Seismic-based Reservoir Characterization of Thin Shaly-Sandstones Reservoirs using Stochastic Inversion and Elastic Attributes Rotation, Study Case: Lower Terumbu Formation, East Natuna Basin, Indonesia

Abstract The main reservoir of the Lower Terumbu formation in the East Natuna Basin, Indonesia is shallow marine bioturbated shaly sandstones. A well-based analysis shows that the ratio between P- and S-wave velocities ( Vp/Vs ratio) and P-impedance are sensitive to lithology, porosity, and fluid saturation. Therefore, a seismic-based reservoir characterization method is carried out through a two-term Amplitude Versus Offset (AVO) analysis of Ursenbach-Stewart and Extended Elastic Impedance (EEI) inversion approaches. However, since the reservoir thickness is mostly below the tunning thickness, the stochastic inversion technique is utilized. The P-impedance and Vp/Vs ratio resulted from the EEI inversion are then combined to form a pseudo-elastic impedance for lithology (PEIL) and a curved pseudo-elastic impedance (CPEI). These two attributes are used to provide estimates of two main petrophysical parameters, namely porosity and fluid saturation. Results show that fluid saturation can be approximated by CPEI with a correlation of 0.83. Meanwhile, a linear relationship between porosity and PEIL is found with a better correlation of 0.85. Three main results representing P10, P50, and P90 are extracted out of 25 stochastic realizations using a hydrocarbon pore volume (HCPV) approach. The results demonstrate that the proposed approach has proven to be a robust and reliable technique to help delineate thin sandstone reservoirs in this study area.