Results of 1992 seismic reflection experiment in Lake Baikal

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95684/1/eost9857.pd

Active destabilisation of gas hydrate accumulations in Lake Baikal by tectonically induced fluid-flow

Multi-channel seismic profiling and deep drilling have evidenced the presence of gas hydrates in Lake Baikal, Siberia. They occur in the deep basins around the large Selenga River Delta. The presence of the hydrates is evident on seismic records by virtue of a distinct high-amplitude, reversed-polarity, cross-cutting BSR. Locally, however, the BSR shows a very anomalous behaviour. In the vicinity of some of the main, active, intra-basin faults, its depth strongly fluctuates, with undulations (positive as well as negative compared to background positions) and vertical displacements of several hundreds of ms TWT. Locally, the BSR is even entirely disrupted by vertical ‘chimneys’ that reach up to the lake bottom.High-resolution deep-tow side-scan sonar mosaics over one of such areas of deformed and disrupted BSR show a cluster of morphological irregularities on the lake floor, in contrast to areas above a regular BSR where the lake floor is absolutely regular and flat. Four large irregularities - aligned parallel to the fault – were discovered, one of them coinciding with one of the ‘chimneys’. They were mapped in detail by bathymetric sounding and proved to be either elevations (mud volcanoes ?) or depressions (craters) at the lake floor. Echosounding has also shown venting associated with these features, which is evidenced by an acoustically non-transparent plume, reaching 10-25 m above the bottom (in other places in a similar context plumes were observed of > 200 m of height). CTD-profiling, which shows very little change in bottom-water temperature at the venting sites, suggests that the plumes represent cold seeps.Heat-flow values measured over the area show a good correlation with changes in BSR depth: values vary between 50-60 mW/m² to 80-90 mW/m². In the craters, heat-flow values are highest, but they do not exceed 165 mW/m². Our observations suggest that the Baikal hydrates are locally - along particular segments (about 15 km long) of active faults - destabilizing by tectonically controlled upward flow of fluid and heat, and that this results in active venting of gasses and/or fluids at the lake floor

Multi-frequency seismic study of the gas hydrate accumulations in Lake Baikal, Siberia

Recently, the presence of methane hydrates has been evidenced in Lake Baikal, Siberia, by means of seismic profiling, deep drilling and shallow coring. This is -up to now- the only reported occurrence of gas hydrates in a confined fresh-water basin. In this presentation, we discuss the frequency-dependent acoustic characteristics of the hydrate-bearing sediments, using 5 different types of reflection seismic data encompassing frequencies from 10 to 1000 Hz. On low-frequency airgun-array data, the base of the hydrate stability zone (HSZ) is observed as a single, high-amplitude, inverse-polarity reflection that often crosscuts the local stratigraphy. Amplitude and continuity of the BSR decrease or even disappear on higher-frequency data. On medium- to high-frequency data (e.g. watergun) the base of the HSZ is no longer expressed as a single reflector, but rather as a facies change between enhanced reflections below and blanked reflections above. The increasing reflection amplitude of the BSR with increasing offset (AVO-analysis), the high reflection coefficient of the BSR (-40 % of lake floor reflection) and the presence of enhanced reflections beneath the BSR suggest the presence of free gas below the HSZ. The observation of some enhanced reflections extending into the HSZ could even indicate that free gas may co-exist with hydrates within the HSZ. Blanking of the reflection amplitudes above the BSR is variable. Instantaneous frequency analyses reveal a low-frequency shadow beneath the BSR. We also collected lake-bottom reflection/refraction data, using GEOMAR's "Ocean-Bottom Hydrophones". Several profiles were recorded with a medium-resolution single airgun with sufficient energy to penetrate below the HSZ. The velocity information obtained from these measurements shows a distinct low-velocity layer below the base of the HSZ. Above, several higher-velocity layers are recognised. Modelling of interval velocities in this zone indicate hydrate presence of 5 to 8 % of pore volume. We also acquired new medium-frequency, single-channel airgun data at the BDP-1997 site (Baikal Drilling Project), providing the first acoustic images from this location. Hydrates (10 % pore volume) were retrieved from 121 and 160 m sub-bottom depth, but still about 200 m above the base of the local hydrate stability field. Remarkably, the seismic data at the drilling site show no indications for the presence of hydrates at the hydrate-recovery depths (no acoustic blanking, no BSR). These results were used to roughly estimate the amount of carbon stored in the Lake Baikal hydrate reservoirs, showing that most probably they do not form a future energy resource

