Organic Geochemistry of Continental Margin and Deep Ocean Sediments Page: 4 of 51
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vi) abundant and vigorously growing ocean bottom biological communities similar to those found in
hydrothermal vent areas that depend on the venting oil and gas as their food source.
vii) a potential source for the large pools of dissolved organic carbon, DOC, that are too old (5000
years) considering their probable recent, marine sources (less than 200 years; Druffel et al., 1992;
Wang et al., 2001, Whelan 1997).
viii)a potential source of the green house gas, methane, venting from the ocean floor to the
atmosphere (Kvenvolden and Lorenson, 2001 & 2004)
This work began with research on the highly productive EI-330 oil and gas province, considered
to be one of the giant oil and gas fields of the world (Carmalt and St. Johns 1986) and has now been
extended to a N-S transect across the Northern Gulf Coast shelf and slope (Fig 1) .
(Summary source and maturity variations thru transect)
Fluid compositions throughout the transect are highly variable with different sands producing a
variety of admixtures of gas, condensate, and oil despite fairly homogeneous biomarker compositions
(Figures 2 and 4), indicative of a relatively homogeneous Early Cretaceous or Jurassic source (Whelan
et al., 1993, 1995, and 2001 and Whelan et al., 2005). Recent modeling combined with organic
geochemistry showed that oil generation probably took place in the Miocene from rocks currently
buried directly beneath their present day Pleistocene reservoirs (Erendi, 2001). Therefore, oil
generation probably occurred considerably before the time of formation of the present day Pleistocene
reservoirs and was initially trapped beneath salt sheets as shown via basin modeling of the area. More
recently, further rapid sediment deposition caused the salt to flow so that the trapped oil was able to
flow rapidly upward through holes in the salt into present day reservoirs, a process that is probably
still in progress at the present time in southern portions of the transect including EI-330 and Green
Canyon 184 (Fig 1; Erendi, 2001). Salt movement is very dynamic in this area during sediment
loading, so that recent development of holes through the salt as it flows and moves would have
allowed oil and gas to flow upward into Pleistocene reservoirs. This scenario is consistent with the all
of the organic geochemical data assembled by our group and with the basin and fluid flow modeling
carried out by the Cornell group. Thus the oil maturities and sources previously deduced from organic
geochemical biomarkers (Whelan et al., 1994) that were previously very puzzling have now become
compatible with the most recent Cornell basin and fluid flow modeling showing oil and gas generation
to be much deeper than previously thought. Furthermore, the Woods Hole organic geochemical data is
also very compatible with the idea that oil and gas became temporarily trapped beneath salt prior to its
migration upward into modern reservoirs.
The combined work of the Woods Hole and Cornell groups has now also demonstrated a number
of surprising effects caused by the rapid upward movement large volumes of gas. Oils in several
intervals show evidence for extensive alteration via flushing and equilibration with multiple volumes
of gas (Meulbroek et al., 1998; Whelan et al., 2001; Losh et al, 2002a). The gas involved in this
process probably comes from deeper and more mature intervals than the oil (Whelan et al., 1993).
However, the mechanics of the process are currently a mystery because oil and gas that are separated
in the subsurface are difficult to remix at subcritical temperatures and pressures. For oil and gas to
mix efficiently, they may have to be generated together. However, our previous measurements show
gas maturities to be significantly higher than those of the associated oil (Whelan et al., 1993).
Alternatively, if gas and oil are generated separately, then some as yet uncharacterized mechanism
must exist for remixing the gas and oil in the subsurface. Our previous work with the Cornell along
with the current data suggests the existence of mixing chambers in isolated subsurface pressure cells
that persist below subsurface depths of about 10,000 ft. (Hunt et al, 1998; Hunt 1996 ; Whelan et al,
1997). At these pressures and temperatures, methane can act as a supercritical fluid that can dissolve
large volumes of oil (Whelan et al., 1997).
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Whelan, Jean K. Organic Geochemistry of Continental Margin and Deep Ocean Sediments, report, October 17, 2006; United States. (https://digital.library.unt.edu/ark:/67531/metadc883306/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.