Unveiling Rare Transgressive Carbonates via Unsupervised Petrophysical Rock Typing in the Menggala Formation, Central Sumatra Basin

The PA Field in Indonesia’s Rokan Block is an actively producing asset with hydrocarbons sourced from the Bekasap, Bangko, and Menggala Formations. While the Menggala Formation is typically interpreted as a fluvial-dominated clastic system, recent core analysis has revealed a previously overlooked thin carbonate layer in its upper section. Misidentified as tight sandstone, this carbonate interval shows distinct log responses and warranted detailed investigation.

A quantitative approach was applied by integrating log and core data to characterize this seismically unresolved layer. Petrophysical Rock Typing (PRT) using unsupervised clustering produced a Carbonate–Non-Carbonate (CNC) model, validated with side-wall and conventional core data. Further refinement led to the development of a Static Rock Typing (SRT) model, identifying four carbonate lithofacies: Foraminiferal Packstone, Coral Rudstone, Foraminiferal Grainstone, and Crystalline Boundstone.

Foraminiferal Packstone dominates the interval (∼74%), with Boundstone at 21%. Facies distribution, diagenesis, and paleo-slope were interpreted using facies geometry, Lucia cross-plots, and matrix texture analysis. Results suggest a carbonate deposition phase linked to post-extensional tectonics in the Late Miocene. This study highlights the value of integrating petrophysical and geological data, offering a robust framework for reservoir characterization in mature fields with limited core coverage.