To obtain the characteristics of the gas hydrate reservoirs at GMGS3-W19, extensive geophysical logging data and cores were analyzed to assess the reservoir properties. Sediment porosities were estimated from density, neutron, and nuclear magnetic resonance (NMR) logs. Both the resistivity and NMR logs were used to calculate gas hydrate saturations, the Simandoux model was employed to eliminate the effects of high clay content determined based on the ECS and core data. The density porosity was closely in agreement with the core-derived porosity, and the neutron porosity was higher while the NMR porosity was lower than the density porosity of sediments without hydrates. The resistivity log has higher vertical resolution than the NMR log and thus is more favorable for assessing gas hydrate saturation with strong heterogeneity. For the gas hydrate reservoirs at GMGS3-W19, the porosity, gas hydrate saturation and free gas saturation was 52.7%, 42.7% and 10%, on average, respectively. The various logs provide different methods for the comprehensive evaluation of hydrate reservoir, which supports the selection of candidate site for gas hydrate production testing.

Recently, shallow gas fields and hydrate-bearing sand in the deepwater area of the northern South China Sea have been successively discovered, and the accurate prediction of shallow sands is an important foundation. However, most of the current prediction methods are mainly for deep oil and gas reservoirs. Compared with those reservoirs with high degree of consolidation, shallow sandy reservoirs are loose and unconsolidated, whose geophysical characteristics are not well understood. This paper analyzes the logging data of shallow sandy reservoirs recovered in the South China Sea recently, which show that the sand content has a significant influence on Young’s modulus and Poisson’s ratio of the sediments. Therefore, this paper firstly constructs a new petrophysical model of unconsolidated strata targeting sandy content and qualitatively links the mineral composition and the elastic parameters of the shallow marine sediments and defines a new indicator for sandy content: the modified brittleness index (MBI). The effectiveness of MBI in predicting sandy content is then verified by measured well data. Based on pre-stack seismic inversion, the MBI is then inverted, which will identify the sandy deposits. The method proposed provides technical support for the subsequent shallow gas and hydrate exploration in the South China Sea.

Research on gas hydrate accumulation in the Shenhu Area of the northern South China Sea has been conducted, highlighting a vertical distribution pattern with gas hydrates at the top and free gas at the bottom, with both coexisting in the middle of the formation. To address the challenge of quantitative evaluation arising from the interaction between coexisting gas hydrates and free gas in the hydrate-bearing zone during well logging, we established an identification method based on the intersection of three porosity logs that directly displays the vertical distribution pattern of the hydrate reservoirs. To quantitatively evaluate coexisting gas hydrates and free gas, we first constructed a model for calculating gas saturation based on the relationship between the resistivity increase rate and water saturation established through core experiments in the target interval. Based on this, we utilized the neutron density normalization difference to calculate the gas saturation in the coexisting zone. By determining the relative contents of bound water and free gas, we obtained the hydrate saturation in the coexisting zone. The hydrate saturation obtained using the above methods was consistent with the results of the core experiments, indicating the reliability of the method. The research results determine the hydrate content in the coexistence zone and provide a basis for the accurate assessment of natural gas hydrate resources in the study area.

Research on gas hydrate accumulation in the Shenhu Gas Hydrate Formation in the northern part of the South China Sea has been conducted, determining a vertical distribu-tion pattern with gas hydrates at the top and free gas at the bottom, with both coexisting in the middle of the formation. To address the challenge of quantitative evaluation arising from the interaction between coexisting gas hydrates and free gas in the hydrate-bearing zone during well logging, we established an identification method based on the intersec-tion of three porosity logs that can intuitively display the vertical distribution pattern of the hydrate reservoirs. To quantitatively evaluate coexisting gas hydrates and free gas, we first constructed a model for calculating gas saturation based on the resistivity increase rate and gas content through sedimentary core experiments in the target interval. Based on this, we utilized the neutron density normalization difference to calculate the gas satura-tion in the coexisting zone. By determining the relative contents of bound water and free gas, we obtained the hydrate saturation in the coexisting zone. The hydrate saturation ob-tained using the above methods was consistent with the results of the core experiments, indicating the reliability of the method. The research results can determine the hydrate content in the coexistence zone and provide a reliable basis for the accurate assessment of natural gas hydrate resources in the area.