QUANTIFYING THE METHANE CONTENT OF NATURAL GAS AND GAS HYDRATE ACCUMULATIONS IN THE DEEP-WATER BASINS OF THE BERING SEA
Ginger A. Barth, David W. Scholl, and Jonathan R. Childs
United States Geological Survey, M.S. 999, Menlo Park, CA 94025
Seismic reflection data from the deep-water (>3500 m) basins of the Bering Sea suggest the abundant presence of natural gas and gas hydrate within the sedimentary section. A gas hydrate BSR (bottom-simulating reflection) near 450 m bsf (below sea floor) suggests that high methane concentrations are present throughout the Aleutian and Bowers Basins. In addition, distinctive velocity pseudostructures are evident in the otherwise horizontal and uniform sedimentary reflection sequence. These velocity-amplitude anomaly structures ("VAMPs") are interpreted as methane chimneys overlain by concentrated gas hydrate caps. A typical VAMP includes a zone of high velocity pull-up (attributed to gas hydrate within the sediment) directly overlying a zone of low velocity push-down (attributed to free gas in the pore spaces). Within the VAMPs, the hydrate BSR roughly separates the pull-up from the push-down. Hundreds of VAMPs have been imaged throughout the Bering Sea; several thousand are inferred to exist.
We endeavor to quantify the size and methane content of typical VAMP structures, primarily through interpretation of observed seismic reflection time anomalies. USGS seismic reflection data coverage from the deep water Bering Sea region includes over 24,000 km of single channel profiles coincident with GLORIA side-scan sonar tracks acquired during the 1986-7 EEZscan program, plus several older 24-channel multichannel lines crossing the Aleutian and Bowers Basins (Figure 1). These airgun-source data all provide images from seafloor to basement, over 3 km of penetration. The imaged abyssal basin fill includes generally horizontal and uniform sedimentary reflection sequences, comprising predominantly mudstone, diatomaceous, and distal turbidite deposits, upon oceanic crust of probable Cretaceous age.
Several VAMPs have been chosen for detailed study. These are typical of the larger anomaly structures seen in the Aleutian Basin, with lateral extents from 2-6 km, cumulative velocity pull-up above the BSR of 20-40 ms, and evidence for both focused gas chimneys (Figure 2) and more distributed gas accumulations (Figure 3) below the hydrate BSR. For each example, key horizons have been interpreted through comparison of positive and negative polarity components, deconvolved and migrated images, and by comparison with DSDP Leg 19 drilling results. Reflection times were auto-picked using Landmark SeisWorks2DTM. It is assumed that sedimentary horizons are parallel to each other and that velocity structure is laterally uniform, with the exception of gas and gas hydrate contributions, within a 16 km study window centered on each VAMP. Background velocity structure is taken from DSDP Leg 19 and regional sonobuoy results. Given these constraints, interval travel time anomalies between seismic reflecting horizons are mapped as velocity anomalies, which are quantitatively interpreted as free gas and gas hydrate indicators. For the hydrate, represented by positive velocity anomalies above the BSR, a three-component (hydrate, water, sediment-frame) time average equation is used. For the gas, represented by negative velocity anomalies below the hydrate BSR, a linear velocity-gas fraction relation is adopted, with 2% gas content assumed for velocities less than 1200 m/s.
The VAMPs studied contain 20-50 m cumulative thickness of gas hydrate. Hydrate distribution appears to be lithologically controlled; it is most concentrated in a zone ~90 m above the hydrate BSR. Assuming 50-60% porosity, based on DSDP results, gas hydrate appears to fill less that 50% of pore space, even in the most concentrated zones. Free gas is present in the section to well below 1 km bsf. Each individual large VAMP is estimated to contain a gas volume (including gas hydrate) similar to that of an economic gas field, >1010 m3 or >0.35 Tcf. Given the abundance of VAMP structures observed, the deep-water Aleutian and Bowers Basins of the Bering Sea are clearly a major storehouse of natural gas. Preliminary volume estimates based on VAMP analyses easily approach 1000 Tcf of natural gas within these basins.
The basin-wide occurrence of a horizontal, laterally persistent hydrate BSR overlying thousands of gas chimneys associated with VAMP and VAMP-like structures testifies to a high basin-wide flux of methane toward the sea floor. VAMPs have been rarely reported from other areas, so why are they so abundant in the Bering Sea basins? Related to this question are the observations that:
- heat flow across the basin floor (55-60 mW/m2 , n= 31, range 42-90; gradient ~50-55 C/km) is much higher than would be predicted for deeply buried basement of Cretaceous oceanic crust,
- the regional upwelling of nutrient-laden deep waters (ongoing since at least 30 Ma), makes the surface waters of the Aleutian Basin among the Earth's most productive.
We explore the notion that beneath the basin's abyssal floor the rapid glacial-age accumulation of a thick (~1 km) blanket of water-rich, diatomaceous turbidite deposits warmed underlying organic- and diatom-rich hemipelagic beds of late Miocene and older age. Warming is hypothesized to have speeded the exothermic conversion of semi-consolidated opaline diatomaceous units to brittle porcelaneous shale. Self-heating of the Neogene section promoted, at depths of 1500-2500 m, the production of thermogenic methane and other hydrocarbons. In a manner analogous to that described for the siliceous Monterey formation of coastal California, silica diagenesis was also attended by release of water, rock-volume contraction, and vertical hydrofracturing. Although the basinal sequence is virtually unfaulted, fracturing provided permeable pathways facilitating the flushing of heat, water, and dissolved and gaseous methane toward the basin floor. Advective ascent of warmed fluids became focused or self-organized into chimney-like columns of enhanced fracture permeability. At the top of the columns gaseous methane entering the upper half of the porous turbidite-diatom section crossed into the P-T field of hydrate stability and nourished the massive deposition of pore-filling hydrate accompanied by the release of heat of crystallization. The vertical flux of large quantities of methane and heat to the basin floor is thus hypothetically linked to the alteration-augmented warming of older diatomaceous beds, and the consequent production of thermogenic gases that, with other fluids, continue to migrate toward the basin floor through diagenetically fractured siliceous shale.
Figure 1. USGS seismic reflection data from the deep water Bering Sea basins includes >24,000 km of single-channel and >1000 km of heritage multi-channel data, covering an area comparable to the states of California and Nevada combined.
Figure 3. Analysis of the travel time anomalies within a VAMP includes (A) identification of key horizons (arrows along right margin), (B) derivation of velocity structure based upon interval time variation between picked horizons and (C) interpretation of the anomalies in terms of equivalent volume of hydrate and gas within the section. Background velocity in the upper section is taken as constant at 1600 m/s. In this example, free gas content is <2% everywhere, and maximum hydrate concentration implied is ~20% of pore space. If the profile is a slice through the midpoint of a structure of cylindrical geometry, the hydrate zone alone contains ~0.87 Tcf of natural gas.