Natural Seepage Systems at Biloxi & Dauphin Domes & Mars Mud Volcano, North East Mississippi Canyon Protraction Area, Gulf of Mexico
Authors: Carragher Peter David , Ross Andrew 
Published: May 2013

In 2011 BP's Gulf Coast Restoration Organization undertook a project to identify and sample seven separate natural hydrocarbon seepage areas associated with salt domes within 30 kms of the 2010 hydrocarbon release from the Macondo well. In addition we investigated a more distant site at the Mars Mud Volcano 102 km to the south-west. A multidisciplinary survey approach identified areas of interest for detailed investigation using historical, pre-incident synthetic aperture radar (SAR) from satellites and geophysical data from 3D exploration seismic, multibeam bathymetry and pre- and post-incident sub-bottom profiles.

Additional SAR, X-band radar, surface slick observations and water column echosounder acoustic profiles were used to identify seafloor locations of active seepage. BP obtained further 2012 SAR data in the MC252 area that provides further insight into the Biloxi and Dauphin Dome and Mars Mud Volcano seepage sites. The effect of subsea currents on the vertical trajectory of methane bubbles and possibly on oil droplets has been mapped from echo-sounder data. A bubble rise technique was developed and has proved useful in the tracking of an oil slick origin point down to its likely seabed seepage site.

Seabed seepage sites have been confirmed by seafloor site investigation and sampling at the Biloxi and Dauphin salt domes, and the Mars Mud Volcano using a Remotely Operated Vehicle (ROV). Although only a few sites were actively seeping during the short time period of each survey, the data reveal a large number of potential natural seepage sites. Some of these were confirmed by the release of oil and gas during coring. A process model of inflation, release and collapse is presented.

The geochemistry of the investigated NE Mississippi Canyon natural seep sites shows close correlation with South Louisana Crude Oil family found in reservoirs in the vicinity of the Macondo well. Geochemical analysis of seep oils from the Mars Mud Volcano are derived from the Mars-Green Canyon Crude oil family and are less correlated with oils collected from seepage areas associated with the salt domes.

Introduction

On April 20th 2010, there was an uncontrolled hydrocarbon release from the Macondo well, leading to an explosion and fire aboard the Deepwater Horizon mobile drilling rig, and the tragic loss of 11 lives and numerous injuries. The drilling rig sank on April 22nd 2010. A significant discharge of crude oil and natural gas into the environment continued until the well was capped on July 15th 2010. The fact that the discharge was entering the environment at a water depth of 1,522 m was unique.

Under the Oil Pollution Act (OPA), the Natural Resource Damage Assessment and Restoration (NRDAR) process is used to identify natural resources that are injured by an oil spill, and restore the resources to their baseline condition, which is the condition they would be in if the oil spill had not occurred. Baseline conditions are defined, in part, by conditions that existed before the accident, in areas potentially, or actually, impacted by the uncontrolled release of hydrocarbons. Baseline also includes natural conditions, such as natural oil seepage.