The lithological characteristics of natural gas hydrates in permafrost on the Qinghai of China

The environment is seriously threatened by the methane emitted as permafrost melts. Studying deposits of natural gas hydrates that include methane is therefore important. This study presents a novel approach based on the rock Archie formula to discover the porosity and saturation of gas hydrates. The relationship between resistivity and porosity and the porosity of hydrates was studied, and the results showed that the resistivity of hydrate reservoirs was closely related to porosity and hydrate saturation, and the polarization rate was only related to the concentration of natural gas hydrates and had nothing to do with porosity. Using the multi-channel time domain induced polarization (MTIP) method, the profile with five boreholes in the Muli area of the permafrost area of the Qinghai-Tibet Plateau was observed, and the thickness of the shallow permafrost distribution and the underground structure were inferred based on the resistivity of the MTIP data. The polarization rate and hydrate saturation of the inversion assessed the presence of hydrates in the Muli region. The results show that the MTIP method can be used to detect the thickness of permafrost distribution, determine fault boundaries, reveal the distribution of natural gas transport paths, and evaluate the presence of natural gas hydrates.

Wavefield Decomposition of Ocean-Bottom Multicomponent Seismic Data with Composite Calibration Filters

Downgoing/upgoing P/S-wave decomposition of ocean-bottom seismic (OBS) multicomponent data can help suppress the water-layer multiples and cross-talks between P- and S-waves, and therefore plays an important role in seismic migration and construction of P- and S-wave velocity models. We proposed novel composite calibration filters by introducing an additional dimension to the calibration of the particle velocity components, extending the wave-equation-based adaptive decomposition method. We also modified the existing workflow by jointly using primary reflections at near-to-medium offsets and ocean-bottom refractions at far offsets in the calibration optimization. The decomposition scheme with the novel calibration filters yielded satisfactory results in a deep-water OBS field data decomposition example. Expected decomposition effects, such as the enhancement of primary reflections and the attenuation of water-layer multiple events, can be observed in the decomposed upgoing wavefields. An experiment illustrated the effectiveness of composite calibration filters that compensated for unexpected velocity errors along the offset dimension.

Impact of Gas Saturation and Gas Column Height at the Base of the Gas Hydrate Stability Zone on Fracturing and Seepage at Vestnesa Ridge, West-Svalbard Margin

The Vestnesa Ridge, located off the west Svalbard margin, is a >60 km long ridge consisting of fine-grained sediments that host a deep-marine gas hydrate and associated seepage system. Geological and geophysical observations indicate the predominance of vertical fluid expulsion through fractures with pockmarks expressed on the seafloor along the entire ridge. However, despite the apparent evidence for an extended free gas zone (FGZ) below the base of the gas hydrate stability zone (BGHSZ), present-day seafloor seepage has been confirmed only on the eastern half of the sedimentary ridge. In this study, we combine the relationships between aqueous phase pressure, capillary pressure, sediment clay fraction, porosity, and total stress to simulate how much gas is required to open preexisting fractures from the BGHSZ towards the seafloor. Data from four specific sites with different lithology and pressure regime along the ridge are used to constrain the simulations. Results demonstrate that fracturing is favored from the FGZ (with gas saturations < 0.1 and gas column heights < 15 m) towards the seafloor. Neglecting the capillary pressure overpredicts the size of the gas column by up to 10 times, leading to erroneous maximum gas vent volume predictions and associated ocean biosphere consequences. Further parametric analyses indicate that variations in the regional stress regime have the potential to modify the fracture criterion, thus driving the differences in venting across the ridge. Our results are in line with independent geophysical observations and petroleum system modeling in the study area, adding confidence to the proposed approach and highlighting the importance of the capillary pressure influence on gas pressure.