Over the past 60 years,
numerous direct and indirect hydrocarbon 
exploration
methods have been
developed. The application of these surface prospecting methods to oil
and gas exploration has resulted in varied success and considerable controversy.
Few question that hydrocarbons migrate to the near surface in amounts that
are detectable, but many are skeptical of how such information can be integrated
into more conventional exploration and development programs. Our understanding
of the process of hydrocarbon migration from source or reservoir to the
near surface is poorly understood and severely limits the interpretation
of surface geochemical data. The past decade has seen a renewed interest
in this topic which, when coupled with developments in analytical and interpretive
methods, has produced a new body of data and insights in this area.
This publication is a direct
outgrowth of the AAPG Hedberg Research Conference held in April 1994 entitled
"Near-Surface Expression of Hydrocarbon Migration." The purpose of this
research conference was to gather international experts from industry and
academia to critically examine the process of hydrocarbon migration and
its varied near-surface expressions. The wide range of topics discussed
is reflected by the papers selected for inclusion in this volume: near-surface
manifestations of hydrocabon migration, hydrocarbon-induced alteration
of soils and sediments, migration mechanisms, hydrocarbon flux measurements,
sampling and analytical techniques, survey design and interpretation, physical
and geological implications of hydrocarbon leakage, and finally, exploration
case studies.
Conference participants engaged
in lively discussion, and despite a lack of consensus on a number of topics,
there was general agreement on the following conclusions:
*
Hydrocarbon accumulations are dynamic; seals are imperfect.
*
All petroleum basins have some type of near-surface hydrocarbon leakage.
*
Surface expression of leakage is not always detectable by conventional
means.
*
Hydrocarbon seepage can be active or passive, and it can be visible (macroseepage)
or only chemically detectable (microseepage).
*
Seepage expression, whether active or passive, is a function of many factors
other than the mere presence or absence of active hydrocarbon generation
and migration.
*
Migration occurs mainly vertically, but it can also occur over long distances
laterally.
*
Hydrocarbons can move vertically through thousands of meters of strata
without observable faults or fractures in a relatively short time (weeks
to years).
*
Relationships between surface geochemical anomalies and subsurface accumulation
can be complex; proper interpretation requires integration of seepage data
with geological, geophysical, and hydrological data.
*
Hydrocarbon migration mechanisms are still poorly understood. Present evidence
favors effusion as the process of macroseepage and bouyancy of microbubbles
as the mechanism for microseepage.
It is our hope that the information
and ideas presented in this volume will assist the formulation of more
effective exploration and development strategies by providing a better
understanding of hydrocarbon migration and its near-surface effects. Only
through a fuller understanding of these processes can surface exploration
technology achieve its full potential, a goal increasingly important as
our industry strives to improve exploration efficiency during these times
of economic uncertainty.
