Significance of hydrocarbon seepage relative to petroleum generation and entrapment

Near-surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source (organic matter type), maturation (organic maturity), migration (migration pathway delineation), and in selected geologic settings, specific prospect hydrocarbon charge. All petroliferous basins exhibit some type of near-surface signal, but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods. Understanding the Petroleum Seepage System, hence petroleum dynamics of a basin, is key to understanding and using near-surface geochemical methods for basin assessment and prospect evaluation. The relationships between near-surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex. The petroleum seepage system contain four key elements: seepage activity (qualitative expressions of relative leakage rates, active vs passive, and episodic vs continuous), seepage type (concentration of migrated thermogenic hydrocarbon relative to in situ material, macro vs micro), migration focus (major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and/or entrapment), and near-surface seep disturbances (near-surface processes which can greatly alter or block seepage signals). The rate and volume of hydrocarbon seepage to the surface greatly control near-surface geological and biological responses, and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively. To properly predict subsurface petroleum properties, interpretation of near-surface geochemical data must recognize many potential problems including recent organic matter input, transported hydrocarbons, bacterial alteration, mixing, contamination, and fractionation effects. Surface geochemical data should always be integrated with other geological data. Calibration datasets to determine the utility of near-surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge. (c) 2005 Elsevier Ltd. All rights reserved.