Seismoacoustic Study of the Shallow Gas Transport and ... - E-LIB

Chapter 5
In addition to the feeder channels, three other types of pathways for gas and fluid
migration could be recognized: columnar zones, acoustic curtains and faults. And three
types of gas reservoirs in the central Black Sea are identified: a) Large gas/gas hydrate
reservoirs around the feeder channels of mud volcanoes (BSZA); b) Large gas/gas
hydrate reservoirs located above the acoustic curtains (BSZB); c) Small gas/gas hydrate
reservoirs at the top or sides of the faults and columnar zones (BSZC). The BSR is absent
in most parts of the central Black Sea, probably due to sedimentary layers being almost
parallel to the seafloor and due to a local gas supply. However, a local BSR segment was
observed at a depth of ~650 ms TWT bsf. Although the accuracy is not very high, the
thermal gradient and heat flow can be approximately estimated from the BSR. Assuming
a sediment velocity of 1600 m/s and the average BSR depth, a temperature gradient of
23°C/km is suggested. All the BSZAs of relatively wide lateral extent appear at the
boundary between Units U1 and U2, which is marked by a change from supposedly more
sandy material to fine-grained turbidites, providing a seal for gas migration due to lower
(vertical) permeability, forming a reservoir-seal combination. The bright spot Zones
BSZB and BSZC are more probably formed by massive gas hydrate layers, converted
originally from free gas, which are now associated with normal polarity reflections.
Mud volcanoes are composed of wide and deep feeder channels, showing several wide
mud chambers, residing on collapsed sediment strata in the center. They show both gas
and gas hydrate accumulations near the feeder channel, where gas may be of biogenic
and thermogenic origin, and partially also from hydrate decomposition during eruptive
phases. Kornev, Goncharov Malyshev, MSU and Vassoevitch mud volcanoes reveal
three major eruption episodes, while at Yuzhmorgeologiya mud volcano only two major
eruption episodes can be identified. All of the eruptive episodes of the mud volcano
might be related to distinct sea level falls, which seem to be one of the main trigger
factors of the mud volcano eruptions in the central Black Sea.
In Chapter 4, we studied the gas seeps in the western part (area 1 in Chapter 3) of the
Kerch Peninsula margin, Black Sea. Strong evidence was observed in seismoacoustic
water column and subsurface data for free gas, distributed in numerous bright spots
within, beneath and above the GHSZ. The presence of gas hydrates in conjunction with
the deformation history of the sedimentary units appears as the main factor for gas
distribution, migration and seepage. No BSR can be clearly identified, which may be
attributed to limited permeability due to finer-grained sediments in the sedimentary units
near the seafloor and at the base of the GHSZ. Instead, other seismic features like the
boundary of zones, where high amplitude reflectors (HATZ) or bright spots terminate
(BSTZ), may indicate the location of BGHSZ, inhibiting lateral migration of free gas by
gas hydrate impregnation. Both types of terminations zones closely coincide with the
calculated BGHSZ. Three anticlinal structures were recognized in the study area (Chapter
3), which probably provide effective structural traps for oil and gas in the study area. For
the special situation at the Kerch seep area, our model suggests that free gas is likely
originating from deeper sources and migrating along steep upthrust faults on the
basinward side of the Anticline b towards their crest. Gas reservoirs are found at the top
of Anticline b. Furthermore, the distal Paleo-Don River fan may inhibit the upward gas
migration, which is leading to lateral migration towards morphological highs or,
alternatively, further upslope. Near the anticlines this may contribute to the builtup of gas
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