Near-Surface Expression of Hydrocarbon Migration

Michael A. Abrams, Dietmar Schumacher

Over the last sixty 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 which are detectable, but many are skeptical of how such information can be integrated into the more conventional exploration and development programs. Our understanding of the process of hydrocarbon migration from source rock or reservoir to the near surface is poorly understood and severely limits the interpretation of surface geochemical data. The 1994 Hedberg Research Conference entitled "Near Surface Expression of Hydrocarbon Migration" critically examined the process of hydrocarbon migration from the deep subsurface to near surface, and to document the varied expression of this migration in shallow sediments and soils.

The purpose of this paper is to review key points from 87 papers presented at the conference. The participants were in agreement that all petroleum basins have some type of near-surface geochemical leakage, but the surface expression of leakage is not always detectable by conventional means. The near surface seepage can be expressed as macro-seepage (visible hydrocarbons) or micro-seepage (chemically detectable). Two types of seepage activity exist: active (Gulf of Mexico) and passive (offshore Alaska). Several papers discussed secondary manifestations which result from the micro-seepage of hydrocarbons (microbiological anomalies, vegetation changes, precipitation of secondary carbonate minerals, radiometrics, trace elements, and changes in electrical and magnetic properties). Most of the participants believe hydrocarbon accumulations are dynamic (seals are not perfect) and that hydrocarbons can migrate vertically through 1000's of meters of strata without observable faults or fractures in a relatively short time period (weeks to years). The relationships between seepage and subsurface accumulations-generation is often complex and therefore requires integration with geological and geophysical data. There is no consensus as to the process of hydrocarbon movement into near surface sediments. The existence of macro-seeps from major migration conduits is easily explainable whereas micro-seepage depends on the ability of hydrocarbons to move through large sections of relatively impermeable sediments. Migration mechanisms discussed during the conference include diffusion, water transport, and micro-bubbles.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995