Distribution of gas hydrates on continental margins by means of a mathematical envelope: A method applied to the interpretation of 3D Seismic Data

Abstract

[1] A 3D seismic volume from the Nankai Trough accretionary wedge (SE Japan) is used to evaluate the subsurface distribution of gas hydrates as a function of structural and stratigraphic complexity, variable heat flow patterns and the presence of subsurface fluid conduits. Eleven equations were modified for depth, pressure, and temperature, modeled in 3D, and compared with the distribution of BottomSimulating Reflections (BSRs) offshore Nankai. The results show that the equations produce overlapping-and thus potentially consistent-predictions for the distribution of BSRs, leading us to propose the concept of a ''BSR Stability Envelope'' as a method to quantify the subsurface distribution of gas hydrates on continental margins. In addition, we show that the ratio (R) between shallow and deep BSRs of seven subenvelopes, which are defined by BSR stability equations, indicates local gas hydrate equilibrium conditions. Values of R < 1 relate to cooler regions, whereas when R > 1 the majority of BSRs are located in warmer structural traps. The method in this paper can be used to recognize any divergence between observed and theoretical depths of occurrence of BSRs on 3D or 4D (time lapse) seismic volumes. In the Nankai Trough, our results point out for equilibrium conditions in BSRs located away from the Megasplay Fault Zone and major thrust faults. This latter observation demonstrates the applicability of the method to: (a) the recognition of subsurface fluid conduits and (b) the prediction of maximum and minimum depths of occurrence of gas hydrates on continental margins, under distinct thermal and hydrologic conditions.

Bottom‐simulating reflector dynamics at Arctic thermogenic gas provinces: An example from Vestnesa Ridge, offshore west Svalbard

“…Global estimates of gas hydrate concentrations rely on the prediction of the extent of the GHSZ at specific geological settings [e.g., Klauda and Sandler , ]. The GHSZ extent can be predicted using thermodynamic models [e.g., Bale et al ., ; Dickens and Quinby‐Hunt , ; Lu and Sultan , ; Sloan and Koh , ] together with constraints from direct sampling and indirect evidence from both geophysical and geochemical data. Several numerical models have been proposed that emphasize controls on hydrate stability by specific parameters [ Bale et al ., , and references therein; Peszynska et al ., ].…”

“…The GHSZ extent can be predicted using thermodynamic models [e.g., Bale et al ., ; Dickens and Quinby‐Hunt , ; Lu and Sultan , ; Sloan and Koh , ] together with constraints from direct sampling and indirect evidence from both geophysical and geochemical data. Several numerical models have been proposed that emphasize controls on hydrate stability by specific parameters [ Bale et al ., , and references therein; Peszynska et al ., ]. The modeling approach by Sloan and Koh [] is particularly widely implemented for hydrate studies in different continental settings due to its applicability for gas hydrates with a mixed composition of microbial and thermogenic methane with higher‐order hydrocarbons (i.e., structures I, II, and H).…”

“…The dynamics of a gas hydrate system at a specific geological setting in response to dominant external factors such as bottom water temperature (BWT), geothermal gradient (GTG), heat flow, pressure, gas composition, and salinity can be assessed by analyzing discrepancies between the predicted BGHS and the observed BSR [e.g., Bale et al ., ; Liu and Flemings , ; Ruppel , ; Xu and Ruppel , ]. For instance, in convergent margin settings such as Nankai Trough and the Hikurangi Margin, the depth of the BSR (300–500 m below seafloor (mbsf)) readjusts primarily to heat flow changes and pressure changes due to uplift and erosion [ Kinoshita et al ., ; Pecher et al ., ], whereas in continental slopes along passive margins, seafloor temperature variability and sea level cycles can be major factors controlling the BSR depth [e.g., Berndt et al ., ; Mienert et al ., ; Skarke et al ., ; Westbrook et al ., ].…”

Gravity complexes as a focus of seafloor fluid seepage: the Rio Grande Cone, SE Brazil

“…Although our data cannot exclude the three-phase system hypothesis, the high resistivity values and the not sufficiently high chloride concentrations found in the study area do not fully support the existence of high-salinity pore water and suggest that free gas in the GHSZ may be due to another mechanism, possibly linked to gas supply in excess of hydrate formation 20 . The gas is supplied via the mapped structures showing chaotic reflections underneath pockmarks; additionally, gas can be supplied by conducts that cannot be imaged on seismic 49 .…”

“…These areas are interpreted as ancient emission loci that are no longer active 48 . In addition, high gas and gas hydrate saturations are observed in areas with no clear connection to seafloor seep structures 48 , and are therefore interpreted as systems yet to be fully developed, i.e., no full bypass of gas occurs through the GHSZ and conduits for gas are not imaged on seismic 49 . Present-day, fully developed systems in the RGC have high gas saturations underneath pockmarks 48 and are commonly accompanied by flares in the water column.…”