Publications

Exploration of the deepwater Oligocene clastics play in the Andaman Sea, offshore North Sumatra, was accelerated by the acquisition of a large multi-client 3D survey by PGS in 2019. Harbour Energy licensed a significant portion of this data with which to evaluate the play potential in its Andaman II concession. The top of the target reservoirs was noted to have variable AVO expression, ranging from Class I, through Class IIP to Class II within the prospective fairway. As such, careful, reservoir focused gather conditioning was carried out to ensure AVA fidelity and fitness-for-purpose before embarking on any pre-drill Quantitative Interpretation (QI) studies. Extended Elastic Impedance (EEI) inversion proved to be very helpful in prospect delineation through separation of reservoir and fluid effects and formed the geophysical basis for the selection of the first exploration well in the play.

With well-log calibration available, post-drill evaluation extended into the simultaneous inversion domain using one-step direct to litho-fluid class (LFC) inversion (PCube+). Of note was the uplift in S:N ratio, lateral continuity and bandwidth/resolution that the PCube+ method provided in comparison to the EEI method. With the LFC prior model being somewhat site-specific, and to exploit these data quality uplifts over the full play fairway, a method was developed whereby the prior model for the PCube+ was simplified so that the principal outputs from the inversion became relative elastic properties, rather than LFC-probabilities. These relative elastic properties became the primary screening tool for the next phase of prospect evaluation and showed extremely good calibration with well log data from the subsequent exploration well in the program. With further well data available for calibration, a more comprehensive prior model has now been built for LFC inversion, including depth trends reflecting the burial history of the basin. This provides a unified basis for the next phase of QI in this play fairway.