Publications

The Upper Sihapas Group has been long recognized as the main productive reservoir in the Central Sumatra Basin. Several integrated regional studies in Rokan Block were conducted by Caltex in 1970’s to 1990’s. Here we attempt to update the Early Miocene (17.5 – 22 Ma) depositional model by incorporating biostratigraphic data, water salinity, cores, wells and seismic data to construct a new paleogeography map. The Upper Sihapas Group consisting of the Duri, Bekasap, and Bangko Formations were deposited in a northwest to southeast trending narrow seaway system in a transgressive sedimentary package. This marine basin was a tide-dominated system with four bathymetric zones: supra - intertidal, intertidal - inner shelf, inner – middle shelf, and middle – outer shelf. Multiple sediment feeders from the Malayan Shield in the northeast were recognized as forming coastal morphology variations. During the ‘Middle’ Early Miocene (21 – 22 Ma), the Upper Sihapas Group deposition in northern and the southern part of the Rokan Block formed positive coastal morphologies delivering higher sand distribution and quality compared to the central portion. During the ‘Upper’ Early Miocene (17.5 – 21 Ma), the coastal line shifted to the northwest at the onset of a transgressive phase. This transgressive phase provided higher shale deposition, which aligned with an increasing trend of formation water salinity from the northeast to southwest of the Rokan Block. In a regional sense, the new paleogeography model is consistent to the distribution of the Upper Sihapas Group reservoirs from northeast to southwest direction. It is is confirmed by individual field reservoir distribution in Rokan Block. This new model will improve reservoir predictability in terms of quality and distribution, and will serve as a guide for further reservoir management initiatives such as EOR, low quality reservoir development, and production optimization strategy in Rokan Block.