Estimation of Limestone Reservoir Porosity by Seismic Attribute and AVO Analysis

Parigi Formation is a group of sedimentary rocks deposited during the Neogen time transgression in Northwest Java Basin. Its lithology comprises mostly limestone biostrome with 10-50 m thick, bioherm of 300-500 m thick. It was deposited in the shallow marine environment during the Late Miocene. Parigi Formation is one of the most important gas reservoirs in Northwest Java Basin.

In the limestone reservoir, the hydrocarbons were trapped within vughs of the formation. The vughy or porosity in the Parigi Formation is studied by seismic attribute and AVO analysis. In this formation the top of porous zone could be first identified by the reflection strength and instantaneous frequency from the seismic attribute analysis. After the spatial location of this top has been identified, the value of porosity is estimated by using AVO analysis. In Shuey‘s AVO equation, the reflection coefficient as a function of angle of incidence depends on P-wave velocity, density and Poisson‘s ratio. The Marquardt iterative algorithm was used to find the Poisson’s ratio which is directly related to the S-wave velocity.

For carbonate reservoir, an empirical relationship exists between the porosity and the Young modulus. Since the Young modulus is related to the density, the P-wave velocity, the S-wave velocity and the Poisson’s ratio, then the porosity of the reservoir can be computed.

The application of these two methods to one structure in the Parigi Formation shows a zone of high reflection strength and low instantaneous frequency in the reef limestone reservoir. Porosity calculation in that zone by AVO analysis give a value of porosity of 36.6 % with a Poisson‘s ratio 0.266. These results compared favourable with borehole data In the location (20 - 38 % porosity). The method has given a satisfactory result in estimating the porosity and delineating its lateral direction.