Characteristics of gas hydrate reservoirs and their effect on petrophysical properties in the Muli area, Qinghai-Tibetan plateau permafrost

Abstract

Gas hydrates occur in the pores of sandstones with low porosity and high clay mineral content and in the fractures of mudstones in the Muli area. This leads to difficulties in the evaluation of gas hydrates using geophysical logs. Understanding the characteristics of gas hydrate reservoirs and the generation mechanisms for geophysical log responses is the basis for log interpretation. In this study, core experiments including scanning electron microscopy, thin section observation, nuclear magnetic resonance, and porosity and permeability measurements were performed to understand the pore structure of gas hydrate reservoirs. X-ray diffraction, X-ray fluorescence, cation exchange, total organic content, and natural gamma spectrometry measurements were conducted to understand the rock composition and its effect on the petrophysical properties of the reservoirs. Reservoir log responses were analyzed and combined with core experimental data. The results showed that pore-type reservoirs with low porosity (0.6%–10.7%) and low permeability (0.005 mD-0.603 mD) belong to tight sandstones and fracture-type reservoirs contain relatively high TOC (0.133%–8.377%). A high resistivity log value is the most significant feature of gas hydrate-bearing reservoirs. An increase in clay mineral content, which is related to more bound water and a higher cation exchange capacity in sandstones, can remarkably reduce resistivity, which results in poor correlation between the resistivity of water-saturated sandstone and porosity. Fractures in mudstone are important storage spaces for gas hydrates. Calcite veins frequently occur in mudstones in the Muli area. If fractures are filled by calcites, the density, resistivity, and velocity of mudstone increase significantly. The velocity log values between gas hydrate-bearing reservoirs and other formations show no significant difference, which is caused by narrow and complex storage spaces, fracture-filling constituents, and poor velocity measurement results related to borehole collapse. This study is helpful to improve the accuracy of evaluation of gas hydrates in the Muli area.