Abstract
The Muli area is the only permafrost region on the Chinese mainland wherein gas hydrates have been discovered. The gas hydrates are present in the fractures and pore spaces of the host rocks with a lamellar or micro-disseminated structure. By combining conventional and image logs, we describe the thickness of the permafrost layer and the well log responses of the gas hydrate reservoir, and calculate the porosity and gas hydrate saturation. We then analyze the advantages and disadvantages of different logging methods for evaluating gas hydrate reservoirs. Our results indicate that (1) gas hydrates are present below the permafrost in the Muli area, (2) gas hydrates predominantly occur in rock fractures, (3) the apparent resistivity is sensitive to gas hydrates present in pore spaces, and both apparent resistivity and acoustic logs are sensitive to gas hydrates present in fractures, (4) a density log is more appropriate for calculating porosity, and (5) gas hydrate saturation can be effectively calculated by the Archie equation, the modified Archie equation, and the Indonesian equation.
Highlights
Gas hydrates are cage-structured compounds consisting of water and natural gas
Regardless of the pattern observed, all temperature logs could be used to determine the thickness of the permafrost region
The pore-type gas hydrate reservoir can be identified by apparent resistivity logs, while the fracture-type gas hydrate reservoir can be identified by apparent resistivity and acoustic logs
Summary
Gas hydrates are cage-structured compounds consisting of water and natural gas They are mainly found in terrestrial permafrost regions and in marine sediments along the outer margins of continents, and they are a potential source of unconventional energy[1]. From 2008 to 2017, the China Geological Survey conducted a scientific drilling project to evaluate the presence of gas hydrates in the south of the Qilian Mountains in Qinghai, which included obtaining gas hydrate samples from boreholes. In permafrost areas and marine sediments, the well log responses of gas hydrate reservoirs are characterized by high resistivity, high P-wave velocity, high neutron porosity, and low density[16,17,18,19,20]. We develop a set of well logging methods that can be used to evaluate high-altitude permafrost gas hydrate reservoirs. There are three coal fields located on the northern syncline and four coal fields on the southern syncline
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