Delineation of reservoir boundary using AVO analysis

Keutapang Formation is a group of sedimentary rocks consisting of sandstone with shale intercalation. It was deposited in the coastal environment during Middle Miocene age in the north Sumatra Basin. The thickness of the formation is in range of 500-1300 m.

In the sandstone reservoir, hydrocarbons were trapped within the porous zone. In this rock formation the top of the porous zone could be identified as the source of AVO anomaly. By applying Shuey’s (1985) equation to the AVO anomaly parameters of velocity of P wave Vp, ΔVp (velocity contrast of two layers), ρ (density), Δρ (density difference of two layers) and σ can be estimated by the Marquardt inversion.

The relationship between ρ and Vp for water-filled sand shale sequences is explained by Gardner’s et al. (1974) equation. The difference between the derivative and the equation itself is defined as the fluid factor.

Application of the fluid factor to the field where Keutapang sandstone located gives a significant change in values. These changes also shown by the value of P wave velocity and density for each CDP location. The prospective reservoir is located in the area of high value compared to the surrounding area.

Borehole data in the area shows the comformable result to the predicted zone, therefor the method has given a satisfactory result in estimating the zone of interest for further delineation of reservoir boundary.