An Alternative Approach to Delineate the Thin Reservoir Pinch-Out Boundary of LKU D Formation in Thung Yai Area (TYI), S1 Onshore Oil Field, Thailand

Abstract The Thung Yai area (TYI) within the S1 Onshore Oil Field presents a challenging yet promising stratigraphic play in the D Unit of the Lan Krabu Formation (LKU), where the primary reservoir is a thin deltaic sand body that pinches out laterally. Due to the reservoir's thickness falling below the seismic resolution, simple seismic interpretation methods have struggled to accurately delineate the reservoir boundary, leading to inconsistencies between predicted and actual well outcomes. A 2019 in-house seismic amplitude study initially guided development. However, subsequent drilling campaigns revealed a lack of correlation between seismic amplitude and oil pay thickness, prompting a need for a revised interpretation approach. This study introduces an alternative methodology that focuses on delineating a broader stratigraphic unit known as the High Acoustic Impedance Package (HAIP), within which the oil-bearing sand consistently resides. The HAIP, distinguishable by its high acoustic impedance relative to the surrounding shale, is a coarsening-upward pro-deltaic silty sequence capped by a thin delta-front sand. Through integrated analysis of wireline logs, sidewall cores, and multi-well seismic ties, we characterize the HAIP and demonstrate that while the oil sand itself is seismically undetectable, the HAIP produces a distinct seismic response, particularly a basal peak event in reverse-polarity seismic data. By extracting the amplitude of this basal peak, we generate a revised amplitude attribute map that more reliably outlines the HAIP extent and, by proxy, the potential reservoir distribution. This approach is further supported by stratigraphic reinterpretation and reservoir pressure analysis, which confirm lateral connectivity and suggest a single hydrocarbon tank. The integration of well data enables the estimation of hydrocarbon column height and the delineation of the prospect boundary. This alternative delineation strategy overcomes the limitations of seismic resolution and provides a more geologically consistent and technically robust framework for reservoir prediction. The successful application of this method in the Thung Yai area has improved well placement and production performance, and it is now being considered for future development in adjacent areas. The findings contribute to enhanced reservoir characterization and strategic field development in complex stratigraphic settings.