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

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.
 

Dietmar Schumacher

Michael Abrams

We sincerely appreciate the assistance given by the following individuals who generously provided critical reviews of the manuscripts published in this volume:

We also wish to thank the editorial and production staff of AAPG, particularly Anne Thomas and Kathy Walker.
 

Dietmar Schumacher and Michael Abrams