§ç B¹T, NGUYÔN QUèC AN, NGUYÔN QUý HïNG, NG¤ XU¢N VINH,
NGUYÔN THÕ HïNG, §ç VIÕT HIÕU, NGUYÔN TRUNG HIÕU

The Malaya – Thæ Chu Basin is situated on the SW Vietnamese continental shelf in the SE part of the Gulf of Thailand, extending in the NW-SE direction with a length of about 300 km and a width of 100 km. Together with the Pattani Depression this basin was formed in a pull-apart basin under the impact of the Three Pagodas Fault.

The fault system in the north of the basin is mainly longitudinally trending, but in the south – mainly NW-SE trending. It forms the following main structures: Northeast Monoclinal, Southwest Monoclinal, Northeast Graben, Central Horst and Central Graben.

Tertiary sediments in the Malaya - Thæ Chu Basin consist mainly of terrigenous sediments with a thickness locally of up to 9-10 km. In the continental shelf part of ViÖt Nam the greatest thickness of Tertiary sediments is only about 4000 m.

In the more than 20 boreholes drilled by the FINA Co (lots 46, 50 and 51) and UNOCAL Co (B, 48/95 and 52) the pre-Tertiary basement occurs only in some uplifts in the N-NE margin of the basin. It consists mainly of rocks metamorphosed to the low greenschist, phyllite and sericite schist facies interbedded with quartzitic silty sandstone and quartzite (boreholes 46-CN, 46-KL and B-KQ). They have been referred to the Mesozoic Khorat Group. Limestone of the Late Carboniferous and Jurassic occurs in the Bunga Raya borehole (lot PM-3). However, correlation with the geology of Hµ Tiªn area and islands of the Gulf of Thailand has allowed to date the limestone and terrigenous sediments as Paleozoic and Mesozoic.

 PALEOGENE

Developed in the basin, the Kim Long Formation comprises mainly claystone interbedded with siltstone, sandstone and some brown coal seams, locally with white micro-grained limestone. In some uplifted areas the lower part of the formation contains much irregular-grained sandstone in comparing with other areas. The above sediments were formed in the deltaic plain and lacustrine-marshy environmental conditions together with the sea influence in the upper part of the formation.

Due to the burial in different depth the described sediments were subjected to secondary alteration from early catagenesis stage (for the beds lying in the depth < 2700 m) to the middle-late stage (for the beds lying in the depth of 2700-3350 m) and late stage (for the beds lying in the depth > 3350 m).

The Kim Long Formation occurs mainly in grabens and structure slopes with the thickness varying from 500 to 1000 m and the distributive areas are controlled by NE-SW and N-S trending faults. On seismic profiles its reflected waves are expressed by the seismic member ML-TC 5 with discontinuous reflectivity, relatively high amplitude and the speed in the beds of about 3500-5000 m/sec. Toward the centre of the basin the reflections are weakly parallelly bedded with from medium to good continuity and relatively high amplitude.

Fossils of the formation are relatively poor and consist only of spores and pollens of the Florschuetzia trilobata Zone and some other species, such as Cicatricosisporites dorogensis, Magnatriatites howardii, Mayeripollis nahakoteris etc. which are those usually met in Oligocene.

The Kim Long Formation rests unconformably upon the basement rocks.

In the whole basin the sections of the formation includes mainly claystone, slightly carbonate claystone, coal-bearing claystone with thin interbeds of siltstone and sandstone. Locally, there are white, white-grey micro-grained limestone and limestone containing terrigenous fragments.

 The formation was formed mainly in the environment conditions of flooded deltaic plain containing in abundance peat beds interbedded with phases of shallow-sea and littoral sea manifested by the presence of glauconite, foraminifera fossils and other marine faunas of different levels. The influence of the sea factor clearly increases in the upper part of the formation. The beds of the formation were subjected to secondary alteration of from the early catagenesis phase to the beginning of the late catagenesis phase for sediments lying deeper than 2800 m.

The sand/clay ratio in the whole formation is in the medium level; the granulity of sandstone tends to become finer upwards. The gamma and resistance lines are of medium to high values. The observation on seismic materials shows that the formation is widely developed in the depth varying from 900 to 1500 m, corresponding to the ML-TC 4 member which consists of parallel reflection lines with medium to bad continuity and low amplitude in the lower part of the formation, but better continuity and higher amplitude in the upper part. It rests unconformably upon the Kim Long Formation in the form of onlap, downlap and, locally, toplap manifestations.

