Publications

The Rokan Block is one of Indonesia’s largest oil producers, contributing over 20% to the nation’s oil output. The Central Sumatra Basin harbors several facies resulting from three main tectonic events spanning from the Eocene-Oligocene to the Plio-Pleistocene periods, encompassing both non-marine and marine depositional environments. The rift period during the Eocene-Oligocene era is evidenced by the deposition of the Pematang Group, comprising the Upper Red Bed Formation, Brownshale Formation and Lower Red Bed Formation, all of which were deposited in non-marine to lacustrine settings. Following this, the Early Miocene witnessed the immediate post-rifting basin sagging, followed by marine incursion and regional transgression, as recorded by the deposition of the Miocene Sihapas Group. This group predominantly consists of marine sediments such as the Menggala Formation, Bangko Formation, Bekasap Formation, Duri Formation and Telisa Formation. The Late Miocene to Pliocene period marked basin uplifting and orogeny, characterized by the deposition of the Petani Group, with the youngest formation being the Minas Formation transitioning to a shallow marine back to a non-marine environment. The age and depositional setting of these formations were characterized using a biostratigraphy approach, a method applied to identify age and bathymetric changes of the depositional environment based on fossil assemblages within the formations. Pollen and spores are common methods for determining the age and depositional settings of non-marine sediments, while planktonic foraminifera are relied upon for age determination in marine sediments.. Benthic foraminifera were used to determine the common water depth range where the sediment accumulated. With over 16,000 wells in the Rokan Block, almost all exploratory wells have acquired various types of rock samples during drilling operations, including conventional core samples, drill cutting samples, and sidewall cores. A significant proportion of these sample types were processed for biostratigraphy analysis. Based on Rokan’s robust biostratigraphy database, a semi-regional sequence stratigraphic framework and a regional Miocene-Pliocene marine sediment age framework were developed in the late 1990s.

Foraminifera fossils, calcareous nannoplankton, pollen and spore assemblage in the Sihapas Group have enabled a detailed biostratigraphic zonation in Rokan, facilitating the recognition of main boundaries of regional geologic ages such as 25.5 Ma, 22 Ma, 21 Ma, 16.5 Ma, 15.5 Ma, and 13.5 Ma sequence boundaries based on the existence of Cassigerinella chipolensis, Globorotalia kugleri, Globigerinoides primordius, Globigerina ciperoensis, Catapsydrax unicava, Orbulina universa, Globorotalia peripheroronda, as well as calcareous nannoplankton. Marine benthic foram assemblages were used to

develop paleobathymetry maps and sections, defining landward and basinward directions of sedimentary deposition in each sequence mentioned above. Some common fossils found in the Sihapas Group are marine mollusk from the pelecypod class.

In the non-marine syn rift deposits of the Pematang Group, which consists of the Upper Red Bed, Brown Shale, and Lower Red Bed, some species of pollen and spore fossils were commonly found. Among them are Meyeropolis naharkontensis and Pediastrum spp , indicative of Eocene - Oligocene age deposited in freshwater environments. In some conventional cores of the Pematang Group, freshwater mollusks were also found, showing imbrication that indicates low energy, non-marine fluvial sedimentation processes. Some core samples in the Brown Shale Formation also exhibit other unique fluvial to lacustrine ichnofossils consisting of higher plant leaves.