Surface and subsurface manifestations of gas movement through a N–S transect of the Gulf of Mexico

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Methane
Hydrate
Seep
Migration
Petroleum
Gas
Biodegradation
Abstract
Large volumes of gas have vented through a north-south transect of the offshore northern Gulf of Mexico. An overview of surface and subsurface manifestations of this gas venting is presented. This gas movement has caused extensive alteration of reservoired oils to the north of the transect which are estimated to have equilibrated with, or been gas washed by, as much as 30 volumes of gas for every volume of oil. This gas washing entrains and carries upward the most volatile oil components depositing them in either shallower reservoirs or venting them to the overlying sediments and the water column. A significant amount of this gas bypasses the reservoirs and vents upward into the overlying sediments and waters. In spite of the significant amounts of the gas involved, the venting at the seafloor appears to occur primarily through highly localized faults and fractures. This gas discharge is spatially and temporally heterogeneous, making it difficult to estimate the actual hydrocarbon fluxes involved. This upward gas movement leaves characteristic signatures at the sediment water interface including carbonate pavements in older seep areas, and chemosynthetic biological communities, methane hydrates, and gas seeps in more recent long-term seep areas. In some cases where gas venting is very recent, massive disruption of surface and subsurface sediments is observed to be occasionally accompanied by mud volcanoes. Venting can be vigorous enough to produce methane gas bubbles, which appear to be injected rapidly into surface waters and which may constitute a significant source of methane, a greenhouse gas, to the atmosphere. In the northern Gulf of Mexico, gas venting is sometimes accompanied by natural oil slicks at the sea surface, which can be tracked for many miles in non-productive areas. These gas-venting signatures are not unique to the Gulf of Mexico; similar seep features are observed in sediments worldwide. The widespread occurrence of these seep features, which may or may not be related to subsurface oil and gas deposits, may explain why use of surface seeps has often proved to be so controversial in oil exploration. Indeed, most seeps are probably not linked with economic subsurface petroleum reservoirs. The relationships between surface seep features and productive subsurface reservoirs along a N-S transect of the Northern Gulf of Mexico are presented as an example of how all surface and subsurface geochemical, geological, geophysical data might be used together to better constrain interpretations regarding the nature and dynamics of subsurface oil and gas deposits and their plumbing in frontier areas.
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Author Posting. © The Authors, 2004. This is the author's version of the work. It is posted here by permission of Elsevier B. V. for personal use, not for redistribution. The definitive version was published in Marine and Petroleum Geology 22 (2005): 479-497, doi:10.1016/j.marpetgeo.2004.08.008.
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