--> Joint Interpretation of geophysical field experiments in the Danube deep sea fan, Black Sea

AAPG Asia Pacific Region Geosciences Technology Workshop:
Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?

Datapages, Inc.Print this page

Joint Interpretation of geophysical field experiments in the Danube deep sea fan, Black Sea

Abstract

Gas hydrates provide a natural deposit for methane gas within almost all sediment-rich continental margins. Due to the high grade of compaction such deposits might perform as future reservoirs for offshore marine gas production. Investigating the reservoir capabilities of such deposits require reliable estimates on the gas and hydrate content and slope stability. Prior to expensive in-situ sampling by drilling, geophysical investigation is the most appropriate tool for the exploration. Regional 2D multichannel seismic (MCS) data provide the broad overview of a hydrate prone area. High resolution 2D and 3D reflection images enable further development of detailed depth distribution models. Streamer systems with group offsets of 1.5 m applied in 3D acquisition like the P-Cable provide the highest resolution but lack of velocity information due to short cable length. Ocean-Bottom Seismometers (OBS) with their four-component refraction seismic acquisition fill in this gap. Both techniques combined allow the development of structural models with compressional (Vp) and converted shear wave (Vs) velocities. New developments allow for the application of common reflection surface (CRS), NIP tomography and full waveform inversion (FWI) techniques to enhance model results for seismic data sets. Deduced porosity values provide the link to 2D and 3D controlled-source electromagnetic measurements. The derived resistivity-depth distribution models further point towards the saturation in the reservoir. Calibration with laboratory experiments on hydrate formation models and resulting velocity and resistivity values guide the joint interpretation into saturation estimation models. A case study from the Danube Fan, Black Sea, shows that involved techniques and disciplines still lack from missing parameters or idealised model conditions. High resolution multichannel seismic measurements image structural changes, variable amplitude strength indicate changes in pore fill and fractures indicating migration pathways. Wide angle seismic measurements provide compressional and shear wave velocities. Detailed analyses of OBS data allows to confirm patchy distribution of gas accumulations in small pockets below the BSR. Vp and Vs analyses points towards lithological density anomalies for inverted reflection events within the hydrate stability zone. Density – depth profiles may be deduced to serve resistivity measurements. Controlled source electromagnetic investigations and seismic experiments are available in 2D and 3D complementing each other. Still physical parameters e.g. salinity and porosity distributions with depth are missing for a detailed conversion. Laboratory experiments on gas hydrate formation and dissociation usually use idealised sediment compositions or substantial material. In situ contaminations or mixtures of different sediment compositions are neglected. In this paper we summarize an individual approach of a multiple parameter study and the joint application fostering the understanding of the development of a hydrate system, its equilibrium conditions and stability. As a summary we point to the limits of concordance between the disciplines. Consequently, joint interpretations combining all available technology still need support by in situ ground sampling.