Geostatistical Pre-Stack Inversion for Thin-Bedded Reservoir Characterization in Bongkot Field, Gulf of Thailand

Abstract

Abstract A geostatistical seismic pre-stack inversion was carried out over a producing gas and condensate field in the Gulf of Thailand, North Malay basin. As the main reservoirs are thin-bedded stacked fluvial deltaic sands of Miocence age, detailed mapping of reservoir distribution was challenging due to limited seismic resolution. To overcome such challenges, a pre-stack geostatistical inversion was initiated. The input dataset consisted of six wells and 250 km2 of 3D seismic data. The well log data passed through rigorous QC and rock physics analysis, while the seismic data were subject to preconditioning to ensure improved CDP gather flatness and signal-to-noise ratio. Starting from geostatistical modeling, the inversion generated multiple detailed realizations of lithology and elastic properties based on Bayesian Inference and Markov-Chain Monte Carlo methods. Statistic of multiple resulting realizations also implied a range of possible solutions of this non-unique inverse problem. In addition, petrophysical properties were simulated by using statistical relationships between inverted elastic and petrophysical properties. By integrating high frequency well logs with low frequency seismic, the geostatistical inversion process provided high vertical resolution and captured spatial variations of the various lithology types and their respective elastic properties. Since a majority of the stacked gas sand reservoirs in the area were below the tuning thickness, the geostatistic inversion results provided significantly improved insight to facies distribution. According to blind well results, precision of net pay estimation provided by the geostatistical inversion improved from 40% to 83%, compared to predrilled prognosis; while, pay estimation uncertainty was reduced by 30%. The generated petrophysical volumes also showed more detailed spatial variation, and can be used to improve in-place volumetric calculations and support field development planning.