OS5B-1:SEA-LEVEL AND GEOLOGICAL CONTROLS ON FORMATION OF HIGH SATURATION GAS HYDRATES ABOVE BSR IN THE SHENHU AREA, SOUTH CHINA SEA

Xin SU 1, Yi LUO 1, Jinqiang LIANG 2, Yiqun GUO 2, Fang CHEN 2, Xinhe YU 1, Jian LIU 2, Yang ZHOU 2
1. School of Marine Geosciences, China University of Geosciences, CHINA; 2. Guangzhou Marine Geological Survey,  CHINA

    In 2007, the first gas hydrate drilling in the Shenhu area, South China Sea recovered high saturation (>20% to 40%) of gas hydrate from silty clay dominated reservoir layer above BSR, being significantly different from world-wide silty clay reservoirs containing low saturation (3 to 10%) gas hydrates. A synthetic study was carried out to understand geological controls.

    (1) Source and transport of methane: few major faults was recognized trough Paleogene to lower Neogene strata, implying major methane flux in gas hydrate zones (GHZ) did not come from underlying Palaogene oil-gas reservoir strata. The dominated structure feature along Shenhu slope was slumping faults in ages of late Neogene to Quaternary.

    (2) Feature of reservoir: abundant foraminifera shells were found in silty clay sediments from gas hydrate reservoir. A number of evidences suggested that these shells provided abundant pore space for accumulation of high saturation gas hydrates (HS-GH).

    (3) Evidence of seal layer: sediments of GHZ along Shenhu slop were deposited during low sea-level periods. Sediment sequences above GHZ were featured with finer grains and low permeability, thus as a seal layer for upward migration of methane. This is supported by evidence of low methane flux in the upper part of the sequences in drilling holes

    (4) A numerical simulation model was developed to reconstruct BSR migration in correlation with sea-level change along Shenhu slope since Miocene. The results suggested BSR might be 20m deeper than the present depth during major low sea-level periods when it dropped about 100m. Dissociation of gas hydrates on shallow slope might cause slope failure and slumping deposits, in consisted with dominated occurrence of slumping along the slope. On deep slope, methane released by dissociation of gas hydrates might be stored in the former reservoir sediments by overlying seal layer. Methane amounts might increase with time due to low but continually biogenic methane input from underlying Neogene strata. Gas hydrate amount thus increased when it reformed in former reservoir during next high sea-level period. Therefore, the model provided reasonable explanation for sea-level and geological control on formation of HS-GH in the Shenhu area.