Tidal Current Energy

282 E. Allison
The most significant drawback of prospecting for hydrate using a BSR is
that it provides no information about the hydrate volume. Determination of a
complete methane hydrate petroleum system and seismic attribute and travel
time analysis are the most reliable techniques for estimating methane hydrate
volumes. These advanced techniques are in their infancy, but are rapidly progressing
based on laboratory studies of methane hydrate properties, modelling
seismic properties of methane hydrate-bearing sediment and comparison of
seismic predictions with drilling.
2.2 . Petroleum system approach
Using a petroleum system approach to predict methane hydrate deposits provides
a more precise prediction of the location and volume of methane hydrate deposits.
Definition of a geological setting in which coarse-grained sediments occur in areas
with methane and within the hydrate stability zone provides a more reliable indicator
of concentrated methane hydrate deposits than BSRs. The petroleum system
includes the source rock, migration path, reservoir rock and seal. For methane
hydrate, the formation temperature and pressure, pore water salinity, and gas
chemistry must also be within the hydrate stability field. The MMS and BLM
assessments that will be published in 2008 use a petroleum system approach.
2.3 .
Types of hydrate accumulations
Methane hydrates have been identified and sampled from sub-sea outcrops
and below sediments in the Arctic and offshore. Arctic deposits are contained
in porous and permeable strata, much as are conventional hydrocarbons. In the
marine realm, methane hydrate may form mounds on the seafloor, fill steeply
dipping fractures or faults, or occupy pores of sub-sea sediments.
The critical fact guiding technology for future production is that hydrate concentrations
are greater in coarser-grained sediment. For comparison, hydrate
deposits occur in fine-grained sediments, at Blake Ridge, Atlantic offshore the
south-east USA, and in coarse-grained sediments, in the Arctic deposits in
Alaska and Canada.
At Blake Ridge, Ocean Drilling Program leg 164 found methane hydrate disseminated
in sediments and filling faults. Disseminated hydrate represented
1–5% of bulk volume. Sediment analysis showed that although essentially all
sediments are silt- and clay-sized, hydrate concentrations were highest, based
on negative chlorine anomalies in pore waters, in the coarsest grained sediments
[17] . The estimated total volume of methane hydrate at Blake Ridge is
2–3%, which is too low to consider commercial exploitation.
In the 2007 methane hydrate stratigraphic test well at Milne Point Field, Alaska,
hydrate pore saturations were 60–75% in sediments ranging from very-finegrained
sandstone to medium-grained sandstone. The Mallik 2002 well in the
Mackenzie Delta, Canada, logged about 110 m of methane hydrate-bearing strata
(sand, pebble conglomerate and minor siltstone), with hydrate concentrations