Analysis of Oil Volume Fluxes of Hydrocarbon Seep Formations on the Green Canyon and Mississippi Canyon: A Study Using 3D Seismic Attributes in Combination With Satellite and Acoustic Data
Authors: Garcia-Pineda Oscar , MacDonald Ian , Shedd William 
Published: May 2013

Analyzing the magnitude of oil discharges from natural hydrocarbon seeps is important for improving our understanding of their role in the carbon cycle and in the Gulf of Mexico (GOM) ecosystem. Hydrocarbon seepage in the deep water of the GOM is associated with deep cutting faults, generated by vertical salt movement, that provide conduits for the upward migration of oil and gas. Seeps transform surface geology and generate prominent geophysical targets that can be identified in 3??D seismic data. Seafloor amplitude anomalies in plan view correlate with the underlying fault systems.

Based on 3D seismic data, detailed mapping of the northern GOM has identified more than 24,000 geophysical anomalies across the basin. In addition to seismic data, Synthetic Aperture Radar (SAR) images have proven to be a reliable tool for localizing natural seepage of oil. We used a Texture Classifier Neural Network Algorithm (TCNNA) to process more than 1200 SAR images collected over the GOM. We quantified more than 1000 individual seep formations distributed along the outer continental shelf and in deep water. Comparison of the geophysical anomalies with the SAR oil slick targets shows good general agreement between the distributions of the two indicators. However, there are far fewer active oil seeps than geophysical anomalies, probably due to timing constraints during the basin evolution.

Studying the size of the oil slicks on the surface (normalized to weather conditions), we found that the average flux rate of oil (per seep) may be affected by the local change in the baroclinic and barotrophic pressure (e.g. warm core eddies, storms, etc). We found that oil slicks in the Mississippi Canyon protraction area tend to be more sensitive to pressure changes than Green Canyon protraction area seeps.

Introduction

In the Gulf of Mexico, hydrocarbon seepage is a natural process on the continental margin (Garcia-Pineda, MacDonald et al. 2010). Where over-pressured fluid and gas deposits exist at depth in the sub-surface, the hydrocarbons migrate vertically and laterally through the sediment column along fault planes, sedimentary bedding planes, or through permeable stratigraphic units to create " seep mounds" or " mud volcanoes" on the seafloor (Roberts 2001). These seep mounds and their associated features are of interest because they occur in areas that may contain substantial accumulations of gas hydrate, a methane ice-like substance that is both a trophic source for chemosynthetic communities and a potential alternative energy source (Sassen, Losh et al. 2001). Additionally, every offshore petroleum field development site in the Gulf must be evaluated for potential geohazards as pressurized fluid and gas deposits as well as seep features pose a threat to offshore structure installations (Roberts and Carney 1997; Fisher, Roberts et al. 2007). Finally, the productive ecology of seep communities must be protected from damage by exploration and production activities. Evaluation and identification typically occurs through geophysical means (e.g. seismic, multibeam and side-scan sonars) as these data are able to show us a great deal about the structure and geology of a seep, which may enhance our understanding of seep processes and the environmental conditions that favor their formation.