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AAPG Bulletin, V.
Evaluation of possible gas microseepage mechanisms
Alton Brown1
1ARCO, 2300 West Plano Parkway, Plano, Texas 75075; email: [email protected]
AUTHORS
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.
ACKNOWLEDGMENTS
ARCO is gratefully acknowledged for release of this article. I thank ARCO reviewers Mark McCaffrey, Paul Willette, and Ron Gajdica for help with clarity and scope. I also thank AAPG reviewers Alain-Yves Huc, Ronald Klusman, and Peter Gretner for constructive reviews of the original manuscript.
ABSTRACT
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.