Print this pageKilometer-Scale Upward Migration of Hydrocarbons in
Geopressured Sediments by 
Buoyancy
-Driven Propagation of Fluid-Filled Fractures
NUNN, JEFFREY A.
Several lines of evidence support kilometer-scale upward migration of fluids
in the Gulf Basin: discharge of hypersaline brines at the seafloor, long-term,
natural hydrocarbon seeps, lead-zinc mineralization in salt dome cap rocks and
allochthonous brines in Cenozoic sediments. Crude oils in thermally immature
Tertiary reservoirs are generally believed to be derived from Paleocene or older
source rocks. Jurassic source rocks would require more than 10 km of vertical
migration on time scales of 1-2 m.y. Oilfield brines sampled from Pleistocene to
Eocene reservoirs in the Gulf Basin yield medium source ages of 53-55 Ma using
{129}I (Moran et al., 1995). The discrepancy between source ages and reservoir
ages implies 2-10 km of vertical migration of brine and associated hydrocarbons.
Gas chimneys above geopressured reservoirs are common features in the Gulf
Basin. These gas plumes appear to originate from depths greater than 4.5 km.
Seismic evidence indicates that hydrocarbons are not migrating up faults because
the chimneys are nearly vertical whereas the faults are at an angle. I explore
the hypothesis that upward fluid transport in geopressured sediments is caused
by buoyancy-driven propagation of isolated fluid-filled fractures. In other
words, instead of fluid migrating along a fixed network of interconnected pores
or fractures, fluid enclosed within an isolated fracture is transported upward
by hydrofracturing the mechanically weak geopressured sediments. Thus, the
fluid-filled fracture propagates upwards through the sediments. Hydrofracture is
driven by the pressure difference (buoyancy
) between the enclosed fluid and the
surrounding sediments.