Variation in Seabed Geochemical Signature Relative to Reservoir Hydrocarbons

Michael A. Abrams¹ and Harry Dembicki^2
1 University of Utah Energy and Geoscience Institute, Salt Lake City, UT
² Anadarko Petroleum Corp, Houston, TX

Site specific seabed geochemical samples are commonly collected along major migration pathways to evaluate subsurface petroleum charge systems. Comparison of sediment interstitial gas (headspace) and hexane extracted C15 plus hydrocarbons indicate surface molecular compositions can differ from the reservoir hydrocarbons even in areas with active petroleum seepage. Several possible explanations for the discrepancies include: migration fractionation, mixing with near-surface hydrocarbons unrelated to seepage (bacterial and transported-reworked), secondary alterations (biodegradation, dissolution/waterwashing, and oxidation), and/or fractionation in sediment collection-extraction process. Recognition of these differences is very important when evaluating seabed derived geochemical data. Conventional interpretation schemes designed for reservoir fluids and gases usually do not work with surface derived seep samples.

Sediment seep gases (anomalous population) can contain gas wetness in excess of 70% (C2-C5/C1-C5) when the reservoir gases are only 20 to 30% wet. Background sediment gases (low concentration samples) also will contain samples with elevated gas wetness (greater than 25%). and are often confused with true seep gases. Carbon and hydrogen compound specific isotopes will assist in evaluating anomalous gas origin but caution should be used since carbon isotopic measurements on sediment gases can be altered by shallow microbial processes and mixed with near-surface derived gases.

Conventional biomarker parameters from sediment extracted seep samples rarely match the corresponding reservoir fluid. In part this is due to dissolvable recent organic matter (ROM) material and secondary alterations. Use of novel biomarker ratios which are less susceptible to interferences by ROM and biodegradation provide values more similar to reservoir fluids.