Predictive Areas For Gas Hydrate

Three areas of likely methane gas hydrate development have been identified. These areas each have similar interval tectonostratigraphic character, in which sediment thickness and gas-generating capability of sediments are suitably disposed between 1 km and 3.5 km water depth. In addition, seismic reflection data indicate the presence of a BSR in all three areas (Fig. 1).

Area I (Fig. 2) is composed of passive margin sediment built-up to between 1 and 3 km thick. Area II is composed of active margin sediment in a forearc trench on the depressed eastern margin of the South China Sea Plate and in the imbricated sediments of the obducted Heng-Ch'un Ridge. Area III is composed of passive margin sediment that has been encroached upon by the active margin with resultant thick sediment build-up (> 8 km).

Sediments along the Area I and III margins have been accumulating since early Mesozoic (Huang, 1993) during which time considerable hydrocarbon source beds were deposited world-wide (Dow, 1978; Jansa and MacQueen, 1978). Sediments in the Heng-Ch'un Ridge, however, are younger than upper Mesozoic (Huang, 1993). They are more thinly bedded than along the continental margin.

It is in Area III that there is the greatest likelihood of gas generation. The thick sediment build up found here is due to a combination of original sedimentation and tectonic thickening. This zone would have been a depocenter subject to sediment influx from both the NW and the NE.

Figure 2. Map of likelihood of hydrate in the sea area immediately south and west of Taiwan. Sediment thickness between 1 km and 3.5 km water depth. Area I, passive margin sediment build-up between 1 and 3 km thick. Area II, active margin sediment and depressed South China Sea Plate trench. Area III, active margin superimposed upon by the active margin.

Figure 2. Map of likelihood of hydrate in the sea area immediately south and west of Taiwan. Sediment thickness between 1 km and 3.5 km water depth. Area I, passive margin sediment build-up between 1 and 3 km thick. Area II, active margin sediment and depressed South China Sea Plate trench. Area III, active margin superimposed upon by the active margin.

Uncompacted passive margin sediments may host significant hydrate concentrations. If these sediments are then brought into subduction complexes and accretionary prisms by tectonic encroachment, the tectonism can accentuate hydrate redistribution and methane concentration. The impingement, and apparent SW migration of the western Pacific Plate prong, which passes through Taiwan obliquely into the passive margin sediments should cause concentrations of gas hydrate to form by tectonic accentuation of the cycle of dissociation and reformation of hydrate (Max and Lowrie, 1996).

The hydrate conservation cycle (Max and Lowrie, 1996; 1997) was originally conceived as a response to long-term sedimentation that would result in the base of hydrate rising as a thermogenic response to the elevation of the seafloor. Tectonic activity that raised seafloor, even without the effect of sedimentation and burial, would have the same effect of lowering the seafloor pressure. In a sedimentary prism such as the Heng-Ch'un Ridge, the HSZ of a bathymetrically deep seafloor will thin as a response to lower ambient pressure when the sediment package is technically elevated. This causes hydrate stable in the HSZ at depth to dissociate. The concentration of methane in the HSZ by this tectonic process could result in formation of concentrated methane deposits because large amount of methane would be brought into a relatively thin HSZ.

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