Malay Basin Oil Exploration Overview

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Questions and Answers

The Malay Basin is located in the southern part of the Gulf of ______, between Vietnam and Peninsula Malaysia.

Thailand

Petroleum exploration in the Malay Basin began in ______.

1968

Esso operated in the area north of ______ N latitude.

The first oil production came from the Pulai and ______ fields in 1978.

Tapis Signup and view all the answers

Seligi is recognized as the largest oil field in the basin, with over ______ MMSTB EUR.

550 Signup and view all the answers

By the end of 1997, around ______ exploration wells were drilled in the Malay Basin.

330 Signup and view all the answers

The Malay Basin is believed to have formed during the early ______.

Tertiary Signup and view all the answers

The Tenggol Arch separates the Malay Basin from the ______ Basin.

Penyu Signup and view all the answers

The Malay Basin shows progression from mainly nonmarine environments during the ______.

Oligocene Signup and view all the answers

The southern part of the Malay Basin contains most of the oil reserves, including giant fields such as Seligi and ______.

Tapis Signup and view all the answers

The effective source rocks in the northern region of the Malay Basin are mainly of ______ origin.

coaly Signup and view all the answers

Hydrocarbon occurrences in the Malay Basin may be categorized according to the structural style of the ______.

traps Signup and view all the answers

Structures formed earlier in the south of the basin were able to trap ______.

oil Signup and view all the answers

The northern region of the Malay Basin is described as being more ______ prone.

gas Signup and view all the answers

On a regional scale, the geographic and stratigraphic distribution of oil and gas appears to be controlled by basin ______.

morphology Signup and view all the answers

Main structural trap styles in the Malay Basin include compressional anticlines and ______ closures.

fault dip Signup and view all the answers

Most prolific trap style comprises E-W trending anticlines formed by inverted ______.

grabens Signup and view all the answers

Compressional anticlines in the south are more ______ while in the north are gas prone.

oil-prone Signup and view all the answers

Main reservoirs are shallow marine and fluvial ______ of Group H, I, J, K.

sandstone Signup and view all the answers

Hydrocarbon in these structures were sourced from interbedded carbonaceous ______ and coals mainly in Group 1.

shales Signup and view all the answers

Compressional anticlines in the Central part of the basin involve reservoirs generally formed by shallow marine sandstones of Group ______ and E.

D Signup and view all the answers

The ______ Fault is a major normal fault zone with a maximum throw of about 2500 m.

Tenggol Signup and view all the answers

The Angsi-Duyong trend includes major gas discoveries of ______ and Besar.

Angsi Signup and view all the answers

Only ______ is currently producing in the Southwestern part which is a major gas trend.

Duyong Signup and view all the answers

The ______ structure is a normal fault bounded N-trending anticline located in the northern part of the basin.

Bergading Signup and view all the answers

Substantial amounts of gas and condensate were found in Group ______ and I reservoirs.

H Signup and view all the answers

Hydrocarbons in the Malay Basin are found in sandstone reservoirs of Group ______ down to K.

D Signup and view all the answers

In older groups (K, L, M) reservoirs are formed mainly from fluvial channels in a nonmarine-______ setting.

lacustrine Signup and view all the answers

Geochemical studies indicated two main depositional settings for source rocks – lacustrine and ______-deltaic.

fluvi Signup and view all the answers

Lacustrine source rocks consist of shales rich in ______ components.

algal Signup and view all the answers

Fluvi-deltaic source rocks are found mainly in the Lower-Middle Miocene groups I and ______.

E Signup and view all the answers

Natural gas is estimated to represent more than half of the hydrocarbon reserves of the ______.

basin Signup and view all the answers

The Tenggol Arch is relatively featureless except for isolated basement mounds of which some become structural closures when Tertiary sediments are draped over them upon ______.

compaction Signup and view all the answers

The unique trap style on the Tenggol Arch has allowed for the discovery of oil in shallow marine ______ at Malong.

sandstones Signup and view all the answers

Hydrocarbon at Malong are probably charged from Group K or older lacustrine ______ in the basin.

shales Signup and view all the answers

The migration distance to the Malong structure is at most ______ km.

10 Signup and view all the answers

The northeastern flank of the Malay Basin has resulted in 19 oil and 15 ______ discoveries.

gas Signup and view all the answers

Lateral seal in the northeastern flank of the Malay Basin is provided by sand-______ juxtaposition.

shale Signup and view all the answers

An example of a trap style formed by basement drapes is the ______ Raya Field.

South Signup and view all the answers

Stratigraphic channel plays can be exemplified by the ______ discovery.

