Reservoir Characterization Using Poisson Impedance and Vp/Vs Inversion Methods in Ngimbang Clastics Sandstone, North East Java Basin, Indonesia

Ngimbang Clastics within the JS Ridge, North East Java Basin is proven to be a productive hydrocarbon interval. The area is surrounded by three possible kitchens (deeps) which are the Central Deep, Tuban Trough, and Lamongan Deep. Apart from the traditional carbonate system of the Kujung Formation as the main reservoir, the occurrence of the older interval of Ngimbang Clastics is also considered as the deeper reservoir target. This interval is characterized by thin-bedded sandstone, conglomeratic sandstone, and shale deposited as syn-rift products on the fluvial-estuarine environments. Rock physics analysis has been carried out by connecting well logs and seismic inversion results in order to discriminate the reservoir from other lithology and to know the fluid content within this reservoir. This process is conducted to find an appropriate model with a reasonable match between the elastic parameter and well data. Fluid content and contact that are accurately estimated by the Vp/Vs ratio can be transformed to seismic data by the Poisson Impedance (PI) inversion method. PI is calculated by subtracting Acoustic Impedance with Shear Impedance ( \({\text{PI}} = {\text{AI}} - c{\text{SI}}\) ), where \(c\) is a term of rotation value of the AI-SI data. The Poisson Impedance (PI) inversion is a type of pre-stack inversion, where this technique enhances the appearance of distribution and fluid content within siliciclastics reservoir rocks of Ngimbang Clastics sandstone interval. In this interval, a low value of Poisson Impedance indicates gas sand with a range of 500–6500 ft/s*g/cc. The low value of Poisson Impedance and Vp/Vs ratio identifies gas content in a reservoir. Therefore, it is applicable to discriminate between sandstone reservoirs and non-reservoirs.