The Top Seal Capacity of Regional Telisa Shale and Intraformational Seals in Central Sumatra Basin

Abstract

Central Sumatra Basin (CSB) is well known as one of prolific basin in the world home for several giant fields, the Minas and Duri super giant fields among others. There are also several large fields and many smaller size fields in the basin. One of the petroleum system elements which contributed for the presence of hydrocarbon accumulation in the CSB is a competent top seal. This paper describes the type and characteristics of the top seals in relation with their capacity to retain giants and smaller size hydrocarbon accumulation. Telisa Formation was deposited regionally as open marine shale dominated unit and in some part was in distributary mouth bar of deltaic system during transgressive cycle in the Early to Middle Miocene Time (22.5-13 Ma). Telisa Formation contains a widespread thick shale and mudstones of condensed section at basinal setting as result of peak transgressive period (16-15 Ma). Telisa Formation plays the role as regional top seal for the hydrocarbon accumulation in Upper Sihapas Group, the Bekasap and Duri Formation reservoirs directly underneath it. The Bekasap and Duri are contributing for more than 80% of discovered oil in place in CSB. The hydrocarbon accumulation at the Duri and Minas fields and several giant fields such as Bangko, Bekasap, Petapahan, and Kotabatak Fields are retained by this regional Telisa top seal. Regional Telisa Shale has the capacity to seal giant accumulations from 0.5 BBO to 9 BBO corresponding to 200 to 550 ft hydrocarbon column. The role of regional top seal is strongly indicated by the single common oil water contact system within each field despite their multiple sand reservoirs interval responsible for accumulation. Several other fields in CSB contain hydrocarbon accumulations in reservoirs those are not directly overlain by the Telisa Shale regional top seal. There are several productive reservoirs in deeper Lower Sihapas Group members, Bangko and Menggala Formation sands, as well as in Pematang Group sands reservoirs those are not directly underneath Telisa Shale. In this case, an intraformational top seal was thought to be responsible for hydrocarbon accumulations in deeper reservoirs. The intraformational shales interval in Bekasap, Bangko and Pematang Formations were thought to contribute as intraformational seal. In some fields, this intraformational top seal can clearly hold hydrocarbon accumulations in multiple reservoirs with a single oil water contact. In some other cases the contribution of these intraformational seals are not that clear due to their concurrent presence with the shallower hydrocarbon accumulation controlled by regional Telisa Shale. The multiple oil water contact systems are likely present where both regional Telisa Shale and Intraformational top seal concurrently working. Intraformational top seals have the capacity to retain hydrocarbon accumulation size up to 400 MMBO and column high up to 300 ft. Based on analog from CSB, from Pager, Petani, and Balam South Fields, Bangko Shale is very likely having enough seal capacity to cap Lower Sihapas interval in Bangko and Menggala sands. The other intraformational top seal type is in the Pematang Group. Three types lithology of Pematang work as top seal are paleosol at the top of Pematang, flood plain mudstone and lacustrine shale as intraformational of Pematang top seal. They were locally deposited as Oligocene fluvial swamp and lake sediment setting. The estimated maximum possible sealing capacity of these non marine top seals in Pematang, based on mercury injection capillary pressure (MICP) measurement data, is 1,750 ft of oil column.

AAPG Datapages/Search and Discovery Article #90354 © 2019 AAPG Asia Pacific Technical Symposium, The Art of Hydrocarbon Prediction: Managing Uncertainties, Bogor, (Greater Jakarta), Indonesia, August 7-8, 2019