RESERVOIR CHARACTERIZATION USING ACOUSTIC IMPEDANCE INVERSION, VARIANCE ATTRIBUTE, AND MULTI-ATTRIBUTE SEISMIC ANALYSIS FOR DEVELOPMENT WELL PLANNING: A CASE STUDY OF EARLY MIOCENE CARBONATE IN TELLA FIELD, NORTHEAST JAVA BASIN

Abstract

The Northeast Java Basin is a proven hydrocarbon-producing area, with Early Miocene carbonates from the Kujung Formation serving as one of its primary reservoirs. Although hydrocarbons have been confirmed, the high heterogeneity of reservoir rocks increases drilling risk, requiring advanced analysis to reduce uncertainty in well planning. To reduce this uncertainty, this study applies model-based acoustic impedance inversion to estimate lithology distribution based on impedance contrasts and uses seismic variance attributes to detect lateral changes related to facies variations. The analysis utilizes 3D PSDM full-stack preserved amplitude seismic data in the time domain, along with data from three exploration wells. The cross plot between acoustic impedance and total porosity clearly differentiates reservoir (carbonate) and non-reservoir (shale) zones. Impedance values below 23,000 ft/s*g/cc represent shale, while higher values indicate carbonates. Carbonates are classified into porous (23,000–30,000 ft/s*g/cc) and tight (>30,000 ft/s*g/cc). The tight carbonate in Kujung I Zone A is selected as the main focus of this study, as it is the only interval with confirmed hydrocarbons. This zone is positioned directly above the porous carbonate, with the spill point located at the contact between the tight and porous carbonates. A rapid drowning event caused by sea-level rise halted carbonate growth, forming a tight layer where hydrocarbons are trapped. Due to insufficient structural elevation, hydrocarbons remain confined within the tight carbonate and will only migrate once the trap reaches maximum capacity (full to spill). Production test results confirmed hydrocarbon presence in this zone, thus establishing it as the primary focus of this study. Inversion results show high-impedance values near Well T2, and shale interbeds around Wells T1 and T3. Since shale reduces productivity, tight zones without shale are prioritized for development. Structural mapping reveals a right-lateral strike-slip fault and a circular high interpreted as a potential patch reef. Seismic variance further supports the presence of patch reef geometries. The integration of these methods enhances the accuracy of future development well placement and reduces drilling risk. Keywords: Reservoir Characterization, Acoustic Impedance Inversion, Variance Attribute