Gauging formation dynamics of structural-seepage methane hydrate reservoirs in Shenhu area of northern South China Sea: Impact of seafloor sedimentation and assessment of controlling factors

Abstract

Abstract As massive methane hydrate reservoirs in Shenhu slope area of northern South China, the structural-seepage hydrate-bearing sediments, which mainly include thick-bedded and disseminated types of hydrates, show favorable advantages of high content, great thickness, and excellent exploitation value. They are generated by thermogenic methane vertically migrated from deep strata through faults, and finally accumulate in shallow sand layers. In order to investigate the formation dynamics of massive methane hydrate reservoirs in Shenhu area, a vertical fluid flow-methane hydrate formation reaction-seafloor sedimentation model is designed to gauge the accumulation mechanism. From the analysis of Peclet number which weighs the relative importance between deposition and formation reaction, local methane hydrate looks more likely to actively gather together in the sediments primarily, then gradually evolve into massive reservoirs accompanying passive seafloor sedimentation. Through choosing three typical formation stages (50 ka, 3 Ma, and 5 Ma) to exhibit the evolution process of these hydrate reservoirs, the change of local pressures, temperatures, dissolved methane and salt, phase saturations, stratum permeability, and pore capillary pressure deduces how the structural-seepage hydrate reservoir operates. The investigation also shows that after 5 Ma theses hydrate-bearing layers can proceed to take on similar appearance with current occurrence when average seafloor sedimentation rate and the initial seafloor are 5 cm/ka and 988 m, respectively. Finally, the performance of five controlling factors, including methane flux, kinetic coefficient, initial fluid position, permeability and seafloor sedimentation rate, has been quantitatively assessed in this formation model. Our work verifies this methane hydrate formation reaction-seafloor sedimentation mechanism is adequate for studying Shenhu structural-seepage hydrate reservoirs. The findings further suggest the combination of small methane flux and small reaction coefficient should be preferentially recommended to breed this type of methane hydrate-bearing layers.