Interpreting Geochemical Data From Seafloor Hydrocarbon Seeps: You Can
Authors: Dembicki Harry 
Published: May 2013

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.

Introduction

The primary objective in hunting for naturally occurring seafloor hydrocarbon seeps in exploration programs is to find evidence of thermogenic hydrocarbons to confirm the presence of a working petroleum system to reduce risk. Once the presence of thermogenic hydrocarbons has been established, the emphasis of the seep study changes to interpreting the detailed charactersistics of the seeped oil and gas, such as the isoptopic signature of the gases and the biomarkers of the oil, to determine if these hydrocarbons are related to previous discoveries in the area (Abrams et al., 2001). These data may also be used to infer some of the characteristics of the source rock (e.g. lithology, depositional setting, kerogen type, and maturity) that generated it (Seifert and Moldowan, 1981; Curiale, 2008). In order to achive these objectives, the source controlled geochemical information in the oil and gas must be preserved and clearly recognizable.

Unfortunately, biodegradation of the seeped hydrocarbons, as well as mixng with in situ organic matter in the sediments, can mask or eliminate some or all of this geochemical information. The in situ organic matter can come from a variety of sources including recent biological activity and reworking of sedimentary organic matter from source rocks, coals, and sometimes other seeps. Biodegradation is pervasive in seeped hydrocarbons at the seafloor and can affect both high and low molecular weight compounds. In the high molecular weight fraction, biodegradation may only impact the n-alkanes and isopreniods, but it can also alter some or all of the classes of the biomarker compounds. In the low molecular fraction, the composition and isotopic signature of the gases can be altered by removal material and isotopic fractionation due to microbial oxidation of the gases. Composition and isotopic signature of these gases may also be changed by methanogenesis associated with the biodegradation of the high molecular weight hydrocarbons.