The Pre-Tertiary Petroleum System in North Sumatra Basin: An Integrated Study from Onshore North Sumatra Outcrops and Subsurface Data from Offshore West Glagah Kambuna
Abstract
The offshore area of North Sumatra Basin is believed to have good hydrocarbon accumulation potential and is considered challenging for hydrocarbon exploration. Several exploratory wells had been drilled in this area with major discoveries mainly in Tertiary age reservoirs such as the Baong and Belumai Formations. The Pre-Tertiary sedimentary deposit of North Sumatra Basin is still underexplored and is believed to have an important role in future hydrocarbon exploration in the area. The Pre-Tertiary play is the main exploration target in West Glagah Kambuna in offshore North Sumatra Basin. Previous regional studies suggested that the basements in this area are mainly the Tampur Formation of Eocene - Early Oligocene age. Based on present study however, the basement of the West Glagah Kambuna area mainly consists of the Batumilmil, Kaloi and Kualu Formations of Permian - Triassic age that are parts of the Peusangan Group. Offshore seismic data clearly indicate angular unconformity contact between Pre-Tertiary and Tertiary stratigraphy. In addition to the clearly defined Tertiary sedimentary-fills section, the seismic also shows good images of Pre-Tertiary packages which can be differentiated based on their internal-external reflection characteristics and reflectivity. By correlating the pre-rift seismic section with onshore geology which is based on outcrop data, it is interpreted that the Pre-Rift stratigraphy found in offshore area belongs to Mesozoic Peusangan Group. The Peusangan Group in the offshore area can be divided into three different formations; from the oldest to the youngest are the Batumilmil, Kaloi, and the Kualu Formations. The Batumilmil Formation is represented by chaotic seismic character; the Kaloi Formation is represented by high amplitude seismic reflection, while the low amplitude seismic reflection above the Kaloi Formation is interpreted as the Kualu Formation. The good match between seismic and well data in WGK Block and surrounding area strongly support the interpretation. Outcrop study in Aceh and North Sumatra Province has confirmed that the pre-Bampo/Bruksah section of the North Sumatera Basin comprising not only the Dolomite Tampur Formation, but also several formations from Meureudu and Peusangan Groups which are Pre-Tertiary in age. The study also suggested that the pre-rift dolomitic carbonate is not just belong to the Tampur Formation since the Peusangan Group also shown similar lithology, particularly in Batumilmil and Kaloi Formations. Extensive tectonism and subaerial exposure have promoted fractures and caving in those formations. Beside the dolomitic carbonate, the study also found clastic rock within the Pre-Tertiary stratigraphy. The clastic rocks are black shale of Batumilmil and mudstone of Kualu Formations. Several fossils could still be found in Batumilmil carbonate and Kualu mudstone. The outcrop sedimentology characteristics are still showing sedimentary rock character where bedding and layering is clear and common. The Pre-Tertiary formations that developed in WGK Block and surrounding area are parts of the Peusangan Group, which based on field observation and laboratory analysis of Kaloi, Kualu, and Batumilmil Formations, they show good reservoir properties and high carbonaceous content in shales in several localities. It indicates the possibility for the Pre-Tertiary formations to have its own petroleum system, separated from the Tertiary formations. This promotes new exploration opportunity in the southern part of North Sumatera Basin, particularly in WGK Block.
Area
Sumatera
Author(s)
A Bachtiar, JB Unir, A Bunyamin, HI Darmawan, FH Darmawan, FH Korah, ...
Year
2014
Publication
Indonesian Petroleum Association, 2014