The Malay Basin is located at the southern part of the Gulf of Thailand between Vietnam and Peninsular Malaysia. The produced gases in the basin contain varying amounts of CO2. High CO2 concentrations (>50 mol%) are, in typical of reservoirs, mainly derived from inorganic sources. In order to develop these fields with reduced or zero greenhouse gas emissions for sustainability development, some geological storage sites were screened to store the future CO2 production. One of the important geological storage sites is the saline aquifer. There are many elements to assess the quality of the CO2 storage sites, such as caprock integrity and storage capacity. This study focused on one of the caprock characteristics, i.e., caprock velocity characteristic, and its significance for CO2 storage in a saline aquifer.

The studied saline aquifer is one of a numerous grabens or sub-basins at the southern part of Malay Basin. There is one abandoned exploration well with 2D and 3D seismic surveys and there are several types of velocity data, including sonic log, checkshot and seismic stacking velocity. The formations are composited of multiple thick sandstones and shales. Based on velocity analysis, including average velocity and inter-val velocity analysis. the interval velocity in caprock zone is related to the lithology, normally expressed by the sandstone percentage in the caprock zone. The lithology variation will affect the geomechanics characteristics, so, the velocity in caprock zone presents the caprock quality. The caprock is confined within the sub-basin and sandstone percentage is less than 40%, which shows a good seal capacity, com-bined with the caprock thickness distribution.

The velocity in shale zone (caprock) is higher than overlying and overlain sandstones. There is strong positive reflection (peak) at the top of shale zone (caprock) and negative reflection (trough) at the bottom of caprock, i.e., top of sandstone aquifer. There would be strong class III AVO feature at the top of sand-stone aquifer on time-lapse 4D seismic reflection once CO2 is injected. These features (negative reflection with bright spots) and class III AVO at the top of aquifer) are useful for MMV to monitor the CO2 plume migration.

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