Potential of porosity waves for methane transport in the Eugene Island field of the Gulf of Mexico basin

The Eugene Island minibasin and hydrocarbon field in the northern Gulf of Mexico basin is a well documented occurrence of anomalously rapid fluid migration. A variety of evidence suggests that hydrocarbons ascended through kilometers of very low permeability sediments separating source rocks and reservoirs at velocities up to kilometers per year. Previous research has shown that porosity waves could transport petroleum at millimeter per year velocities under a narrow range of low permeabilities. The purpose of the current research was to test the hypothesis that porosity waves could transport methane much more rapidly than oil. To test this hypothesis, a one-dimensional numerical model of porosity wave behavior was constructed for Eugene Island sediments saturated with methane. The results show that gradual rates of pore fluid pressure generation typically caused by diagenesis are too slow for porosity waves to transport methane at kilometer per year rates. Instead, essentially geologically instantaneous pore fluid pressure increases are needed, which then could allow porosity waves to ascend 1-2 km at velocities >10's of m/year. Thus, porosity waves could only reach the lower reservoirs at Eugene Island, but could transport methane orders of magnitude faster than the background Darcian flow regime. Based on their predicted size, 10's of porosity waves would have been needed to charge these lower reservoirs with their original amount of methane. Whether a mechanism for instantaneous pore fluid pressure generation exists at Eugene Island is unclear. Earthquakes are capable of generating essentially instantaneous pore fluid pressure increases of order 1's of MPa or greater, although Eugene Island's seismic history is thought to have been relatively quiet. Thus, despite their high velocities, porosity waves are unlikely to have played a major role in transporting methane at Eugene Island, but could have in other more seismically active locations where required methane transport distances are smaller. (C) 2016 Elsevier Ltd. All rights reserved.