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AAPG Bulletin


AAPG Bulletin, V. 86, No. 5 (May 2002), P. 907-918.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Kilometer-scale upward migration of hydrocarbons in geopressured sediments by buoyancy-driven propagation of methane-filled fractures

Jeffrey A. Nunn,1 Peter Meulbroek2

1Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, 70803-4101; email: [email protected]
2Department of Geological Sciences, Cornell University, Ithaca, New York, 14853; email: [email protected]


Jeffrey A. Nunn received a B.A. degree in history and geology from Amherst College and an M.S. degree and Ph.D. in geology from Northwestern University. He is a professor of geology and geophysics at Louisiana State University. His research interests include fluid flow, mass transport, and associated diagenesis in the Gulf of Mexico; fluid flow and deformation on faults; heat flow, mass transport, and the evolution of foreland basins; rheology of continental lithosphere; deep migration of hydrocarbons by propagating fractures; and microfabric analysis of fine-grained rocks in geopressured basins.

Peter Meulbroek received a B.S. degree in mathematics from the University of Chicago and a Ph.D. in geology from Cornell University. He is currently working at Caltech at the Molecular and Process Simulation Center. His interests there include modeling macroscopic chemical behavior with equations of state, microscopic behavior using molecular dynamics and quantum mechanics models, and developing data-centric models for all scales.


This research was supported by the Donors of the Petroleum Research Fund, administered by the American Chemical Society and the Global Basins Research Network. Constructive comments by Bruce Hart, Deet Schumacker and an anonymous reviewer greatly improved the manuscript. Jeff Hanor and Jean Whelan provided useful comments on a previous version of this article.


Several lines of evidence support kilometer-scale upward migration of fluids in the Gulf of Mexico Basin: discharge of hypersaline brines at the sea floor; long-term, natural hydrocarbon seeps and microseeps; gas chimneys; lead-zinc mineralization in salt dome cap rocks; and allochthonous brines in Cenozoic sediments. We explore the hypothesis that upward fluid transport in geopressured sediments is caused by buoyancy-driven propagation of isolated methane-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 upward through the sediments. Hydrofracture is driven by the pressure difference (buoyancy) between the enclosed methane and the surrounding sediments. Our results show that methane-filled fractures with half-lengths of a few meters should propagate upward through geopressured sediments with velocities of hundreds of meters per year or higher. As methane-filled fractures increase in volume and decrease in density with decreasing confining pressure, they develop the potential to entrain and transport more than 1000 kg/m3 of oil or brine. Methane-filled fractures should propagate to the surface unless they are trapped beneath a layer that has high fracture toughness, such as salt, or are absorbed when they intersect a permeable (>10 md) sand layer.

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