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Publication 2014

(ICGES 2014)

Proceeding of 2014 3rd International Conference on Geological and Environmental Sciences

(ICGES 2014)

Singapore, 6-7 August, 2014

Sedimentology and Petrography of Selected North Sumatra Pre-Tertiary Formations: Anticipating New Petroleum Systems in Western Indonesia

 Andang Bachtiar (1), Prihatin Tri Setyobudi (1), Siti Asyiah (2), Armein Suleiman(2), Purnama Ary Suandhi(1), Andi Kriyunianto (1)

Affiliation (“1” PT. Geosain Delta Andalan; “2” PT. Paleopetro)

 Abstract

This research is focused on studying the depositional system and petrographic characteristics of petroleum system that form in North Sumatera, Indonesia to reveal the diagenesis, metamorphism, alteration indicators related to reservoir and source rock potential. Field work was integrated with paleontology analysis and comprehensive petrographic study of selected thin sections. Paleozoic and Mesozoic Stratigraphy of North Sumatra can be divided into Tapanuli Group (Alas Fm, Kluet Fm, Bohorok Fm) and Peusangan Group (Pangururan Bryozoa Bed, Batumilmil Fm, Kaloi Fm, Kualu Fm). Pre-Tertiary Rocks were variously deposited in deep marine (ie: Sibaganding Limestone) with Radiolarian Limestone; shallow marine (ie: Batumilmil Fm, Kaloi Fm) with Limestone and dolomitic limestone; moraine glacier and till (ie: Bohorok Pebbly Mudstone); and shallow water (ie: Kualu Mudstone, Pangururan Bryozoa Bed). Batumilmil Fm can be divided into Dolomitic red limestone, Light-grey Limestone, Dark-grey Limestone, and Carbonaceous Claystone. Reservoir potential was formed by farctures, disslutions and dolomitization (ie: Kaloi and Batumilmil Dolomitic Limestone). Further more, source rock potential was found in Batumilmil and Kualu Mudstone.

Keywords: petroleum system, North Sumatera, diagenesis.

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PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION

Thirty-Eighth Annual Convention & Exhibition, May 2014

 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

Andang Bachtiar*, Jaafar B Unir**, Amin Bunyamin**, Hendra Iriansyah Darmawan**, Fithra Harris Darmawan**, Frederick Herbert Korah**, Bobby Mazmur Sihole**, Lismawati***, Prihatin Tri Setyobudi****

* Institute of Technology Medan, GDA Consultingand Exploration Think Tank Indonesia, ** Petronas Carigali West Glagah Kambuna Ltd., *** Institute of Technology Medan, ****GDA Consulting

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 plays of the North Sumatra Basin are still underexplored and are believed to have an important role in future successful 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.

The offshore seismic data clearly indicate angular unconformity contact between the Pre-Tertiary and the Tertiary stratigraphy.  In addition to the clearly defined Tertiary sedimentary-fills section, the seismic also shows good images of the 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.

The 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 ages. The study also suggested that the pre-rift dolomitic carbonate are not just belong to the Tampur Formation since the Peusangan Group also shown similar lithology, particularly in Batumilmil and Kaloi Formations. The extensive tectonic and subaerial exposure has 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 in Batumimil and mudstone in Kualu Formation. Several fossils could still be found in Batumilmil carbonate and Kualu mudstone. The outcrop sedimentology characteristic 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 our field observation and laboratory analysis of Kaloi, Kualu, and Batumilmil Formation, they show good reservoir properties and also with high carbon content in their shales in some places. It shows a possibility of the Pre-Tertiary formations to have its own petroleum system, separated from the Tertiary formations. This promotes new exploration opportunity in southern part of North Sumatera Basin, particularly in WGK block.

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