Publications

The Gabus Formation is a producing reservoir in the Lapang Field, West Natuna Basin, Indonesia. The reservoir consists of inter-bedded sandstones and claystones deposited in a delta front environment. The sandstone reservoir thicknesses are below seismic tuning and hence challenging to interpret. Sensitivity analyses showed that the parameters Lambda-Rho and VpVs ratio are particularly sensitive to the distributions of sandstones and gas-sand. An Extended Elastic Impedance (EEI) reflectivity volume was generated and used as an input for Sparse Spike Inversion. In addition to the deterministic sparse spike seismic inversion, a stochastic seismic inversion was performed using the same EEI data. Both the deterministic and stochastic inversions output Lambda-Rho and VpVs ratio volumes as guided by the sensitivity analysis. The stochastic seismic inversion delivered multiple solutions of the impedance model to help address the non-uniqueness and uncertainty ranges not addressed in the deterministic seismic inversion method. This is especially relevant when dealing with a thin bed layer below the seismic tuning thickness. By integrating the EEI, deterministic seismic inversion, and the stochastic seismic inversion, we were able to map the distribution of the thin bed sandstone and gas-sand calibrated to two wells. The stochastic impedance model realizations were then used to generate a probability model to determine the distributions for sandstone lithology and the gas bearing sands. Keywords: Tuning Thickness, Extended Elastic Impedance, Sparse Spike Inversion, Stochastic Seismic Inversion, Sandstone, Gas-sand, Indonesia.