Stratigraphic successions in a sedimentary basin can be related to the basin history and latter to petroleum system. An unconformity-bounded stratigraphic unit approach is expected to provide significant contribution in improving the understanding of geological framework definition, also to determine basin type and history related to the petroleum system in Central Sumatra Basin. The unconformity-bounded stratigraphic unit is known as bodies of rocks bounded above and below by significant unconformities preferably of regional or interregional extent and represents sedimentation cycle in response to changes in relative sea level or tectonic event. It employs to define basin history in Central Sumatera Basin, constrains Bengkalis Trough, based on compilation from previous stratigraphic studies using term “Synthem” as basic of unconformity-bounded stratigraphic units. Stratigraphically, the sediment fills in the area can be divided into three synthems. Each synthem is well-identified by significant unconformities and correlated by seismic and well data across the basinal area. Kelesa Synthem (Synthem One) is the oldest unit, with sediment age range from Middle Eocene to Late Oligocene and bounded by a regional nonconformity with at least 30 Ma time gap with pre-Tertiary rocks. This synthem consists of Kelesa Group (equivalent with Lower Red Beds, Brown Shale and Upper Red Beds) which represents sediments of early basin forming sequence (syn-rift), with an association of alluvial, shallow to deep lacustrine and fluvio-deltaic facies. This group was deposited in the pull-apart basin at opening of Indian Ocean spreading time in Middle Eocene (c. 43 Ma). Regional angular unconformity of Upper Red Beds Event was formed at base of synthem two, which acts as evidence of basin uplifting due to back–arc deformation when initiation of Indian Ocean subduction rollback and opening of South China Sea in the Late Oligocene (c.32 Ma). This synthem classifies Kelesa Play with elements such as deep lacustrine Brown Shale as principal source rock and fluvio-deltaic of Upper Red Beds as reservoir with stratigraphic, structural, and combination trap types. Sihapas Synthem (Synthem Two) consists of sediments equivalent with Lakat, Tualang and Telisa Formations which represents transgression and subsidence sequence (post-rift). The sediments of this synthem have a gradual transgression from fluvial, deltaic and latter to deep marine facies with interior sag basin setting while the subduction rollback continues in the Late Oligocene to Middle Miocene. This transgression sequence was terminated by the structural inversion of Binio Event in the Middle Miocene along with Andaman Sea spreading and Barisan Mountains uplift at 13 mybp. This synthem classifies early Sihapas Play with Lakat and Tualang sediments act as main reservoirs and marine shale of Telisa as regional seal with dominantly structural trap type. Petani Synthem (Synthem Three) comprises sediments equivalent with Binio and Korinci Formations which represents inverted basin sequence (syn-inversion) in the Middle Miocene to Recent. This synthem records regression shallow marine facies to deltaic in a foreland basin setting. In several areas, Binio Event recorded as local unconformity (diastem) on anticline of Telisa Formation, which was formed by wrench fault due to advance movement of subduction. This synthem classifies Petani Play with Binio Sand acts as reservoir with Binio and Korinci shales as intra-formational seal. The Minas Event is the last deformation in the Plio-Pleistocene (c.5 Ma), while uplift and erosion formed angular unconformity along with the younger sediment of Nilo/Minas Formation across the basinal area.
Author : V. Bonny Indranadi, Larryvo Sitohang, Wibisono