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1 ARCO, 2300 West Plano Parkway, Plano, Texas 75075; abrown{at}arco.com
Alton Brown has worked with the exploration research group at ARCO for the last 20 years on various topics of carbonate sedimentation and diagenesis, petroleum migration, hydrodynamics, and inorganic gas geochemistry. He is now retired.
Petroleum microseepage anomalies over petroleum accumulations are commonly explained by rapid, vertical migration of colloidal gas bubbles through fracture networks. This article is a theoretical anal ysis of this mechanism and of continuous gas-phase flow in frac tures. The gas-bubble ascent mechanism is much slower than re ported microseepage velocities, so it cannot account for observed microseepage. In contrast, continuous gas-phase flow through frac tures can equal or exceed reported microseepage velocity, while maintaining total flux low enough so that petroleum accumulations can exist for geological lengths of time. Fracture entry pressure for bubbles is more than twice that of a continuous gas phase, so con tinuous gas-phase migration also requires a lower pressure threshold before initiating seepage. Vertical microseepage is therefore best explained by the same mechanism interpreted for macroseepage.
Although this article provides a theoretically justified mecha nism for microseepage, it also shows why interpretation of surface microseepage signals is problematic. Fracture geometry controls seepage velocity and flux, so geochemical anomalies may indicate an increase in fracture aperture, as well as possible subsurface ac cumulations. Larger fractures require very low gas capillary entry pressures, so in some settings, surface seepage could result from fractures over stratal migration pathways, as well as over petroleum accumulations.
This article has been cited by other articles:
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  J. Cartwright, M. Huuse, and A. Aplin Seal bypass systems AAPG Bulletin, August 1, 2007; 91(8): 1141 - 1166. [Abstract] [Full Text] [PDF]
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 H. M. A. van der Werff, M. F. Noomen, M. van der Meijde, and F. D. van der Meer Remote sensing of onshore hydrocarbon seepage: problems and solutions Geological Society, London, Special Publications, January 1, 2007; 283(1): 125 - 133. [Abstract] [Full Text] [PDF]
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W. Harbert, V. T. Jones, J. Izzo, and T. H. Anderson Analysis of light hydrocarbons in soil gases, Lost River region, West Virginia: Relation to stratigraphy and geological structures AAPG Bulletin, May 1, 2006; 90(5): 715 - 734. [Abstract] [Full Text] [PDF]
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 G. Ciotoli, G. Etiope, M. Guerra, S. Lombardi, G.A. Duddridge, and P. Grainger Migration of gas injected into a fault in low-permeability ground Quarterly Journal of Engineering Geology and Hydrogeology, August 1, 2005; 38(3): 305 - 320. [Abstract] [Full Text] [PDF]
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A. Gay, M. Lopez, P. Cochonat, N. Sultan, E. Cauquil, and F. Brigaud Sinuous pockmark belt as indicator of a shallow buried turbiditic channel on the lower slope of the Congo basin, West African margin Geological Society, London, Special Publications, January 1, 2003; 216(1): 173 - 189. [Abstract] [PDF]
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Kilometer-Scale Upward Migration of Hydrocarbons in Geopressured Sediments by Buoyancy-Driven Propagation of Methane-Filled Fractures AAPG Bulletin, May 1, 2002; 86(5): 907 - 918.
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