3.1 Classification of Gas Hydrate Deposits

Natural GH accumulations are divided into three main classes [124] based on sim­ple geologic features and the initial reservoir conditions. Class 1 deposits are com­posed of two layers: the HBL and an underlying two-phase fluid zone containing mobile gas and liquid water [129, 125] . Class 2 deposits comprise two zones: an HBL, overlying a zone of mobile water (hereafter referred to as WZ). Class 3 accu­mulations are composed of a single zone, the hydrate interval (HBL), and are char­acterized by the absence of an underlying zone of mobile fluids. In Classes 2 and 3, the entire HBL may be well within the hydrate stability zone and can exist under equilibrium or stable conditions. A fourth class (Class 4) pertains specifically to oceanic accumulations, and involves disperse, low-saturation hydrate (SH < 10%) deposits that lack confining geologic strata [140].