Hydrocarbon Seepage at Campeche-Sigsbee Salt Province, Southern Gulf of Mexico (Detection, Mapping, and Seafloor Manifestation)

Abstract

Hydrocarbon seepage is a process during which hydrocarbon fluids are emitted from the seafloor into the water column. This phenomenon has been observed globally from continental margins to the deep abyssal. Hydrocarbon seepage has significant impacts on the marine environment such as (a) influence on sediment composition and dynamics at the seafloor, (b) increasing the habitat heterogeneity on seep biodiversity and (c) contributes to the global carbon cycle. However, the occurrence, distribution, and dynamics of hydrocarbon seepage in the marine environment, especially in the deep ocean remains unclear due to limited investigation. The northern Gulf of Mexico is a well-known prolific petroleum-producing region where numerous gas and oil emissions, associated with salt tectonism, have been observed. The Campeche-Sigsbee salt province in the southern GoM is considered to be an analog to the salt province in the northern GoM, but there has been very little research conducted in this region. Based on the occurrence of natural oil slicks on the sea surface resolved by satellite images, previous studies suggested that there is a widespread distribution of oil seeps in the Campeche-Sigsbee salt province. However, there is still a lack of direct evidence for the presence and the distribution of gas emissions. In addition to gas and oil seepage, Chapopote asphalt volcanism, a novel type of hydrocarbon seepage was first introduced in 2003. Since then, submarine asphalt deposits have attracted considerable research interest. This study aims to have a comprehensive understanding of the hydrocarbon seepage system and dynamics in the southern GoM. The objectives are to investigate the distribution of gas emissions and to understand the controlling factors on the distribution. Furthermore, detailed investigations were carried out at Challenger Knoll and Mictlan Knoll to gain a better understanding of the diverse hydrocarbon seepage system including gas and oil emissions, as well as asphalt deposits. Consequently, the research questions about the fate of the methane bubbles and the quantity of gas bubble released from gas emission site are finally addressed in this study. During research cruise M114 of R/V METEOR, a multidisciplinary approach was conducted, including hydroacoustic surveys, visual seafloor observations, and sampling of gas bubbles. Ship-based multibeam echosounder was used for seafloor bathymetry, backscatter and water column flare mapping in the Campeche-Sigsbee salt province. In addition, multibeam echosounder mounted on Autonomous Underwater Vehicle (AUV) was utilized to obtain high-resolution seafloor bathymetry, backscatter, and water column data at Mictlan Knoll. Remotely Operated Vehicle (ROV) and TV-sled were applied for investigating and documenting seafloor manifestations of hydrocarbon seepage at the seafloor. Gas bubbles were collected by pressure-tight gas bubble samplers operated by ROV at the seafloor of Mictlan Knoll for gas analyses, quantification of gas bubble emissions, and finally gas flux calculation. In summary, gas emissions are found in large numbers in the Campeche-Sigsbee salt province. Their distributions are controlled by the present geological structures. The case study in the Sigsbee Knolls suggests that the edges of flat-top knolls might provide an effective migration pathway for hydrocarbons. As there is no direct evidence for the presence of current oil seepage in the Sigsbee Knolls, we suggested that oil seepage occurs intermittently. Gas, oil seepage and asphalt volcanism are found to occur close together at the Mictlan Knoll, indicating that this diverse hydrocarbon seepage system might be a common phenomenon in the Campeche Knolls. This thesis shows the complex association between the dynamics of diverse hydrocarbon seepage and the geological controls in the southern GoM.