Lower Miocene Duri Formation Sands, Central Sumatra Basin

Abstract

Abstract The Duri Formation consists of interbedded fine to medium grained sandstones and shales. This formation contains the uppermost unit of the Lower Miocene Sihapas Group which includes the most important hydrocarbon reservoirs in the central Sumatra basin. The Sihapas Group consists of fine to coarse grained sandstones and interbedded shales which were deposited in a delta system. The Duri Formation sands appear to have been deposited in a marginal marine delta front environment. P.T. Caltex Pacific Indonesia has recently completed regional biostratigraphic studies based on planktonic foraminifera and calcareous nannoplankton. These studies have resulted in a subdivision of the marine Tertiary sediments of the central Sumatra basin into local stages. These local stages have been given alphabetic designation. They are, in descending order, the G, H, J, K, L, M, N, O, and P stages, covering a time span between Plio-Pleistocene and Paleogene, respectively. The Duri Formation sands include those within the Lower Miocene H, J, K, and L stages in the northeastern part of the basin. They interfinger or grade laterally toward the south and southwest into prodelta silts and shales of the Telisa Formation. These lithologic units were deposited as parts of a delta system adjacent to the Malayan Shield. Sediments were derived from the north-northeast with main sand transport through the Rokan-Pinang area. Sedimentation was controlled by the configuration of a regionally southwestward gently sloping depositional surface. An additional controlling factor was local topographic highs and lows such as the Siak Kecil Syncline and the Sembilan High. In general, the delta system retreated north-northeast at the end of Early Miocene time in association with a continuing marine transgression. Consequently, the delta front sands deposited during the H, J, K, and L stages also migrated northward. Deltaic sedimentation in the basin ended or shifted position at the end of the Early Miocene due to infra-Miocene tectonism. P.T. Caltex Pacific Indonesia has discovered twelve oil and gas fields in Duri Formation reservoirs in the central Sumatra basin; eight of these fields have been put on production. Cumulative production from these reservoirs is more than 112 million barrels of oil. Most of these fields are located within the south and southwestern portions of the delta front sand bodies. It is believed that the oil was generated in the prodelta shales of the Telisa Formation and migrated into the adjacent Duri sands. Two factors appear to have controlled early petroleum accumulation. The first factor was subsidence in the north, which was contemporaneous with rapid sedimentation. The second was arching to the south and southwest. These movements produced a gentle anti-regional tilt toward the north-northeast which prevented petroleum migration northward into massive sand bodies where the hydrocarbons could have been lost to the surface.