AAPG InternationaL Conference d &hibition '94 AugUJt 21-24, 1994, Kuala Lumpur, MalaYJia Petroleum geology of the Nam Con Son Basin NGUYEN TRONG TIN AND NGUYEN DINH TY Vietnam Petroleum Institute (VPI) Yen Hoa - Tu Liem - Hanoi Abstract: The Nam Con Son Basin is situated within 6°6'-9°45'N and 106°0'-109°30'E. Its southern and southeastern boundaries are in the Vietnamese waters which border the neighbouring countries, and its eastern, northern and western boundaries are on the Vietnamese continental shelf. The exploration history of the Nam Con Son Basin can be divided into four periods: the Pre 1975 period, the 1976-1980 period, the 1981-1989 period, and the 1990-present period. The heterogeneous basement of quartz diorite, granodiorite and Mesozoic metamorphic rocks is unconformably covered by Paleocene-Quaternary sediments composed of the Cau, Dua, Thong-Mang Cau, Nam Con Son and Bien Dong Formations. All the geological formations in the Nam Con Son Basin can be divided into two complexes of major structural elements, the basement composed of Pre-Cenozoic strata and the cover composed of Cenozoic sediments. The Nam Con Son Basin can be divided into the following tectonic zones, namely, the Western Differentiated Zone, the Northern Differentiated Zone and the Dua-Close-to-Natuna Zone. Good source rock sequences are developed in Oligocene lacustrine claystones and in Miocene fine grained clastics. Paleogene reservoir rocks include Oligocene clastics and Lower Miocene reservoir rocks. There are Oligocene and Miocene cap rocks. Potential for both structural and stratigraphical traps are considered to exist in the N am Con Son Basin. INTRODUCTION 3. 1981-1989 period The N am Con Son Basin is situated within During this period, Vietsovpetro - a 50:50 6°6'-9°45'N and 106°0'-109°30'E. Its southern and joint venture between Vietnam and the former southeastern boundaries are in the Vietnamese Soviet Union - shot a reasonably dense grid of waters which border the neighbouring countries, seismic lines and drilled wildcat and production and its eastern, northern and western boundaries wells in some blocks. are on the Vietnamese continental shelf (Fig. 1). 4. 1990-present period The exploration history of the N am Con Son Basin can be divided into four periods (Operators for blocks A number of oil companies are presently of Nam Con Son Basin, 1978-1993). undertaking geological, geophysical studies and are involved in drilling activities such as ONGC, 1. Pre 1975 period Enterprise Oil, BP, Shell, BG, Petrocanada, IPL, During this period, the US Oceanographic Cairn, TG, Lasmo, Pedco, AEDC, Astra Petranusa, Service undertook an aeromagnetic survey on a BHP and MJC (Le Van Cu et al., 1992; Nguyen scale of 1:250,000 covering the entire area. Trong Tin, 1993). In 1968-1969 a regional seismic survey was conducted by the British Overseas Development GEOLOGY Department (Alpine). Then during the 1969-1974 period, a number of oil companies such as Mobil Kaiyo, Pecten etc., carried out geological and Sedimentology and stratigraphy geophysical studies in some concessions. Pre-Tertiary formations 2. 1976-1980 period Out of 36 wells drilled in the N am Con Son A series of companies such as Geco, Deminex, Basin, 19 wells encountered the basement which is Agip, Bow Valley etc. conducted detailed surveys heterogeneous in its composition and composed of and drilled 8 wildcat wells in various blocks in the quartz diorite, granodiorite and Mesozoic basin. metamorphic rocks. Geol. Soc. MaLaYJia, BuLLetin 57, July 1995; pp. 1-11 f\.) TECTONIC ELEMENTS OF SOUTH VIETNAM OFFSHORE 14° .... :/ :·:· .. ~ci· ~OCEAN .... ' .... CF\UST OF EAST .... 1 ..... J VI~TNAM . 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T " dUll1l: .,I"lll"ut,:; U[ ::;uuLh VldllCllll uIT::.hur". PETROLEUM GEOLOGY OF THE NAM CON SON BASIN 3 Cenozoic formations seismic sequence. It unconformably overlays the Thong-Mang Cau Formation. This formation is The heterogeneous basement is unconformably covered by Paleocene-Quaternary sediments composed of: • Lower part: Clay, calcareous clay and grey composed ofCau, Dua, Thong-Mang Cau, Nam Con sandstones. Son and Bien Dong Formations. • Upper part: Whitish grey carbonate and quartz Cau Formation (Oligocene, Eocene?, Fig. 2). sandstones. This Formation corresponds to the 'A' seismic At the western margin of Blocks 12,20,21,11- sequence composed of the Bawahm, Keras, and 1 and 11-2 this formation is mainly composed of Barat Formations. It is lithologically characterized calcareous clastic sediments. from bottom to top by: The thickness of this formation varies between • Brownish