydrate Ridge, offshore Oregon, USA (ODP Sites 1244-1252). Down-core temperature anomalies with temperature reductions ranging from -0.3 to -9°C were used to estimate hydrate occurrence and compared to other proxies such as resistivity logs using the Archie Relationship to estimate pore water saturation (S_w), chloride concentration of interstitial water, and gas composition. The IR images also provide information on cm-scale textures of hydrate occurrences. Observed textures include lenses or veins (conformable and cross-cutting), nodular, and disseminated features. Dissection of selected samples revealed that individual hydrate lenses commonly have adjacent fine (<1 mm) veinlets oriented in 2 to 3 mutually orthogonal directions.

Initial analysis of data from the IR images provides a previously unavailable means of estimating the abundance of hydrate lenses (Tréhu et al. 2004). The concentration of hydrate in the gas hydrate stability zone beneath and adjacent to Hydrate Ridge ranges from 1.2 to 5.0% by volume and averages 2.3% by volume, except for a small region near the summit, where the concentration is significantly higher, 20% to 30% by volume. A large fraction of the sites (5 of 9) have relatively low hydrate concentrations (1.4 to 2.0% by volume). IR anomaly data from five sites (1244, 1245, 1246, 1250, and 1251) indicate that the down-core distribution of hydrates within Hydrate Ridge is dependent on both lithology and structure. Silty and sandy basal layers of turbidite deposits comprise less than 1% of the overall lithologic section at every site, yet on the flanks and crest of the ridge, 32 to 65% of the identified thermal anomalies are associated with these sand horizons. Additionally, the IR data reveal a pattern of increasing up-dip concentration of gas hydrate into these sand horizons with 70% of the sand layers hosting gas hydrate at the ridge crest, decreasing to 30 to 57% on the ridge flanks, and 29% in the slope basin. This pattern is likely controlled by the complex interplay of fluid flow through vertical fractures that intersect the lateral permeable pathways presented by the sand horizons. The observed pattern identifies the lateral pathways rather than the vertical ones because our cores are oriented vertically, and are therefore likely to miss steeply-dipping structural flow conduits.

Our initial estimates of gas hydrate abundance are based on a simple parameterization of the relationship between IR anomalies and gas hydrate lenses and do not entirely account for pore-scale or microfracture-filling hydrate. Consequently, we are currently developing an approach to better quantify gas hydrate abundance based on the IR images, including estimating the amount of hydrate whose thermal signature may have been lost during coring and core retrieval. Specifically, we combine detailed temperature and pressure data from the Temperature, Pressure, Conductivity (TPC) Tool attached to the top of the Advanced Piston Core barrel (APC). These data are collected every second during the descent, coring, and ascent of each core and provide the boundary conditions for forward thermal modeling of typical cores containing varying amounts of gas hydrate. Model results are then compared with IR images of core ends to validate the relationship between gas hydrate abundance and temperature of core liner surfaces at a range of times since coring and over a range of water depths. Thermal anomalies on the surface of core liners can then be systematically interpreted using image analysis.

Preliminary analysis of core-end IR images indicates that discrete hydrate features in IR images can be treated as the two-dimensional area equivalent of volume provided appropriate corrections are made for thermal spread and for curvature of the liner surface. Estimating the abundance of gas hydrate grains less than a few mm in size will be more challenging and ordinarily would be difficult to distinguish from larger nodules that occur near the center of the core. However, two IR scans were commonly collected several minutes apart on Leg 204 cores. Gas hydrate features not in direct contact with the core liners show up as diffuse IR anomalies in the second scan, whereas disseminated gas hydrate produces a subdued IR anomaly in the first scan that thermally decays by the time of the second scan. Accurate estimate of the amount of gas hydrate whose thermal signature may have faded prior to IR imaging is even more uncertain, but can be constrained by thermal modeling. In addition to significantly improving the accuracy of gas hydrate abundance estimates from IR imaging, it will be possible to incorporate our approach into future IR imaging of gas hydrate cores, enabling real-time analysis of gas hydrate abundance during coring operations.

Reference

A.M. Trehu, P. E. Long, M.E. Torres, G. Bohrmann, F.R. Rack, T.S. Collett, D.S. Goldberg, A.V.Milkov, M. Riedel, P. Schultheiss, N.L. Bangs, S.R. Barr, W.S. Borowski, G.E. Claypool, M.E. Delwiche, G.R. Dickens, E. Gracia, G. Guerin, M. Holland, J.E. Johnson, Y-J. Lee, C-S. Liu, X. Su, B. Teichert, H. Tomaru, M. Vanneste, M. Watanabe, J.L. Weinberger. 2004. Three-dimensional distribution of gas hydrate beneath southern Hydrate Ridge: constraints from ODP Leg 204. Earth and Planetary Science Letters (in press).