Quantitative seismic interpretation for gas hydrate and free gas-bearing sediments in the Shenhu area, South China Sea

Abstract

During the first gas hydrate drilling expedition conducted by Guangzhou Marine Geological Survey (GMGS1) in 2007, a total of eight Downhole Wireline Logging (DWL) boreholes, and five coring and in situ testing boreholes were completed in the Shenhu area, South China Sea. The DWL boreholes were drilled to collect continuous downhole logs of seismic data relevant for gas hydrate characterization. Seismic data were used to aid in the setup of the five coring and in situ testing boreholes. The logging tool was deployed to measure the natural gamma, in situ density, neutron porosity, sonic velocity, and formation resistivity. Gas hydrates have been identified from logging data at three of the eight sites. Results from DWL, in situ temperature measurement, pore water sampling, and pressurized and nonpressurized coring indicate that a 25-meter-thick layer of gas hydrate-bearing sediment at Site SH2 and gas hydrate commonly occurred in the fine-grained sediments with a maximum saturation of 47% of the pore volume. To quantitatively interpret gas hydrate and free gas-bearing sediments at Site SH2, in the Shenhu area, South China Sea, the multi-channel seismic (MCS) data with preserved amplitude processing and logging data were used for seismic interpretation, AVO analysis, and the estimation of gas hydrate and free gas saturations. In the seismic data, a clear and continuous BSR, a top of gas hydrate-bearing sediments (TGHBS), and a bottom of free gas-bearing sediments (BFGBS) as well as polarity reversals at BSR have been identified. Because of the presence of gas hydrate in the gas hydrate stability zone and free gas beneath the gas hydrate stability zone, some reflectors with polarity reversal often appear at the BSRs. However, polarity reversals also occur at the TGHBS. We use seismic modeling to verify that polarity reversal can occur either at the BSRs or at the TGHBS. The polarity reversal at the BSR is caused by the opposite reflection coefficient sequences from gas hydrate in the sediment pore space within the gas hydrate stability zone (GHSZ) to free gas in the pore space beneath GHSZ with different saturations. The polarity reversal at the TGHBS is due to the opposite reflection coefficient sequences from water in pore space above TGHBS to gas hydrate in the sediment pore space within the gas hydrate stability zone with different saturations. Therefore, the polarity reversal at the top and bottom of the hydrate layer is a unique phenomenon when the hydrate layer is cross-cutting strata. P-wave velocity well log was used to estimate gas hydrate and free gas saturations from an isotropic model of effective medium theory (EMT). The estimated gas hydrate saturations, close to the saturations calculated from pressure core and chlorinity data, have an average value of 22% with a maximum value of 46% at 208 mbsf. The estimated free gas saturations decrease below the BSR with a maximum value at 220.5 mbsf of ~0.3% assuming a uniform distribution and ~10% for a patchy distribution. The calculated density in the uniform distribution are much closer to the measured density than that in the patchy distribution which suggests that the uniform model is probably preferred. We also analyze the AVO of the TGHBS and BFGBS to estimate the gas hydrate saturation at the TGHBS and free gas saturation at the BFGBS, respectively. The gas hydrate saturation at the TGHBS is ~18% and free gas saturation at the BFGBS are ~0.25%.

AAPG Datapages/Search and Discovery Article #90348 © 2019 AAPG Asia Pacific Region Geosciences Technology Workshop, Gas Hydrates – From Potential Geohazard to Carbon-Efficient Fuel?, Auckland, New Zealand, April 15-17, 2019