Active Fault Mapping Reveals Four Major Fault Zones Interact and Cause Compartmentalization of Eocene Reservoirs in Eastern Borneo

Tarakan Basin, located in North-East Borneo, is the largest proven hydrocarbon field in contract with the neighbouring Berau Sub-Basin. The geology of the two basins is similar; however, the lack of hydrocarbons in one poses questions about the role of geological structures in trapping hydrocarbons. Therefore, we have used 8m spatial resolution imagery of the Digital Elevation Model Nasional (DEMNAS) dataset to map the geological structures, including faults, folds, and primary sedimentary structures such as the trace of bedding planes. The mapping was done in ArcGIS Pro, a competent mapping and information managing tool. Faults are identified by mapping geomorphic features such as triangular facets, ridge axis, alluvial fans, deflected streams, etc. The bedding planes are mapped using the Rule of Vs, and the traces of fold axes are inferred from the bedding orientations. The relative age relationship is used to differentiate active from inactive structures, including mapping faulted Quaternary landforms such as fans, river terraces, volcanic landforms, etc. The mapped geological structures are overlaid on the existing geological maps to map the faulted reservoir rocks. The results show a dominance of NW-SE trending reverse faults bordering the north plunging anticlines. The faults are active, as suggested by the young, faulted topography and back-tilted Quaternary fans. A prominent ~E-W trending reverse fault has pierced the earlier NW-SE trending structures, indicating a young faulting phase. The faults interact, drill through the Eocene to Quaternary stratigraphy, and form reservoir compartments.