Bindu Signup and view all the answers

Study Notes

Malay Basin Introduction

Located in the southern part of the Gulf of Thailand.
Covers an area of 80,000 km².
Filled with approximately 14 km thick sediment.
Continue northwestwards to merge with Thailand’s Pattani Trough and southeastwards with the Indonesia’s West Natuna Basin.
Petroleum exploration began in 1968.
Numerous oil and gas accumulations have been discovered, several of which are producing.

Exploration History

First concessions were awarded to Esso and Conoco in 1968.
Esso operated in the area north of 5°N latitude, while Conoco was given the area to the south, which includes the Penyu Basin.
From 1974, exploration was awarded to oil companies under production-sharing contracts.
In 1969, Esso drilled its first well, Tapis-1, followed by Tapis-2 (1974) in the southern part of the basin.
Tapis-1 found gas in sandstones, Tapis-2 found oil.
Continuous exploration led to the first significant oil discoveries at Seligi and Bekok.
The first oil production came from Pulai and Tapis field in 1978.
Seligi is the largest oil field in the basin, with over 550 MMSTB EUR.
Further drilling resulted in the discovery of the Angsi, Besar, Palas, Guntong, Irong, Irong Barat, Semangkok, Tinggi, and Dulang fields.
By the end of 1997, approximately 330 exploration wells were drilled.
Exploration discovered around 50 oil and 30 gas accumulations.

Tectonic Framework

Located at the centre of Sundaland.
One of the deepest continental extensional basins in the region.
Believed to have formed during the early Tertiary.
The Tenggol Arch separates it from the Penyu Basin, the Narathiwat High separates it from the Pattani Basin.
Elongate NW-SE trending basin, underlain by a pre-Tertiary basement of metamorphic, igneous, and sedimentary rocks.
Basement rocks are believed to be the offshore continuation of the geology of eastern Peninsular Malaysia.

Stratigraphy

Reconstruction of the palaeogeographic development of the Malay Basin shows a progression from mainly nonmarine (alluvial to coastal plain) environments during the Oligocene (Groups L and M) to increasingly marine environments (coastal fluviomarine to inner neritic) during the Miocene and later (Groups K to A/B).
The Malay Basin was a narrow seaway or gulf that received sediment from its northeastern and southwestern flanks.
The southern part of the basin contains most of the oil reserves, including several giant fields such as Seligi and Tapis (both have EURs of almost 600 MMSTB).
Hydrocarbons occur in reservoirs from Group L to D.
Groups E, I, J, and K are the most prolific.
Geographically, the basin can be generally subdivided into a northern gas-prone province and a southern oil-prone province, although there are exceptions.

Hydrocarbon Occurrences

On a regional scale, the geographic and stratigraphic distribution of oil and gas appears to be controlled by basin morphology.
On a semi-regional scale, the main factors controlling oil and gas distribution include source-rock quality and maturity, and the relative timing of generation and structuration.
The effective source rocks are coaly in the north (mainly in Group I) and lacustrine in the south (Groups K, L, and M).
Relative timing between structuration and hydrocarbon migration varies from south to north.
- Structures were formed earlier in the south, thus they were able to trap oil, whereas late structuration in the north resulted in more gas being trapped.
Hydrocarbon occurrences in the Malay basin may be categorised according to the structural style of the traps.

Hydrocarbon Plays and Trap Styles

Main structural trap styles are mostly compressional anticlines and fault dip closures.
In the following slides, trap styles and hydrocarbon plays are categorised based on structural features, geographic and stratigraphic distribution, and source-reservoir relationships.

Compressional Anticlines

The most prolific trap style.
Comprises E-W trending anticlines formed by inverted grabens.
Located mainly along the central/axial part of the basin.
Formed during the basin inversion phase in the Middle Miocene (beginning of Group F times.
Most of the anticlines are the result of wrench movement associated with transpressional deformation of the underlying fault-bounded half-grabens.
Clearly observed in the southern part of the basin.
Many major discoveries in the Malay basin are of this play, e.g. Tabu, Irong, Jambu, Seligi, Tapis.

Compressional Anticlines - Hydrocarbon Occurrences

Compressional anticlines in the south are more oil-prone, while in the north are gas-prone.
Main reservoirs are shallow marine and fluvial sandstone of Structures are sealed by intra-group H, I, J, K. group claystone and shale beds.
Hydrocarbons in these structures were sourced from interbedded carbonaceous shales and coals mainly in Group 1.

Compressional Anticlines – Central Part

Compressional anticlines in the Central part of the basin involve reservoirs generally formed by shallow marine sandstones of Group D and E.
Seals are interbedded claystone and shale units within Group D and E.
Source rocks are deeply buried rocks in Group 1, migrating vertically through fault conduits.
Examples of this play are Bintang, Lawit, Jerneh, Dulang, Sepat, Noring, Bujang, Ular, Tangga, Bergading, Inas.

