Appendix L

Gas hydrate stability models of core sites

Gas hydrate stability models were generated for core sites using various
data sets. Data used in the models included measured bottom water
temperature (Labails and others, this vol., ch. 6), measured geothermal
gradient (Labails and others, this vol., ch. 6), measured or estimated
gas composition (Lorenson and others, this vol., ch. 9) and measured or
extrapolated pore-water salinity (Ussler and Paull, this vol., ch. 8).
The models were generated using the CSMHYD program available in Sloan
(1998); however, this program accounts for hydrate formation in pure
systems without a porous media effect. As such, gas hydrate stability
boundaries calculated in this appendix are slightly deeper then they
would be in situ for the fine-grained clay and silty-clay sediment
present in the northern Gulf of Mexico. Each model is graphed as a
function of depth versus temperature, and the appropriate predicted
depth of the base of gas hydrate stability (BGHS) is indicated. For each
model two depths of BGHS are shown, one for pure methane gas and one for
a representative wet-gas composition. Wet-gas compositions, present at
Bush Hill and the Mississippi Canyon region, were taken from those
indicated in Sassen and others (1994), and Anderson and others (1992),
respectively. The gases are composed mainly of methane, with 0.5 to
4-mole-percent concentrations of ethane, propane, butanes, pentanes, and
carbon dioxide. Only gas hydrate recovered from core MD02-2569 was used
in the calculation of a site-specific BGHS in this report (Lorenson and
others, this vol., ch. 9; table 4).

Geothermal gradients were measured according to Labails and others (this
vol., ch. 6; table 1) where reported geothermal gradients ranged from 20
to 38 degrees Celsius per kilometer (ºC/km). For core MD02-2569, results
using two geothermal gradients of 25 and 35 ºC/km were plotted because
the geothermal gradient could not be measured. At this site (MD02-2573),
gas hydrate deposits near the surface prevented our attempts to core
deeply enough to accurately measure the geothermal gradient.

Pore-water salinities were presumed to be equivalent to those of
chlorinity measured onboard the ship as reported in Ussler and Paull
(this vol., ch. 8; table 1). The greatest depth of any measurement was
38 mbsf; thus, chlorinity values had to be extrapolated to determine the
BGHS. To do this, known chlorinity values were plotted with depth, then
a best-fit polynomial curve was generated. The equation of the
polynomial was then applied to predict the chlorinity at subbottom
depths greater than those measured.

# References

Anderson, A.L., Sloan, E.D., and Brooks, J.M., 1992, Gas hydrate
recoveries in the Gulf of Mexico---What is the shallow water depth limit
for hydrate occurrence?: Proceedings of the 24^th^ Offshore Technology
Conference, Houston, TX, 1, p. 381--385.

Sassen, R., MacDonald, I.R., Requejo, A.G., Guinasso, N.L., Kennicutt,
M.C., II, Sweet, S.T., and Brooks, J.M., 1994, Organic geochemistry of
sediments from chemosynthetic communities, Gulf of Mexico slope:
Geo-Marine Letters 14, p. 110--119.

Sloan, E.D., Jr., 1998, Clathrate hydrates of natural gases (2d ed.):
New York and Basel, Marcel Dekker, Inc., 705 p.