Amount and Fate of Gas and Oil Discharged at 3400 m Water Depth From a Natural Seep Site in the Southern Gulf of Mexico

Abstract

This multidisciplinary study of the hydrocarbon seepage system at Tsanyao Yang Knoll (TYK) in the southern Gulf of Mexico illustrates the amount and fate of hydrocarbons (mainly oil and methane) emanating from the seafloor structure and rising through a 3400 m water column. TYK forms part of the Campeche Knolls and was found to be one of the most active seepage structures at such an exceptional depth. Combining ship-based and AUV-based hydroacoustic mapping with direct seafloor observations and investigations, which used a TV-sled and a remotely operated vehicle with gas and water sampling devices provided an integrated view for the various transport pathways of hydrocarbons from the seafloor to the sea surface. In total, 32 acoustic 'flares,' indicative of gas bubble emission sites, were detected emanating from depressions on top of the knoll. Most of the emission sites were concentrated in two depressions that comprised a main seep field. An estimated volume of 550-4650 L of hydrocarbons per hour (or 8300-70,600 mol CH 4 per hour) are released in the form of gas bubbles, which dissolve almost entirely during their rise in the water column. However, echograms showed gas anomalies to about 500 m below sea surface and some bubbles were seen to burst at the sea surface. Concentrations of dissolved methane were highly elevated (∼30,000 nmol/L) directly above the seafloor emission site, but decreased to background concentrations (3-5 nmol/L) within the lowermost 100 m. Smaller volume flow rates of oil also escaped from the seafloor, rose to the sea surface and generated natural oil slicks visible from the ship and in satellite images. This study shows that hydrocarbon seepage at ∼3400 m water depth can be followed to the sea surface. However, most of the methane dissolves in deeper waters, whereas oil reaches the sea surface.

Volumetric Mapping of Methane Concentrations at the Bush Hill Hydrocarbon Seep, Gulf of Mexico

“…The descending dives place an effective ceiling for reliable methane detection over Bush Hill area at 100 m. This is consistent with prior hydrocast ex situ sampling (Solomon et al, 2009) and is typical for other ex situ sampling efforts above seeps releasing methane bubbles (e.g., Römer et al, 2019). However, the lateral glider operations in the spring study suggest that this result may reflect the methane detection height only near the source as dives more offset from the source, and lateral traverses, indicate lower maximum detection height (Figure 5).…”

“…The study of the release of methane from thermogenic and biogenic seeps in the world's oceans (and lakes) has expanded from simple recognition of the extent of the sources to efforts to characterize methane release mechanisms and quantify fluxes. These efforts include studies of: the exchange across the sediment water interface (Tryon and Brown, 2004;Kastner and MacDonald, 2006), bubble fluxes using bottom imaging (Leifer and MacDonald, 2003;Leifer, 2010;Thomanek et al, 2010;Römer et al, 2019;Di et al, 2020;Johansen et al, 2020), bubble fluxes using acoustic imaging (Weber et al, 2014), dissociation of hydrates (Lapham et al, 2010;Lapham et al, 2014), and inferred fluxes to the seafloor based on shallow thermal gradients (Smith et al, 2014). We now know that methane bubble release rates can vary on time scales of seconds, minutes, hours, and days (e.g., Greinert 2008;Leifer, 2019 (and references therein); Johansen et al, 2020).…”

Dynamics of Live Oil Droplets and Natural Gas Bubbles in Deep Water

“…Comparison of TAMOC predictions with laboratory observations reported by Pesch et al23,37 for live and dead oil droplets (Experiments[1][2][3][4][5][6][7][8][9][10][11][12]. Also shown are model values when assuming suppressed aqueous dissolution of methane.…”

“…10 During such events, the coupled chemical and physical dynamics of the released gaseous and live liquid petroleum phases largely determine the trajectories and fates of these fluids in the near environment. Whereas the physical behavior of gas bubbles are well-documented, 6,[11][12][13][14][15][16][17][18][19][20] few direct observations have revealed the chemical and thermodynamic behavior of live liquid petroleum under shallow or deep-water conditions in either the field or the laboratory. [4][5][6][21][22][23][24][25] Limited field observations have been made on composition, dynamic behavior, ebullition, and oil-to-gas phase transfer, [4][5][6]25 and laboratory observations have been made on methane-saturated live oil with and High-pressure laboratory measurements reported by Pesch et al 23 provide the first direct, quantitative observations of free rising live oil droplets undergoing the coupled physical and chemical processes that are relevant in deep water.…”

“…Whereas the physical behavior of gas bubbles are well-documented, 6,[11][12][13][14][15][16][17][18][19][20] few direct observations have revealed the chemical and thermodynamic behavior of live liquid petroleum under shallow or deep-water conditions in either the field or the laboratory. [4][5][6][21][22][23][24][25] Limited field observations have been made on composition, dynamic behavior, ebullition, and oil-to-gas phase transfer, [4][5][6]25 and laboratory observations have been made on methane-saturated live oil with and High-pressure laboratory measurements reported by Pesch et al 23 provide the first direct, quantitative observations of free rising live oil droplets undergoing the coupled physical and chemical processes that are relevant in deep water. In repeated experiments, these researchers studied an oil droplet that was initially saturated with methane under high pressure conditions and then placed within a counter-current flow channel filled with artificial seawater at high pressure (151 bar).…”