Massive Gas Deposits (Clathrates) in the Gulf of Mexico
LOWRIE, ALLEN, Consultant, Picayune, MS, and MICHAEL MAX, SACLANT ASW Research Centre, La Spezia, Italy
Geophysical reconnaissance of ocean basins, including the Gulf of Mexico, reveals extensive distribution of bottom simulating reflectors (BSRs), generally regarded as evidence of massive gas hydrates (clathrates). These BSR reflections represent pronounced velocity changes associated with the base of the clathrate stability zone (CSZ). Thickness of the CSZ depends on the local pressure depth and geothermal gradient. The BSRs mimic the sea-floor morphology.
Preliminary estimates by USGS geologists indicate that there may be as much hydrocarbon in the deep and adjacent margins, in the form of clathrates, as has been found so far on the continents, including coal. In the Gulf of Mexico, there could be from 200 to 2000 tcf of gas. As the global economy goes through a transition to gas based from oil based, these clathrates and related gas deposits may provide the main non-nuclear energy resources for the planet.
Clathrates are compound crystalline substances formed from
natural gas (mainly methane) and water, which are stable at the moderate to high pressures commonly found in oceans below about 400 m water depth (pressure increases at 1 atm/~10 m depth). Clathrates occur in a surface-parallel zone of thermodynamic equilibrium (CSZ) in which heat input from the warmer earth below and heat transfer into cold bottom water are in equilibrium and pressure-temperature fields in which clathrates are stable are maintained.
Clathrates can form and accumulate in any ocean sediment having suitable gases and appropriate thermodynamic conditions. Optimum conditions for formation of clathrates are found in thickly sedimented areas of the deep oceans where large quantities of gas can be produced petrogenetically in the lower part of the sediment column. Regions of high heat flow can produce more gas than comparable areas of low heat flow. A thickly sedimented area that has a high rate of sedimentation and considerable buried organic carbon debris has a high gas-producing potential. Such an area is characteristic of passive continental margins as along the Gulf of Mexico.
BSR commonly imaged on seismic reflection profiles, is an acoustic impedance marker associated with the base of the clathrate blanket. Where free gas occurs beneath the clathrate layer, a strong negative impedance contrast occurs because of the pronounced velocity and density change. The BSR is usually subparallel to the sea floor.
AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)