Compressional Anticlines – Southwestern Part

The southwestern part is a major gas trend close to the Tenggol Fault.
It is the Angsi-Duyong trend, which includes major gas discoveries like Angsi and Besar.
These are large compressional anticlines and are structurally similar to those in the main oil province to the north.
Underlain by synrift half-grabens controlled by normal faults.
Only Duyong is currently producing.

Tenggol Arch

Relatively shallow and flat, NW-trending pre-Tertiary basement that separates the Malay and Penyu basins.
Its northeastern boundary is marked by the Tenggol Fault, a major normal fault zone with a maximum throw of about 2500 m.
Source of sediment for the half-grabens in the north.
Relatively featureless except for isolated basement mounds of which some become structural closures when Tertiary sediments are draped over them upon compaction.

Tenggol Arch Plays

The basement drapes form a unique trap style in which oil has been discovered in one of the structures at Malong, where it occurs in the Group J shallow marine sandstones.
Hydrocarbons at Malong are probably charged from Group K or older lacustrine shales in the basin across the Tenggol Fault, migrated up-dip onto the Tenggol Arch.
Interbedded shales provide the top seal for drape structures.
Sedimentary succession on the Tenggol Arch is generally less than 1500 m thick, any potential rock will be immatured.
Thus, the Malong oil may have come from deeper half-grabens, situated in the northeast of the Tenggol Fault.
The migration distance to the Malong structure is, at most, 10 km.
The Malong discovery spurred interest in the other basement structures on the arch with the assumption that oil migrated over long distances (30-60 km) from the Malay Basin to fill up the structures.
However, some of the basement structures were tested and found to be dry, thus the migration model needs to be revised.

NE Ramp Margin Play

19 oil and 15 gas discoveries were made in the northeastern flank of the Malay Basin.
The major trap styles include subtle stratigraphic/fault traps in Group I, J, and K, such as Larut and Bunga Raya field.
Faulted anticlinal traps in Group I, J, K reservoirs occur in the northeastern and eastern parts of the basin.
Lateral seal is provided by sand-shale juxtaposition.
Top seal is provided by the interbedded shale.
Hydrocarbons are sourced either from in situ interbedded source beds or from beds down-dip (Group K and L shales) via long-range migration.
Examples of this trap style are Bunga Orkid, Bunga Kekwa, and Bunga Raya oilfields (PM3 CAA area), Lerek, Pantai, Lumut, east Belumut, Larut, Abu oilfields in PM5 and PM8.
Other trap styles include traps formed by basement drapes, similar to those on the Tenggol Arc. An example is the South Raya Field, where oil and gas have been found in Group I, J, K reservoirs.
Stratigraphic channel plays, e.g. Bindu discovery.
Stratigraphic pinch-out trap.
Onlap trap.

Deep Reservoir Play

This play type involves reservoirs within or below the overpressured zone, thought to be present beneath existing discoveries/fields.
The Bergading structure, an example of a normal fault bounded N-trending anticline, located in the northern part of the basin (near JDA).
Bergading-1 well found gas in Group B, D, E.
Bergading Deep-1 well was drilled to about 3100 m, successfully penetrated the overpressured zone in Group F.
Substantial amounts of gas and condensate were found in Group H and I reservoirs.

Reservoir Rocks

Hydrocarbons in the Malay Basin are found in sandstone reservoirs of Group D down to K.
Depositional environment of the sandstones vary with stratigraphy.
In older groups (K, L, M) reservoirs are formed, mainly fluvial channels in a nonmarine-lacustrine setting.
In J and younger groups, sandstones are predominantly shoreface and subtidal shelf sands (esp in J) and fluvial-deltaic to estuarine channel complexes (I group and younger).

Source Rocks

The abundant oil and gas reserves in the Malay Basin give testimony to the presence of effective source rocks.
Geochemical studies indicated two main depositional settings for source rocks – lacustrine and fluviodeltaic, with varying degrees of mixing between the two end-members.
Lacustrine source rocks:
- Consist of shales rich in algal components.
- Occur in Oligocene/Early Miocene K, L, M, and pre-M/synrift groups.
- Have only been penetrated at shallow depths on the flanks of the basin.
Fluviodeltaic source rocks:
- Found mainly in the Lower-Middle Miocene I and E groups.
- Mainly in coastal plain shales and coal/carbonaceous shales.
- Are encountered in the basin centre.

Natural Gas

The Malay basin is not purely an oil province, but also contains several large gas accumulations.
Natural gas is estimated to represent more than half of the hydrocarbon reserves of the basin.
Total gas reserve in place is estimated to exceed 60 TSCF.
A large proportion of the reserve is in the Jernih, Lawit, Duyong, and Seligi fields.

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Description

This quiz provides insights into the Malay Basin, located in the Gulf of Thailand, detailing its geological features and exploration history since 1968. Discover key events, key players, and significant findings in the history of petroleum exploration in this area.