Best Practices for Detecting, Identifying and Characterizing Near-Surface Migration of Hydrocarbons within Marine Sediments
Authors: Abrams M.A. , Segall M.P. , Burtell S.G. 
Published: October 2008

Abstract
Seabed geochemical sampling programs specifically designed for detecting, identifying and characterizing the presence of near-surface hydrocarbons greatly enhance conventional exploration and production programs. In addition, these geochemical sampling programs allow operators to evaluate the genesis, spatial and temporal relationships and physical effects of gassy sediments on foundation materials for drilling and production programs. Reproducible results that can be compared within different systems in a basin and among several basins can only be obtained by using standardized methods. Routine techniques that will result in reliable, reproducible results include:pre-survey basin screening that evaluates interrelationships of sediment fill, sedimentation rates and tectonics;pre-survey basin screening that uses existing seismic surveys to evaluate the presence and character of any hydrocarbon seeps;analysis of remote-sensing data to assist the operator in locating potential seep sites;real-time core site selection based on high-resolution seismic data collected immediately prior to seabed sampling;coring programs appropriately designed for a specific environment that optimize the amount and quality of material obtained; andstandardized subsampling and analytical laboratory programs that provide maximum information on any thermogenic, biogenic or contaminant components recovered from an area.

Introduction
All petroliferous basins exhibit some type of nearsurface signal, but the surface geochemical leakage is not always observed with conventional detection methods. The presence of hydrocarbon seeps in nearsurface sediments or on the ocean surface provides evidence that three essential petroleum system elements are present:Source: Organic rich rocks capable of generating hydrocarbons have been deposited and preserved.Maturation: The source rock has reached sufficient temperatures to generate and expel hydrocarbonsMigration: The hydrocarbons have migrated from the source to a potential trap and subsequently leaked or migrated directly to the near-surface sediments. The application of near-surface prospecting methods in offshore oil and gas exploration has resulted in varied success and considerable controversy due to the complex nature of hydrocarbon leakage in marine environments1. Numerous case studies demonstrate the effectiveness of near-surface geochemical methods in offshore exploration1,2,3,4. Understanding when these methods will work, which sampling technique or analytical procedure is best suited for a particular marine setting, and most importantly, how to properly interpret the results is critical to a successful seabed geochemical survey. The purpose of this paper is to delineate key principles important for the successful use of nearsurface geochemistry in petroleum exploration and production programs.

Seepage Activity
Near-surface hydrocarbon seeps range from large concentrations of visible hydrocarbons known as macroseeps to very subtle chemically detectable concentrations known as microseeps1,5. The rate and volume of hydrocarbon leakage to near-surface marine sediments is controlled by hydrocarbon generation, migration mechanisms/pathways, and near-surface conditions. The terms active and passive5 are defined as qualitative expressions of the comparative rates at which hydrocarbons leak to the ocean floor: Active seepage describes areas where subsurface hydrocarbons actively seep in large concentrations within and above the surface marine sediments. The Gulf of Mexico's prolific ongoing leakage represents the acti