Interpreting Geochemical Data From Seafloor Hydrocarbon Seeps: You Can

Dembicki, Harry (Anadarko Petroleum Corp)


Naturally occurring seafloor hydrocarbon macro-seeps are important indicators in deepwater exploration programs. They provide strong evidence for the presence of a working petroleum system and in theory should provide insight into the contents of the subsurface reservoir, its relationship with previously discovered hydrocarbons, and some of the characteristics of the source rock that generated the oil, all before any wells are drilled. However, in practice the hydrocarbons need to be relatively intact and free of any chemical interference to accomplish these tasks and this is not always the case. There are many physical, chemical, and biological processes in the marine environment that can obscure, diminish, or destroy the geochemical information carried by seeped hydrocarbons. In the light hydrocarbon fraction (C1-C4), microbial processes such as anaerobic oxidation of methane and methanogenesis can alter both the composition and isotopic signature of the seeped gases. For the high molecular weight hydrocarbons (C12+), their concentration is an important consideration. At low concentration, these hydrocarbons can be diluted by contributions from recent organic matter, reworked source rock organic matter, and transported hydrocarbon seepage. At higher concentrations, biodegradation of the high molecular weight hydrocarbon fraction may alter or completely eliminate the biomarker compounds used in deciphering the characteristics and source of the seeped oil.
This report will discuss methods used to recognize these interferences with the geochemical information contained in seafloor macro-seepage and how best to distinguish the seep's geochemical signal from the background geochemical noise. Application of these techniques should greatly enhance the ability to utilize hydrocarbon seep data for maximum benefit.