grey sandstones intercalated with 100 m and 500 m, locally up to 600 m (Block 04-3). several beds of silty clay. Bien Dong Formation (Pliocene-Quaternary, • A thick bed of dark grey clay intercalated with Fig. 6). Following AGIP this formation corresponds light coloured oilstone, locally fine grained and to the Muda Formation. It constitutes the 'C' seismic calcareous. sequence and is developed throughout the basin. It • Quartz sandstone intercalated with greyish­ black, firm claystone. is Pliocene-Quaternary in age and can be subdivided • Dark grey, greenish grey clay, intercalated with into two parts: beds of sandstone, and siltstone. • Upper part: Quaternary in age, composed of The thickness of this formation varies between quartz sand (angular to rounded) in the lower 200 m and 800 m. section gradually changing to silty clay in the middle section and quartz sand containing shell Dua Formation (Lower Miocene, Fig. 3). The fragments in the top section. sediments of this formation, unconformably • Lower part: Pliocene in age, composed of overlying the Cau Formation, correspond to the siltstones, friable claystones, and grey lower part of the Arang Formation and constitute calcareous clays intercalated with white or light the 'BI' seismic sequence. This formation is yellow quartz sandstone, rich in carbonate and composed of . light grey, bright-grey sandstone, containing glauconite. interbedded with blackish-grey silty clay. The main components of the sandstone are fine to medium The sediments of this formation were deposited grained, angular, quartz, with carbonate cement, under coastal/marine, shallow to deep marine and glauconite associated with fossil fragments. conditions. Their thickness varies from several The thickness of this formation varies from 200 meters to 200 meters. m to 800 m, locally up to over 1,000 m (Block 05-3). Tectonic-structural features Thong-Mang Cau Formation (Middle Miocene, Fig. 4). The sediments of this formation correspond Faulting to the middle part of the Arang Formation and constitute the lower part of the 'B II' seismic In the Natp. Con Son Basin the following major sequence. The sediments are of clastic origin, fault system exist: interbedded with biogenic carbonates. The lithology • N-S Fault System: Concentrated in the of this formation can be divided into two distinct western differentiated zone, sub-zone next to parts: Natuna • Lower part: Quartz sandstone and calcareous • NE-SW Fault System sandstones interbedded with clays and • E-W Fault System siltstones. Fine to medium grained sandstones • NW-SE Fault System with carbonate cement and glauconite and fossil fragments. Structural elements • Upper part: Light grey, whitish/grey, locally All the geological formations in the Nam Con reddish brown dolomitized carbonates with Son Basin can be divided into two complexes of thicknesses varying from 10 m to 100 m. major structural elements (Nguyen Giao and Intercalated with these carbonates are clays Nguyen Trong Tin, 1990). friable siltstones and fine grained, calcite­ Basement. The basement is composed of Pre­ cemented sandstones. Cenozoic strata. The Nam Con Son Basin developed Nam Con Son Formation (Upper Miocene, Fig. on the surface of a basement which is heterogeneous, 5). This Formation corresponds to the Tereembu and it is displaced and strongly incised by faults Formation and constitutes the top part of the 'B II' into subsidence troughs at various levels. July 1995 4 NGUYEN TRONG TI N AN D NGUYEN DIN H Ty DISTIRIBUTION OF OLIGOCENE SEDIMENTS NAM CON SON BASIN N CAMBODIA , VIETNAM z o 26 27 z 38 33 20 28 39 34 o 40 35 29 , 30 " ,, D PEDIASTRUM W MAGNASTRIATITES = MANGROVE P·.t \) I ALLUVIAL, COASTAL o WELL CONTROL DRAW No AM03IP OZ.COA Do.. 2 August 94 Figure 2. Distribution of Oligocene sediments, N am Con Son Basin. Ceo i. Soc . MalaYJia, Bulletin 37 PETROLEUM GEOLOGY OF THE NAM CON SON BASIN 5 DISTIRIBUTION OF EARLY MIOCENE SEDIMENTS NAM CON SON BASIN lOON lOON CAMBODIA VIETNAM 126 z o o 26 = = 27 z 36 33 134 26 0 39 34 135= 40 35 29 '-- \ 30 \ \ D PEDIASTRUM MAGNASTRIATITES ~ MANGROVE D SHALLOW MARINE MIXED CLASTICS AND LIMESTONES 0 WELL CONTROL ()RAW No AM03POI COR D Date 2 Al.CJUlI c:w Figure 3. Distribution of Early Miocene sediments, Nam Con Son Basin. JuLy 1!J!J5 6 NGUYEN TRONG TIN AND NGUYEN DINH Ty DISTIRIBUTION OF MIDDLE MIOCENE SEDIMENTS NAM CON SON BASIN N N CAMBODIA VIETNAM . , 128 \ .. ' o 38 33 20 28 o 21 o 29 22 CAIRN o t:I--------' o 30 , \ , _ COASTAL PLAIN D SHALLOW MARINE - COASTAL BAR D MIXED CLASTICS AND LIMESTONES I I LIMESTONE I I o WELLCONTROL DRAW No AMOJ P 03 COR Oa,'" 2 AUJUSI 9-' Figure 4.