Publications

North Sumatera Basin has long history of hydrocarbon exploration and production of Tertiary deposits, while Pre-Tertiary deposits are still overlooked. Several publications already proposed possible concepts of Pre-Tertiary sediment as new exploration target in the basin. Recent 3D seismic survey in Langkat area of the basin shows thick sediment layers below Paleogene aged Tampur Formation, which exceed 4000 ms seismic time. These layers have structural configuration different from the layers above Tampur Formation, therefore yielding a petroleum system different from existing Tertiary petroleum system. Our study combines outcrop analysis and subsurface interpretation to define Pre-Tertiary petroleum system in Langkat Area. Outcrop of Pre-Tertiary rocks are sampled and lab analyses including petrography, biostratigraphy, and geochemistry are conducted. Subsurface interpretation consists of horizon interpretation, fault interpretation, and geochemistry analysis from well data. Our analyses and interpretations identify 5 basic elements of petroleum system in the area, which are source rock, reservoir, closure, timing and migration, and containment. The source rock is black shale of Kaloi Formation. Geochemistry analyses showed evidence of indigenous vitrinite with high Ro and low TOC, suggesting mature source rock and already expelled hydrocarbon. Burial history model suggests Kaloi Formation has entered gas generation window. Reservoir rocks are lacustrine sandstone and limestone, which is Mesozoic based on biostratigraphy analysis. Flower structure of strike slip origin has generated closure bordered with faults, the faults are older than 16.38 Ma. Based on burial history model, migration occurs after faulting, hydrocarbon is possibly trapped in the structure. For containment, Kaloi Formation has strong and blurred reflectors indicating shale layers and tight limestone as seals. From our study, Pre-Tertiary petroleum system in Langkat Area does exist, it is recommended to be the next exploration target in North Sumatera Basin.