Identification and the differential accumulation of leakage-typed gas hydrate in the Qiongdongnan Basin, northern South China Sea

Leakage-typed gas hydrates occurs in the form of block, vein, nodule in the core and are developed widely in tubular leakage passage in the Songnan low uplift in the central depression of the Qiongdongnan Basin in the northern South China Sea. To explore a comprehensive identification method for the leakage-typed gas hydrate and their differences in the accumulation for various ore bodies of leakage-typed gas hydrate. Identification for multiple types of gas hydrate reservoirs and fluid leakage distribution were studied combining seismic, logging well drilling, and borehole data. In the comprehensive identification methods, seismic reflection phase axis pull-up and variance seismic attribute anomalies indicate fluid leakage channels; the separation of lateral and induced resistivity logging curves, the bright and high-angle deformation layers seen in the resistivity image are the logging responses of leaky hydrates. Based on the borehole data and seismic profiles, three gas-hydrate ore bodies related to leakage gas hydrates were identified, and one of them encountered high-saturation diffusion gas hydrates in sandy sediment. The deposition of sand bodies significantly weakened the fluid leakage intensity of the hydrate ore bodies, during which sand sediments served as good reservoirs, and also energy storage with pressure relief. In hydrate ore bodies without developed sand bodies, seepage-type hydrates are significantly developed in seepage channels, while far away from the seepage channels, the hydrate drilling indication is poor. However, in the absence of sand sediment, leakage gas hydrates well developed in the leakage channels, while far away from the channels, the hydrate drilling indication is poor. Therefore, sand bodies and the leakage channels are the main controlling factors for the differential accumulation of leakage-typed gas hydrate in the Qiongdongnan Basin.