Paleontological assemblages found in the formation consist of Magnastriatites howardi - Echiperiporites estaela Assemblage; Florschuetzia levipoli Zone; and NN4 Zone.

Spores and pollens dating the Early Miocene age of the formation is abundant with: Magnastriatites howardi, Echiperiporites estaela, Verrucatosporites, Barringtonia, Calamus, Fl. trilobata, Fl. levipoli, Fl. semilobata, Retimonocolpites, Pinus and Rhizophora, some dinoflagellates, and Pediastrum, Acrostichum etc..

Nannofossils consist of Discoaster drugii, Helicosphaera ampliaperta having LAD in NN4 (Early Miocene); they occur only in some boreholes.

The Ngäc HiÓn Formation was formed in the marshy, coastal deltaic environment. It rests unconformably upon the Kim Long Formation.

The §Çm D¬i Formation occurs largely in the basin. Apart from above described sediments its section still contains thin interbeds of dolomite or micro-grained limestone containing fragments of white-grey to brown-yellow terrigenous sediments. These sediments were deposited mainly in coastal flooded deltaic environment strongly influenced by, or intercalated with some stages of shallow-sea or littoral sea stages. They were subjected to secondary alteration at the early catagenesis stage with claystone and sandstone of weak cementation by clayey cement, or of medium cementation by carbonate.

On the logging curves the sand/clay ratio is usually medium to high. The sand tends to be coarser upwards, with the low gamma value and medium electric resistance.

The §Çm D¬i occurs everywhere in the basin with the thickness varying from 300 to 1200 m. It is well expressed by seismic materials. This ML-TC 3 seismic member is characterized by parallel reflections with the medium to good continuity, medium to high amplitude and high frequency. The lower part has worse continuity, and locally is characterized by a mixed wave field. The contact with the Ngäc HiÓn Formation (the ML-TC3 seismic member) is expressed by conformable signs, but locally the onlap, downlap and toplap manifestations also occur, related to local unconformities.

The age of the formation has been determined on the basis of following paleontological assemblages: Florschuetzia meridionalis - Fl. levipoli Assemblage; N9-N13 zones; and NN6-NN9 zones.

The palynological composition differs from older assemblages by the presence of Fl. meridionalis, Eugeisonia insignis and Comptostemon which are the forms having FAD in Middle Miocene. Similar to the west of the South C«n S¬n Basin the palynomorph Magnastriatites howardi still frequently occurs. Besides, there are much Pinus, Alnipollenites, Stenochlaena, Acrostichum and Fl. trilobata together with dinoflagellates. The Pediastrum alga is almost absent.

In foraminifera assemblages the benthonic forms still occupy the main composition, however in boreholes the Middle Miocene forms of the N9-N13 zones, such as Orbulina universa, Globorotalia mayerii, Gl. foshi and Gl. continuosa have been found. Nannofossils characterizing the NN6-NN9 zones are: Discoaster deflandres, Disc. hamatus and Disc. kugleri.

Based on above lithological and paleontological features, the §Çm D¬i Formation is considered as formed in the deltaic environment strongly influenced by the littoral sea factor. It rests conformably upon the Ngäc HiÓn Formation and has been dated as Middle Miocene on the paleontological basis.

The Minh H¶i Formation was described on the basis of materials from the Minh H¶i Borehole (51-MH-1X). In this hole, from the depth of 690 to 1097 m its section comprises light grey, bluish-grey claystone interbedded with light grey, dull-grey, fine- to coarse-grained sandstone and some brown coal. The thickness of the formation in this hole is 407 m.

In general, the sand/clay ratio is low, the coarse-grained sand tends to be developed in the upper part. The gamma line is of medium value, while the resistance line – of low value.

In the seismic profile the Minh H¶i Formation is characterized by the ML-TC 2 member having parallel reflections with bad continuity, medium amplitude and frequency, reflecting the coastal marshy facies of sediments. In the contact with the underlying §Çm D¬i Formation there are signs of downlap showing the unconformable relation between the two formations. The thickness of the formation varies from 30 to 500 m.