Three-dimensional seismic investigations of the Sevastopol mud volcano in correlation to gas/fluid migration pathways and indications for gas hydrate occurrences in the Sorokin Trough (Black Sea)

New 3-D seismic investigations carried out across the Sevastopol mud volcano in the Sorokin Trough present 3-D seismic data of a mud volcano in the Black Sea for the first time. The studies allow us to image the complex three-dimensional morphology of a collapse structured mud volcano and to propose an evolution model. The Sevastopol mud volcano is located above a buried diapiric structure with two ridges and controlled by fluid migration along a deep fault system, which developed during the growth of the diapirs in a compressional tectonic system. Overpressured fluids initiated an explosive eruption generating the collapse depression of the Sevastopol mud volcano. Several cones were formed within the depression by subsequent quiet mud extrusions. Although gas hydrates have been recovered at various mud volcanoes in the Sorokin Trough, no gas hydrates were sampled at the Sevastopol mud volcano. A BSR (bottom-simulating reflector) is missing in the seismic data; however, high-amplitude reflections (bright spots) observed above the diapiric ridge near the mud volcano at a relatively constant depth correspond to the approximate depth of the base of the gas hydrate stability zone (BGHSZ). Thus we suggest that gas hydrates are present locally where gas/fluid flow occurs related to mud volcanism, i.e., above the diapir and close to the feeder channel of the mud volcano. Depth variations of the bright spots of up to 200 ms TWT might be caused by temperature variations produced by variable fluid flow

Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal, Siberia

In this paper we present and discuss the frequency-dependent behaviour of the acoustic characteristics of methane hydrate-bearing sediments in Lake Baikal, Siberia. Five different types of seismic sources (airgun-array, two types of single airguns, watergun and sparker) are used, encompassing a frequency bandwidth from 10 up to 1000 Hz. On low-frequency airgun-array data, the base of the hydrate stability zone (HSZ) is observed as a high-amplitude bottom-simulating reflection (BSR) with reversed polarity. The amplitude and continuity of the BSR decrease or even disappear on medium- to high-frequency data, a feature explained in terms of vertical and horizontal resolution. The increasing reflection amplitude of the BSR with increasing offset, the calculated reflection coefficient of the BSR and the occurrence of enhanced reflections below the BSR suggest the presence of free gas below the HSZ. The observation of some enhanced reflections extending above the BSR may be interpreted as an indication for free gas co-existing with hydrates within the HSZ. Amplitude blanking above the BSR is highly variable while the BSR itself appears to act as a low-pass frequency filter for medium- to high-frequency data.New single-channel airgun profiles provide the first seismic information across the Baikal Drilling Project (BDP-97) deep drilling site, at which hydrate-bearing sediments were retrieved at about 200 m above the base of the local HSZ. At the drilling site there are no seismic characteristics indicative of the presence of hydrates. Combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of Lake Baikal, which lead us to conclude that the Lake Baikal gas hydrate reservoirs do not form a prospective energy resource

Active hydrate destabilization in Lake Baikal, Siberia?

In this paper, we present new seismic and heat-flow data that show the base of the hydrate stability zone (BHSZ) in Lake Baikal to be locally characterized by abnormal variations in depth, with distinct regions of deeper-than-normal and regions of shallower-than-normal BHSZ. These variations are related to strong lateral variations in heat flow, and occur in close association with important rift-basin faults. Areas of shallow BHSZ are also characterized by the presence of several methane seeps and mud volcanoes at the lake floor. We infer that the seeps are the surface expression of escape pathways for overpressured fluids generated by the dissociation of pre-existing hydrates, in response to a thermal pulse caused by an upward flow of hydrothermal fluids towards the BHSZ. It thus seems that present-day hydrate dissociation in Lake Baikal is modulated by the tectonic activity in the rift rather than by – climatically controlled – changes in lake level or water temperature

Gas hydrates and gas seeps in Lake Baikal, Siberia

C