Publications

In the offshore East Java Basin, complex carbonate platforms within Wonocolo level have enormous effect on deep reservoir imaging. Accurate anisotropy estimation is a key factor for velocity model updating in carbonate platforms. For most cases carbonate rocks are essentially isotropic, especially at high net pore pressures. In the offshore East Java Basin, however, well data and seismic results both support negative estimates for Thomsen parameter delta in carbonate zones. The common image gathers (CIG) at the well locations show downward curvature when exact well velocity is used in the migration. This means the seismic velocity should be lower than well velocity, implying that negative delta is necessary. In this study, we combined Full Waveform Inversion (FWI) and well calibration to obtain accurate anisotropic models. We used the following steps: 1) FWI to obtain accurate seismic velocity. 2) Well calibration to match the well velocity and introduce negative delta. 3) Further update of the velocity based on the anisotropic model. After these steps, the final velocity model matches velocities at well locations, the CIGs are flat, and the structures beneath carbonate platforms are geologically plausible. We envisage that the lack of fine layering and low pressure are the main causes of the seismic negative delta. In this survey the carbonate depth is relatively shallow, with geological age from Late Miocene to Pliocene, so the carbonate rocks were deposited from 5.3 million years ago. Additionally, the carbonate platforms show poor stratification. The low pressure and poor layering make negative values for delta possible in these carbonate platforms. This study concludes that negative anisotropy values are possible in the offshore East Java Basin, and shows the effect of the negative anisotropy on the seismic imaging results. With a combination of the FWI and well calibration, negative anisotropy is confirmed and pre-carbonate imaging is improved with the negative anisotropy model.