Spores and pollens, foraminiferas and nannofossils occur in different beds of the formation, belonging to Florschuetzia meridionalis - Stenochlaena laurifolia - Anthocerisporites Assemblage; N16-N18 zones; and NN10-NN11 zones.

Similar to other basins on the southern continental shelf the palynological assemblage contains much Carya, Altingia together with such alpine components as Pinus and Podocarpus. The Late Miocene age has been determined based on the FAD of Anthocerisporites, Stenochlaena laurifolia side by side with Florschuetzia meridionalis, Fl. levipoli and Fl. trilobata.

Among foraminiferas, together with benthonic forms of the Pseudorotalia- Asterotalia group, there are many species of Ammonia, Trochammina and the planktonic assemblage with Globigerinoides immaturus, Gds. ruber, Gds. acostaensis and Sphaeroidinellopsis semiluna. This assemblage occurs everywhere in the Vietnamese continental shelf with the characteristic Globorotalia acostaensis species of Late Miocene (N16), that can belong to the N16-N18 zones of Late Miocene according to the historical relation between these species.

Nannofossils with such characteristic forms of the NN10-NN11 zones as Discoaster quinqueramus and Disc. bergreni are of Late Miocene age.

The Minh H¶i Formation was formed in the shallow-sea environment with strong terrigenous influence. It rests unconformably upon the §Çm D¬i Formation and has been referred to Upper Miocene on the paleontological basis.

PLIOCENE

The BiÓn §«ng Formation largely occurs in the basin with a relatively stable thickness of about 400-600 m. Collected fossils belong to: Dacrydium Assemblage, N19-N21 zones, and NN12-NN15 zones.

The most characteristic feature of the spores and pollens assemblage is the presence of Dacrydium and the absence of old species of Florchuetzia (Fl. trilobata and Fl. semilobata). Besides, there are Graminae, Anthocerisporites, Stenochlaena laurifolia, Rhizophora, many dinoflagellates and almost no Magnastriatites howardi.

The foraminifera assemblage includes mainly Pseudorotalia and Asterorotalia. Besides, there are some species of Cibicides, Elphidium, Eponides, Bigeriana and Textularia. The planktonic group of the N19-N21 zones of Pliocene exists with small quantity.

Among nannofossils there are species of Discoaster, Helicosphaera, Sphenolithus of the NN12-NN15 zones, but Discoaster quinqueramus does not occur.

Depositional environment: marine.

In the Malaya – Thæ Chu Basin the BiÓn §«ng Formation rests unconformably upon the Minh H¶i Formation. Based on collected fossils and on the stratigraphic correlation, it has been referred to Pliocene.

Basing on the above results from stratigraphic research, we can figure the evolution process of Tertiary sediments in the Southwest ViÖt Nam continental shelf in 4 stages:

1) The forming stage of the Malaya - Thæ Chu Basin (related to spreading activities at Eocene (?) and mainly in Oligocene), with the formation of conglomerate, sandstone and claystone related to throughs, that grade upwards into lacustrine sediments with limited interbeds of marine sediments. Claystone has a TOC content of 1-5%, kerogene of I-II categories, so this is the petroleum producing formation.

2) The developing and enlarging stage of the distribution area of Tertiary sediments (related to the subsiding activities during Early-Middle Miocene) creating deltaic sediments interbedded with marine sediments. The carbonaceous shale with a TOC content varying from 2 to 5% is an important gas producing source.

3) The development diminishing stage caused by the movement in the region during Late Miocene, creating deltaic sediments.

4) The forming stage of the East ViÖt Nam Sea continental shelf (Pliocene-Quaternary) creating shelf sediments related to the formation and development of the East Sea continental shelf.

The results of prospecting and exploration drilling work has been finding oil-bearing, and especially, gas-bearing formations of Oligocene and Early Miocene ages in the Southwest ViÖt Nam continental shelf.

Acknowledgment. This paper is completed in the framework of the Project "Naming and correlating Tertiary stratigraphic units on the Vietnamese continental shelf" and the Basic Research Project No. 721101 "Evaluating the validity of Vietnamese stratigraphic units". The authors would like to express their deep thanks to the Prof. Dr. Tèng Duy Thanh, Prof. Dr. Vò Khóc, Dr. NguyÔn Ngäc Hïng and other colleagues which have been helping and creating good conditions in the amelioration and publication of this paper.

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