Publications

The pre-carbonate layer in the East Natuna area, offshore Indonesia, forms important and complicated hydrocarbon reservoirs in the deep section. It has proven to be a challenging area for seismic imaging due to the presence of velocity heterogeneity in a more than 1.5 km thick carbonate overburden. An accurate velocity model is required to correctly describe the wave propagation and properly image the targeted pre-carbonate structures. Full-waveform inversion (FWI) is the natural choice for building velocity models in areas of complex structures. However, narrow-azimuth (NAZ) streamer data with limited offsets makes the FWI application challenging for these 3.5 km deep pre-carbonate structures. The diving-wave penetration is limited to the top carbonate level (1.8 km depth) due to an available maximum offset of only 6 km. Moreover, the poor signal-to-noise ratio (SNR) of the low frequencies and the complex wave propagation in the areas with sharp impedance contrasts, like faulted carbonate banks, may also cause some instabilities and lead to cycle-skipping issues during FWI. In this paper, we demonstrate how an advanced model building workflow can overcome these challenges and improve the imaging of the deep targets. Time-Lag FWI (TLFWI; Zhang et al., 2018) was chosen to handle the poor SNR of the low frequencies to allow us to start as low as 3 Hz. By incorporating both reflections and refractions, TLFWI was able to produce a detailed and geologically consistent velocity update that correlates with both the thick carbonate overburden and the pre-carbonate reservoir. To further improve the velocity model beyond the diving-wave penetration depth, enhanced high-definition tomography (EHD tomography; Reinier et al., 2021) was then applied. EHD tomography exploits offset-dependent dip information and dense residual moveout (RMO) information to produce an accurate velocity update for the deep section. The imaging uplift brought by this advanced model building workflow unlocks the interpretation of the pre-carbonate reservoirs, revealing their potential in the Natuna field despite the limitations of the existing NAZ data.