Surface and subsurface manifestations of gas movement through a N–S transect of the Gulf of Mexico
Figure 1: Map of oil and gas seeps/chemosynthetic communities and recent petroleum discoveries (adapted from Sassen et al., 1993a and 1993b). (924.2Kb)
Figure 2: Summary of all gas seep related processes observed in the vicinity of Green Canyon area in the northern Gulf of Mexico, at the south end of the transect shown in Figure 1. (921.4Kb)
Figure 3: A three-dimensional seismic profile of a gas chimney rising from depths greater than 15,000 ft (4573m) up through Plio-Pleistocene sediments in the South Marsh Island area, offshore Louisiana. (3.485Mb)
Figure 4: Generalized subsurface profile of Gulf Coast transect where Miocene and younger sediments overlie salt (brown) and deeper, older “unknown” formations. (342.0Kb)
Figure 5: Map of occurrence of some of gas seeps described to date worldwide. New areas of seepage continue to be discovered. (1.260Mb)
Figure 6: Mixture of methane hydrates and petroleum accidentally dredged from the sea-bottom in the Canadian Cascadia margin. (777.9Kb)
Figure 7: Gas and water continuously rise up from one of numerous mud volcanoes in Azerbaijan (reproduced from Hovland & Mortensen, 1999). (6.309Mb)
Figure 8: North Sea: a violet coral and various sponges living on carbonate mound of the Haltenpipe (6.409Mb)
Figure 9: A sketch suggesting how light hydrocarbons, methane, ethane, and butane (natural gas) seep upward along dipping sedimentary rock layers and through the seafloor in the North Sea Haltenbanken area. (1.704Mb)
Figure 10: Algerian Sahara: fossilized carbonate mound, a giant coral reef-like structure, previously buried in sand. (8.343Mb)
Figure 13 (a-d): a.Typical C10+ n-alkane pattern for Gulf Coast oil that has not undergone gas washing. b. The relative amount of each n-alkane decreases exponentially with increasing carbon number (from Losh et al., 2002). c. Same as Figure 13a), for oil that has experienced gas washing (from Losh et al., 2002a). d. Relative amount of each n-alkane versus carbon number for oil in 13c). (436.6Kb)
Figure 14: Percent of C10+ depletion for Gulf Coast oils in study transect indicated in Figures 1 and 4 assuming that maturity and origin for all oils is the same as is the case for these oils (see text). (421.7Kb)
Figure 15: Gulf Coast transect: general schematic diagram of subsurface oil and gas flow consistent with geochemical, geological, and fluid flow modeling (Whelan & Eglinton, 2001b). (1.848Mb)
Whelan, Jean K.
MetadataShow full item record
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.
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.