WP3 - deliverable No. 1: Mapping of shallow gas and physical characterisation of gas bearing sediments
Deliverable
Jørn B. Jensen and Rudolf Endler
Mapping and quantification of shallow gas by seismo-acoustic techniques
During the Baltic Gas project a large seismic dataset has been collected as a combination of data mining of existing data and acquiring of new data in the numerous cruises carried out under the Baltic Gas project flag.
Contributors of existing seismic data has mainly been the Baltic Gas project members, who also collected additional data during the project
The collected seismic data has been loaded on seismic workstations by the data owners, the distribution of free gas has been digitized and the data has been compiled at GEUS, as basis for GIS-mapping carried out by Alfred Wegener Institute for Polar and Marine Research (AWI).
In order to understand the mechanism of gas production in the seabed; it was decided to make detailed mapping studies in few key areas.
Figure 1: Bornholm Basin key area mapped areas with acoustic blanking caused by scattering effects due to gas bubbles in the sediment and deeper structures represented by the general fault pattern framework (black stippled lines). Depth from seabed to top of acoustical gas front is mapped as well.
The Bornholm Basin was one of the key areas, well known for gas charged sediments and also covered by existing archive seismic data to an extent that it was possible during the Baltic gas project cruises to supplement and get full coverage of the basin.
Observation of shallow gas on seismic profiles is related to acoustic blanking, caused by scattering effects, due to gas bubbles in the sediment (Fig. 4). The surface of the gas front has been digitized and a gas distribution map with indication of depth from seabed to top of acoustical gas front has been produced.
From mapping experience a critical thickness of organic rich Holocene mud must be reached in order to produce free gas bubbles. Characteristic for the Bornholm Basin with water depths in the order of 90m is that the acoustic blanking starts, when the Holocene organic rich mud reaches a critical mud thickness of 6 – 8m.
Physical characterization of gas-bearing sediments
An important part of characterization of gas-bearing sediments has been by physical properties in cores. Multisensor core logging was used to estimate basic physical properties of gas free and gas charged sediments. The results were used for interpretation of sediment echo-sounder records. From this data the thickness of the Holocene mud (deposits of the Littorina Sea) and of the older deposits from the previous Baltic Sea Stages can be estimated.
Figure 2 Results of multisensor logging of a gravity core in (Bornholm Basin. The parameters are:" vp" - pwave velocity, "dwb" -wet bulk density, "vsh" - vane shear strength torsional moment , "conductivity" - electrical conductivity, "Water cont" - gravimetric bulk water content, "suszeptibility" - magnetic volume suszeptibility, "Ignition loss" - loss of ignition, "colorvalue H S V" - from core photo extracted color values of the HSV model. A short sediment echosounder record (SES) is attached at the right side for comparison.
High porosity of the mud is the reason for its high storage capacity for gas bubbles.
Therefore, the gas bubbles are trapped in their "source deposit" and will presumably not move by convection out of the sub bottom into the water column. Minor bubbling may occur along local weak zones, e.g. caused by benthic activities. Only if the gas content of the pore space would overcome 20 % then the particle bonding of the mud will break up leading to a fluid mud. This will open the way for a stronger convection driven gas bubbling out of the sea floor.
Fig 4 Acoustic images (SES 10 kHz),of a shallow gas structure in the Bornholm Basin. Red colours indicate high amplitudes, green-blue colours represent low amplitudes.The surface of the bubbly layer is located about 1m below the sea-bottom (narrow high backscatter stripe). Deeper layers are hidden
The influence of gas bubbles is high on acoustic properties like sound velocity and attenuation. The behavior of acoustic properties is very complex and controlled by environmental parameters like pressure and temperature, by the sound frequency and the physical properties of the different sedimentary components like solid grains, frame, pore space etc
The resonance frequency of the single size bubble is different for each sediment type and is depending on the strength of the sediment. The most obvious feature is the very strong increase of attenuation, mainly caused by strong backscatter which is clearly seen in sediment echo sounder records. Above the resonance frequency the sound velocity is about the same as for gas free sediments. But below the bubble resonance frequency sound velocity drops down to values of less than 500 m/s which is in good agreement with multichannel seismic velocity modeling.