Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Surface geochemistry applied to hydrocarbon exploration has gained little acceptance due to the lack of a satisfactory mechanism for vertical migration that is plausible and explains all observations. Acceptance of surface geochemistry is also hampered by a widely held belief that reservoirs do not leak. A satisfactory mechanism must apply in a wide variety of geologic environments and must be verified by laboratory and field observations. Three mechanisms are proposed for vertical migration of light hydrocarbons: diffusion, transport in aqueous solution, and buoyancy of microbubbles.

Diffusion fails to explain the rapid disappearance of surface anomalies after production from a reservoir begins. Diffusion is sufficiently rapid to dissipate gas reservoirs quickly in the geologic sense. As a vertical migration mechanism, it also cannot account for the resolution observed in surface anomalies. Diffusion as a mechanism for primary migration of hydrocarbons from source rocks and as a transport mechanism in the near-surface unsaturated zone have been demonstrated. Solubilities of light hydrocarbons in water are low at ambient temperatures, but increase dramatically with increasing temperatures at depth in basins. Transport with water, either in solution or as a separate hydrocarbon phase, is important in secondary migration. Computer modeling of the process using finite-difference techniques fails to explain the observed resolution and rapid disappearance of surface anomalies.

We favor the vertical migration mechanism of displacing water by ascending gas bubbles, that is, the "buoyancy of microbubbles." Computer modeling of this mechanism does explain surface observations. The close correspondence of surface anomalies with surface projections of a reservoir and the rapidly disappearing surface anomalies after the start of production are predicted by this model.