Publications

The Basal Sandstone was recognized as an equivalent unit to the Parapatmruksah Formation of Eocene- Oligocene Age. It is an alluvial and fluvial deposit system that developed locally in low areas. In several exploration wells, the Basal Sandstone facies has proved to be hydrocarbon bearing. Infact, it was more frequently discovered than previously predicted, and is often drilled as a secondary objective.Seismic criteria studies have been conducted in the Batang Sarangan, Langkat and Gebang Areas for recognition of the Basal Sandstone facies and prediction of sand distribution. Seismic attributes have been utilized to predict these sands directly, including variation in amplitude, instantaneous phase and continuity. The reflections are described as concordant, onlap divergentlparallel, medium continuity, medium amplitude events, and can be mapped or used to determine the geometry of the basal sandstone and to predict sand prone facies.Deposition of basal sandstones was controlled by tectonic events. There are two kinds of basal sandstone depositional origin: as a syn-rift deposit (Batang Sarangan Type), and post-rift deposit (Langkat- Gebang Type). The sediment was derived from the older sediment in the surrounding area and from the Proto Barisan of the Sumatran Fault System. Both of these had developed along Northeast-Southwest alignment shears at that time.The exploration strategy for basal sandstones is focussed on the edges between relative highs and lows at the present time, with north-south direction. Hydrocarbons within the basal sandstone were sourced from its finer clastics and the Bampo shales and reached maturity 11 million years ago. Hydrocarbons migrated and were trapped during Late Middle Miocene Upper Miocene tectonic period.