Fate of Natural and Anthropogenic Oil Slicks in the Global Oceans
O'Reilly, Carrie (author)
MacDonald, Ian R. (Ian Rosman) (professor directing dissertation)
Abichou, Tarek (university representative)
Chanton, Jeffrey P. (committee member)
Stukel, Michael R. (committee member)
Hsu, Shi-Ling (committee member)
Meurer, William P. (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Department of Earth, Ocean, and Atmospheric Science (degree granting department)
2023
text
doctoral thesis
The Gulf of Mexico is a hydrocarbon-rich region characterized by the presence of floating oil slicks from persistent natural hydrocarbon seeps, which are reliably captured by synthetic aperture radar (SAR) satellite imaging. Improving the state of knowledge of hydrocarbon seepage in the Gulf of Mexico improves the understanding and quantification of natural seepage rates in North America. We used data derived from SAR scenes collected over the Gulf of Mexico from 1978 to 2018 to locate oil slick origins (OSOs), cluster the OSOs into discrete seep zones, estimate the flux of individual seepage events, and calculate seep recurrence rates. In total, 1618 discrete seep zones were identified, primarily concentrated in the northern Gulf of Mexico within the Louann salt formation, with a secondary concentration in the Campeche region. The centerline method was used to estimate flux based on the drift length of the slick (centerline), the slick area, and average current and wind speeds. Flux estimates from the surface area of oil slicks varied geographically and temporally; on average, seep zones exhibited an 11% recurrence rate, suggesting possible intermittent discharge from natural seeps. The estimated average instantaneous flux for natural seeps is 9.8 mL s−1 (1.9 × 103 bbl yr−1 ), with an annual discharge of 1.73–6.69 × 105 bbl yr−1 (2.75–10.63 × 104 m3 yr−1 ) for the entire Gulf of Mexico. The temporal variability of average flux suggests a potential decrease following 1995; however, analysis of flux in four lease blocks indicates that flux has not changed substantially over time. It is unlikely that production activities in the Gulf of Mexico impact natural seepage on a human timescale. Of the 1618 identified seep zones, 1401 are located within U.S. waters, with 70 identified as having flux and recurrence rates significantly higher than the average. Seep zones exhibiting high recurrence rates are more likely to be associated with positive seismic anomalies. Many of the methods developed for this study can be applied to SAR-detected oil slicks in other marine settings to better assess the magnitude of global hydrocarbon seepage. The new approach used to quantify flux oil fluxes from natural seafloor seepage sites in the Gulf of Mexico based on the dynamic characteristics of floating oil slicks is presented. It enables rapid analysis of interpreted satellite imagery and permits large datasets to be analyzed. The method uses the drift path of the oil on water surface, termed the "centerline" of the slick, along with average current and wind speeds to estimate the age of seepage slicks. The results of the CL (centerline) method compare favorably with the hindcast modeling approach which requires time-resolved analysis of wind and current for each slick. The CL method yields an average flux estimate within the range of the flux documented by on-bottom capture for a seafloor oil pollution source. A survey of published flux estimates using a variety of techniques reveals a wide range with on-bottom estimates (0.08–12 ml/s) generally lower than estimates from slicks (1.6–125 ml/s). Determination of fluxes from three seepage sites in the Gulf of Mexico (GC600, Bush Hill, and TYK) reveals a wide range in seepage rates from 2.4 to 52.9 ml/s and flux distributions. Comparison of the observed fluxes from seeps with potential fluxes from a source region can be used to constrain source rock parameters (e.g., thickness, richness, area) and to understand the cumulative oil supply represented by seeps over geologic time scales. Physical processes involved in the ascent of naturally seeped oil from the seafloor and its persistence as a slick are also considered. Simplified, physics-based models are developed, drawing in part from the extensive literature concerned with anthropogenic releases of oil at sea. The first model calculates the ascent of oil droplets or oil-coated gas bubbles as they ascend to the sea surface from the seep source. The second model calculates slick longevity as a function of the effect of wind-driven breaking waves. Both models have simplified inputs and algorithms making them suitable for Monte Carlo-type analysis. Using the oil ascent model, we find that slicks from shallower seeps are offset farther relative to their water depth than those from deeper sources. The slick longevity model reveals four growth modes for seepage slicks: persistent (low wind speeds), ephemeral (high wind speeds), reset (all slicks are cleared from an area by high wind speeds), and aging (slick growth after a reset). A year's worth of modeled winds from the Gulf of Mexico indicate average slick ages of ~ 12 hours. Taking account of the expected oil release duration implied by slick recurrences yields average slick longevities for high recurrence seeps of ~6.5 hours and ~ 5 hours for low recurrence seeps. Seep flux estimates that include the length of individual slicks and the constraints of local currents and wind implicitly take into account the impact of wind-speed history. Those that assume a slick age should be re-evaluated in light of the current findings. The approaches used to quantify natural seepage can be applied to anthropogenic oil slicks on the sea surface. Ship generated oil slicks (i.e. bilge dumping) are a chronic source of oil pollution to the global oceans. Anthropogenic inputs are the largest contributor to oil slicks in the global oceans, with approximately 20% generated by operational discharges of vessels. These slicks are generated by bilge dumping, the illegal discharge of untreated oily wastewater directly into the sea, in violation of international regulations (i.e., MARPOL). Bilge dump oil slicks are routinely detected in satellite imagery; however, responsible parties are not prosecuted due to lack of surveillance, uncertainty of extent of MARPOL violations, and difficulty tracing the origin of slicks back to the responsible vessel. This study analyzed 70 SAR satellite detected bilge dump oil slicks to estimate the oil concentration of the bilge water required to generate the observed slicks, as well as estimated persistence time of the slicks, as a function of wind speed. Bilge dump slick volumes were estimated assuming a range of slick thicknesses (0.04 – 1 μm) and compared to recommended bilge tank volumes for various ship sizes. In all slick thickness cases, for all bilge tank volumes, the observed bilge dump oil slicks were discharged from bilge tanks with oil concentrations that exceeded the 15 ppm MARPOL oil content limit by at least 3 orders of magnitude. Slicks generated at wind speeds less than 5.6 m/s were estimated to persist for over 24 hours and those generated at higher wind speeds were estimated to persist for less than 16 hours. Persistence time was added to the deposition time of each slick, providing a time window to look back at automatic identification system (AIS) data to identify potential responsible parties. MARPOL regulations have been accepted international standards since the 1970s, but increased satellite surveillance and enforcement would improve its efficacy and the health of the marine environment.
July 7, 2023.
A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Ian R. MacDonald, Professor Directing Dissertation; Tarek Abichou, University Representative; Jeff Chanton, Committee Member; Michael Stukel, Committee Member; Shi-Ling Hsu, Committee Member; William P. Meurer, Committee Member.
Florida State University
OReilly_fsu_0071E_18157