Natural Oil Seepages : Detection, Monitoring and Relationships with Submarine Morphologies

Detection of hydrocarbon shows in marine areas is of primary importance for oil and gas exploration since they confirm hydrocarbon generation and prove the presence of an active petroleum system. The use of spaceborne Synthetic Aperture Radar (SAR) images serves as an ideal technology for the imaging of hydrocarbon seeps as it is cost effective, provides large ground coverage with continuous acquisitions and operates day and night and in all weather conditions. Here, we present results on the interpretation of radar images for seepage detection on the West African margin. Long-term monitoring of 150 SAR scenes during 20 years allowed the recognition of more than 1400 oil seepages.

Seabed morphologies associated to oil leakage correspond to clusters of small sized pockmarks, 50 to 200 m in diameter, and high-reflectivity mounds. The correlation between the location of the impact point of the oil plume at the sea surface with the seabed features reveals that oceanic drift of the oil is less than 1000 m through a water column of 1800 m, with a rising speed of 10 cm/s.

In order to address the question of the seeps lifetime, we set up a short-term monitoring through the acquisition of one radar scene every 12 hours during 10 days in a specific area recognized for active oil leaking. Our main observations are: (i) the number of detected seeps varies spatially and temporally, and (ii) oil seep dissipation is effective in less than 12 hours. The variation of the hydrostatic pressure in the water column associated to the oscillation of the tide has been firstly considered as a possible mechanism controlling the expulsion of oil at the seafloor in relation with the number of seeps detected on each image. However, the correlation between the regional mean wind field and the amount of oil seeps strongly suggests that the wind is a primary factor to be considered for seepage detection. In addition, the age of the seepage on the sea surface is undetermined when the SAR imagery is solely used. As a result, it was not possible to confirm or invalidate the tidal hypothesis with the data collected. Future work towards the assimilation of SAR-based oil contour in a drift model is outlined. It will allow the retrieval of the age of the seepage and hence enable the proper assessment of the tidal hypothesis.