Mineralogy and pore characteristics of marine gas hydrate-bearing sediments in the northern South China Sea

Abstract

Properties of host sediments are critical to understanding the formation and occurrence of gas hydrates in nature. This study investigates the mineralogy and pore characteristics of marine gas hydrate-bearing sediments (GHBS) within and below two high-saturation zones cored in drilling site W01 in the Qiongdongnan Basin in the South China Sea. The study involves an original application of high-resolution SEM imaging and automated mineralogy to fields-of-view (FOVs) large enough to undertake quantitative analyses at micro- to nano-scales. The samples are mainly fine-grained (<63 μm) and mineralogically dominated by illite, montmorillonite and quartz, significantly higher in clay mineral content than GHBS cored elsewhere in the South China Sea. Porosities determined from thin sections are from 27.6% to 33.0% and the pores are dominated by diameters of 0.1–1 μm. A sample from the upper high-saturation GHBS is rich in clay minerals and clay-sized grains with pores mainly 0.1–1 μm in size, while a sample from the lower zone of high-saturation GHBS has less clay minerals, more sands, and pore diameters of >1 μm are as important as those 0.1–1 μm in size. A third sample from an underlying zone has similar mineral compositions, grain-size and pore-size distributions to those in the uppermost high-saturation hydrate bearing zone. The largest pores are due to microfossil cavities, but their contribution to total porosity is <5%, suggesting their role in fine-grained sediments should not be overestimated. It is inferred that grain size and pore size, affected by variations in mineral content, are important factors influencing differences in hydrate accumulation. The geological source of gas and its migration are also factors determining the distribution of gas hydrate in the study area. Our approach using large FOVs to obtain quantitative micro- and nano-scale data from SEM images facilitates correlation to centimeter- and meter-scale information from well logs and seismic data, and offers opportunities to obtain new insights into the in-situ occurrence and accumulation of gas hydrate and its effects on physical properties of gas hydrate reservoirs.