Description: Highlights • BSR position does not match BGHS as predicted based on regional TP conditions. • Use steady state and transient models to determine extent of hydrate stability. • Investigate the influence of topographic focusing on hydrate stability. • Variable thermal properties of sediment impact hydrate stability. The Danube Fan in the western Black Sea shows many features indicating the presence of gas and gas hydrates, including a bottom simulating reflection (BSR), high-amplitude anomalies beneath the BSR and the presence of gas flares at the seafloor. The BSR depth derived from 3D P-cable seismic data of an older slope canyon of the fan (the S2 canyon) suggests that the BSR is not in equilibrium with the present-day topography. The Danube Fan was abandoned ∼7.5 ka, and the S2 canyon was likely incised ∼20 ka, suggesting that the gas hydrate system has had at least 7.5 ka years to equilibrate to the present-day conditions. Here we examine the extent and position of the hydrate stability zone through constructing both steady and transient state models of a 2D profile across the S2 canyon. This was done using inputs from mapping of the 3D P-cable seismic data and geochemical analysis of core samples. Using these models, we investigate the effects of different factors including variable thermal properties of heterogeneous sediments in the vicinity of the canyon and, topographic focusing on the geothermal gradient on the extent of the hydrate stability zone. Our results indicate that both factors have a significant effect and that the hydrate system may actually be in, or approaching equilibrium.

Description: A bottom-simulating reflector (BSR) occurs west of Svalbard in water depths exceeding 600 m, indicating that gas hydrate occurrence in marine sediments is more widespread in this region than anywhere else on the eastern North Atlantic margin. Regional BSR mapping shows the presence of hydrate and free gas in several areas, with the largest area located north of the Knipovich Ridge, a slow-spreading ridge segment of the Mid Atlantic Ridge system. Here, heat flow is high (up to 330 mW m-2), increasing towards the ridge axis. The coinciding maxima in across-margin BSR width and heat flow suggest that the Knipovich Ridge influenced methane generation in this area. This is supported by recent finds of thermogenic methane at cold seeps north of the ridge termination. To evaluate the source rock potential on the western Svalbard margin, we applied 1D petroleum system modeling at three sites. The modeling shows that temperature and burial conditions near the ridge were sufficient to produce hydrocarbons. The bulk petroleum mass produced since the Eocene is at least 5 kt and could be as high as ~0.2 Mt. Most likely, source rocks are Miocene organic-rich sediments and a potential Eocene source rock that may exist in the area if early rifting created sufficiently deep depocenters. Thermogenic methane production could thus explain the more widespread presence of gas hydrates north of the Knipovich Ridge. The presence of microbial methane on the upper continental slope and shelf indicates that the origin of methane on the Svalbard margin varies spatially.

Description: Gas migration pathways in the Gulf of Mexico are strongly influenced by the extensive formation and time evolution of salt canopies, welds and sheets. This multi-level salt system (known as the Louann Salt formation) deposited mostly within Callovian age (upper Middle Jurassic) and mobilized during late Miocene up to Pliocene-Pleistocene times controls the extension and direction of petroleum components migration over the entire history of the basin which, in return, has a major impact on potential gas transportation into the gas hydrate stability zone (GHSZ). In the context of gas hydrate formation, presence of extensive salt deposits tends to bend gas migration pathways from vertical (typical for the Gulf of Mexico region) towards rather horizontal and dispersed. However, amalgamation of two or more salt structures often results in re-focusing of the flow towards the local topographic subsalt heights. Together with the formation of local sediment discontinuity structures such as faults developing at the rims and tops of rootless salt deposits related to further stages of allochthonous salt mobilization, new high-permeability migration pathways develop and act as direct connection for the thermogenic gas to the GHSZ. Our study presents the 3D modeling solution quantifying and exploring the gas hydrate accumulation potential in the marine environment experiencing salt tectonics such as the Green Canyon, Gulf of Mexico. This modeling study evaluates the potential of bio- and thermogenic gas hydrate formation within Pliocene-Pleistocene reservoir layers based on full basin re-construction which accounts for depositional and transient thermal history of the basin, source rock maturation, petroleum generation, expulsion and migration, salt tectonics and associated faults development. Based on a numerical study calibrated with the existing field data, we present a new distribution pattern of gas hydrates attributed to both microbial and thermogenic origin. We present here an explanation for a formation mechanism of large gas hydrate amounts (〉 70 vol. %) wide-spread at the base of the stability zone as a result of the gas hydrate-free gas recycling process enhanced by very high Neogene sedimentation rates in the region. We suggest that the rapid development of secondary intra-salt mini-basins situated on top of the allochthonous salt deposits and following sediment subsidence caused a consequent dislocation of the GHSZ lower boundary and led to efficient gas hydrate dissociation process followed by a free gas re-charge into the GHSZ.

Description: The accumulation of methane hydrate in marine sediments is controlled by a number of physical and biogeochemical parameters including the thickness of the gas hydrate stability zone (GHSZ), the solubility of methane in pore fluids, the accumulation of particulate organic carbon at the seafloor, the kinetics of microbial organic matter degradation and methane generation in marine sediments, sediment compaction and the ascent of deep-seated pore fluids and methane gas into the GHSZ. Our present knowledge on these controlling factors is discussed and new estimates of global sediment and methane fluxes are provided applying a transport-reaction model at global scale. The modeling and the data evaluation yield improved and better constrained estimates of the global pore volume within the modern GHSZ ( ≥ 44 × 1015 m3), the Holocene POC accumulation rate at the seabed (~1.4 × 1014 g yr−1), the global rate of microbial methane production in the deep biosphere (4−25 × 1012 g C yr−1) and the inventory of methane hydrates in marine sediments ( ≥ 455 Gt of methane-bound carbon).

Description: This study provides new insights into the complex three- phase system of the Blake Ridge site, offshore South Carolina. A new numerical reaction- transport model has been applied to better understand: 1) the co- existence of dissolved and gaseous methane within the Gas Hydrate Stability Zone (GHSZ) (Guerin et al., 1999, Taylor et al., 2000), and 2) the origin and nature of high methane fluxes associated with the presence of cold seeps and venting sites (Paull and Matsumoto, 2000). Based on our numerical simulation accounting for sediment deposition and compaction, microbial processes of methane oxidation via sulfate reduction (AOM), methanogenesis and organic matter degradation, we would like to present a new scenario of gas hydrate crystallization within the time frame of a basin history.

Description: In order to evaluate and understand the results of sedimentary basin numerical modeling including non- homogeneous layer structures (e.g. faults or tectonically active salt domains), it is essential to prescribe clear and scientifically-based rules of fault-like structure implementation. For this purpose, we examine the most important parameters and aspects that influence modeling of fluids and gas migration pathways within porous sedimentary layers, including model resolution, gridding of the fault/salt-edge structure, 3D continuity, fault properties, migration model, timing of opening/closing, etc. The results of series of model-runs covering the common ranges of parameter values reported from natural marine settings will be presented during the conference. Results of the specific modeled configurations for the Green Canyon (Gulf of Mexico) and Hydrate Ridge (Oregon margin) will also be discussed.