Natural Oil Seeps in the Gulf of Mexico: Distribution, Magnitude, and Variability

Natural hydrocarbon seeps are commonly found on continental margins throughout the worlds oceans. Hydrocarbon-rich regions, such as the Gulf of Mexico, are characterized by the presence of persistent surface oil slicks that are frequently ecologically significant as indicators of deep-sea chemosynthetic communities and may also point to reservoirs available for commercial exploitation. Synthetic aperture radar (SAR) satellite imaging reliably captures surface expressions of persistent natural hydrocarbon seeps. We analyzed data derived from SAR images collected over the Gulf of Mexico to locate oil slick origins (OSOs), to cluster the OSOs into discrete seep zones, to examine the temporal variability of oil discharge, and to estimate flux of individual seeps. SAR data was acquired from three remote sensing vendors: Maxar, Airbus, and NPA as well as satellite scenes previously analyzed by our laboratory. Satellites utilized by the vendors included ERS-1, ERS-2, RADARSAT-1, ENVISAT, Sentinel-1A, and TSX-1 for a total of 2,504 unique scenes collected between 1992 and 2018 and excluding scenes taken during the 2010 oil spill. Weather compliance was verified from modeled wind strength and scene texture. A total of 49,516 OSOs were identified within the total dataset. Cluster analysis identified 1,618 seep zones within radial distances of 1 km. Seep zones were most common on the central and western margin of the U.S. and in the Campeche Knolls region of the Mexican offshore. The large dataset allowed for examination of activity rates for individual seep zones, that is whether or not a surface slick was detected by a weather compliant scene covering an specific zone. Overall, we found an average activity rate of 11.3% (max 73%, stdev 8.11), which demonstrates that discharge of oil sufficient to generate SAR-detectable surface slicks is highly variable when considering individual seep zones. Flux estimates varied geographically and exhibited temporal variability, but the average flux for natural seeps was 9.7 mL s-1, with an annual estimated discharge for all Gulf seeps of 2.75 10.63 x 104 m3 yr-1. These data agree well with previous estimates based on SAR detection of surface slicks. Improving the precision and accuracy of satellite estimates of seepage rates will require better constraints on the thickness of floating oil layers in the ocean. Updating the state of hydrocarbon seepage in the Gulf of Mexico contributes to the understanding and constraint of natural seepage rates in North America. The methods developed for this study can be applied on a global scale to all SAR detected oil slicks and their OSOs, which would more accurately assess the magnitude of global hydrocarbon seepage.