An Improved Weichselian Seismic Stratigraphy, Subsurface Fluid Migration Patterns and Methane Hydrate Dynamics, Offshore West Svalbard

Abstract

The impacts of ocean warming are increasingly noticeable in the Arctic, such as the retreat of ice sheets and the accelerated melting of glaciers. The marine sediments in the Fram Strait gateway, connecting the Arctic and Atlantic Oceans, host methane hydrates (600 to 2000 m water depth). Methane hydrate is an ice-like substance that forms when methane in gaseous form is trapped within the crystalline structure of water molecules. It remains stable under low temperature (-10 °C–25 °C) and high-pressure (3–38 MPa) conditions. However, the stability of the hydrates is sensitive to ocean warming and relative sea-level (RSL) changes. In 2008, methane bubbles were observed above the present-day upper limit of hydrate stability (~ 400 m water depth) in the western continental margin of Svalbard, suggesting hydrate dissociation, but the exact cause remained undetermined. This study examines the influence of RSL changes and variations in bottom water temperature (BWT) on methane hydrate dynamics using numerical simulation code TOUGH+Hydrate (v 1.5, 2014). To assess the impact of RSL fluctuations on hydrates over the past 11,000 years, two different reconstructions of local ice history were used to predict RSL changes. The University of Tromsø (UiT) model suggested a fall in RSL, while the ICE-6G_C model predicted a continuous rise. The modelling revealed that hydrates had dissociated before the bottom water cooling phase that began in the late 1970s, during which hydrates could reform. During the period of accelerated bottom water warming from 1978 to 2016, the newly formed hydrates underwent complete dissociation. Therefore, recent ocean warming played a critical role in newly formed hydrate dissociation completely regardless of long-term sea-level history. The seismic stratigraphic framework for the upper continental slope (top 80 m below seabed) between the Kongsfjorden and Isfjorden Trough Mouth Fans is refined. The study identified four distinct episodes of shelf edge glaciations occurring around 120–110, 90–85, 70–54 and 38–24 ka. On the uppermost continental slope, the Weichselian glacial units are characterized by glacial debris materials transported by a slow-moving ice sheet. Prominent V-shaped indentations carved out by icebergs indicate distinct calving episodes and erosional trough as a result of glaciogenic debris flows. Below the methane seepage area on the upper continental slope, the debris flows exhibit an oblique retrogradational stacking pattern. At water depths > 500 m, the debris flow units transitioned into stratified turbidite deposits, indicating a shift from debris flows to turbidity currents due to progressive dilution with water. Based on the seismic interpretation and numerical modelling, a comprehensive analysis of fluid accumulation beds and migration patterns in the upper continental slope was conducted. Present-day extent of the base of gas hydrate stability was calculated, which is in a transient state due to the migration of the base of methane hydrate stability over the past 11,000 years due to changes in RSL and BWT. Although the SYSIF seismic data did not reveal a bottom-simulating reflection (BSR) on the uppermost continental slope, numerous gas pockets (~ 10 to 30 m below seabed) represented as bright spots with negative polarity below the present-day base of methane hydrate stability were observed. Beneath the methane seepage area, two distinct sets of reservoirs were identified (first ~8 to 12 m and second ~ 20 to 33 m below seabed), comprising fragmented turbidite beds with gas occurrences. These beds are separated vertically by low-permeability glaciogenic debris (~ 12 to 33 m below seabed), that prevents fluid flow. Based on seismic evidence and modelling, it is shown for the first time that methane hydrate dissociation, induced by ocean warming, results in the release of frozen methane. This gas then migrates through fractures, disrupting the sealing capacity of the debris stacks, leading to fluid focusing toward the seepage locations. Summarsing, the findings underscore the impact of recent ocean warming on the dissociation of methane hydrates offshore west Svalbard. High-resolution seismic data analysis improved our understanding of glacial stratigraphy, fluid migration patterns and methane hydrate dynamics in the rapidly changing Arctic region. Advanced imaging methods such as 3D seismic and SYSIF enhance our understanding of glacial reservoirs. This knowledge can optimize hydrocarbon exploration by providing insights into complex reservoir architecture and fluid movement within the fractured system, thereby improving exploration methods.