Subsurface Vents Under the Gulf of Mexico Shelf?—Characteristics and Significance for Hydrocarbon Migration and Trapping

W. Charles Barnes
Stone Energy Corporation, Houston, TX

Mud volcanoes throughout the world erupt with mixtures of mud, brine, and hydrocarbons. As the apex of expulsion systems, these features give insight into the process of hydrocarbon expulsion and shale dewatering at depth. Many eruptions are believed sourced from overpressured shales lying at depths greater than 10,000'.

This paper explores possible roots of Gulf of Mexico styled expulsion systems. Compact structures observed on seismic data near the top of geopressure are interpreted as vents that expel fluids into the transitional and normally pressured section above. The structures are located on the downthrown sides of deeply rooted faults. Collapse topographies surrounding the vents appear created by fluid withdrawal from geopressured shales subjacent and upthrown to the vents. As fluid expulsion is often linked with fault movement, the vertical reach of conducting faults above these vents may be governed by the effective fountainhead of the ascending pressured fluids. Bright spots occasionally stream from the faults and may evidence actively migrating hydrocarbons.

The significance of subsurface vents to petroleum exploration is two-fold. First, subsurface vent structures appear to be almost always charged and often hold reserves in the range of 10 to 50 BCF. Second, subsurface vents may be important point sources of hydrocarbon migration into larger fields nearby. A better understanding of subsurface vents and their evolution may aid in predicting hydrocarbon accumulations in neighboring structures and lead to a more specific knowledge of migration pathways within a basin.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005