Dynamics of Natural Hydrocarbon Seeps in the Northern Gulf of Mexico

Since the discovery of the Gulf of Mexico, it has become an area of extensive exploration. The Gulf of Mexico harbors specific commodities that are essential in our modern economy. Oil companies provided money and technology to find and study locations for oil exploitation. With the onset of seismic data acquisition, it was possible to gain a more comprehensive understanding of the formation and structure of this unique basin that is in constant dynamic disequilibrium, and facilitates hydrocarbon leakage. The natural seepage of oil and gas to the sea floor is of interest because these “leaks” expel methane which is potentially a significant factor in the global Carbon cycle. Determining the migration pathways through the sedimentary strata to the various primary conduits at the sea floor provides a comprehensive understanding of the large scale dynamic “plumbing system” in the Gulf of Mexico. One of the objectives in this research was to quantify the rate and volume of oil and gas released from two natural seeps in lease blocks GC600 (1200 m depth) and MC118 (850 m depth). Our purpose was to determine variability in bubble size and release rates at three individual vents and to estimate how changes in pressure affect bubble release rates. Observations with autonomous video cameras (VTLC) captured the formation of individual bubbles as they were released through gas hydrate outcrops. Image processing techniques determined bubble type (oily, gaseous, and mixed: oily and gaseous), size distribution, release rate, and temporal variations (observation intervals from 3 h to 26 d). One vent at GC600 (Birthday Candles) released oily bubbles with an average diameter of 5.0 mm (std. 1.30) at a rate of 4.37 bubbles s⁻¹. A second vent at GC600 (Mega Plume) released mixed oil and gas bubbles with an average diameter of 3.9 mm (std. 1.19) at a rate of 103 bubbles s⁻¹ (std. 24.6). A third vent at MC118 (Rudyville) released gaseous bubbles with an average diameter of 3.0 mm (std. 1.99) at a rate of 127 bubbles s⁻¹ (std. 34.1). To quantify bubble release, a robust image processing technique was developed that is adaptable to the various environments found in deep-sea oil and gas vents.   Our second objective was to constrain the migration of hydrocarbons from the source rock to the sea floor. A compilation of data sets from the macro to micro scale were used to describe the overall sequence of hydrocarbon migration and discharge from ~15 kmbsf to the water column. Geochemical similarities were found by fingerprinting oil samples from reservoir, active vents and sea-surface to show migration connectivity from source to seafloor. To support the geochemical data, measurements of fluxes and the magnitude of fluid flow indicators (e.g. bacteria mats, hydrate mounds, etc.) were compiled, and we have attempted to categorize and quantify the various processes that sequester hydrocarbons. Different stages of upward hydrocarbon flow were characterized by visual and morphological tracers. Varying reflectivity values in seismic and subbottom profile data delineate salt distribution, fault position, and acoustic blanking zones. Local geomorphological features such as hydrate mounds, carbonate hardground cover, and chemosynthetic communities, suggest passive/focused fluid flow. VTLC records and acoustic targets detected by swath mapping were used to determine the number of vents in the seep zone. We used a systems approach to combine the various data sets at different scales and resolutions to quantify values for the hydrocarbon budget at GC600. Finally, our third objective was to describe the type of benthic communities that frequented the vents in our study and to determine the evolutionary stage of the seep zone. Natural seeps provide a source of energy to chemosynthetic communities in seep areas. We used the autonomous VTLC that was deployed for extended periods of time for an “uninterrupted” view of the behavior of animals within this particular seep zone. We observed a number of metazoans including ice worms burrowing in hydrate outcrops, fish feeding on thick bacteria mats, swarms of annelids, and curious eels and crabs visiting bubble streams. By analyzing the type of organisms within the seep zone, we could determine that these seeps were immature based on the community composition. More matured seeps would include more authigenic carbonate hard grounds, less focused fluid flow (bubbling) and increased abundance of mussels and possibly tube worms. Of particular interest was the sheer abundance of ice worms (~2.8 x 10⁴ in GC600) that inhabit the gas hydrate outcrops and don’t seem to have any predators. Where they go once the gas hydrate dissociates is still an open question.