Tran Canh, Do Van Ha
Petrovietnam
Hanoi
Halfdan Carstens, Sverre Berstad
Nopec AS
Naersnes, Norway
Vietnam Oil & Gas Corp. (Petrovietnam) has, through 1993 and since the Vietnamese government first declared its new open door policy (doi moi) more than 6 years ago, issued 27 production sharing contracts to Asian, Australian, European, and North American companies (Table 1).
The most prospective part of Viet Nam's shallow water acreage is now being explored intensely. This acreage is concentrated in the Nam Con Son basin, the Cuu Long basin (previously known as the Mekong basin), the Malay-Thochusia basin (afl off southern Viet Nam) and the Quang Ngai graben, the Song Hong basin, and the Red River delta (all off northern Viet Nam).
With 6.3 million metric tons of crude oil pumped in 1993 from White Tiger (Bach Ho) field, and with Dragon (Rong) and Big Bear (Dai Hung) fields soon to be brought on stream, offshore Viet Nam is considered to be very attractive by the international oil industry, and the country's potential is still far from being fully appraised (Table 2).
Current exploration has been limited to shallow water predominantly less than 200 m. A new licensing round offering deep water and frontier acreage (as provided for in the Petroleum
Law that passed the Parliament in 1993) off central Viet Nam was announced in October 1993 at the exhibition "Petrovietnam '93" in Ho Chi Minh City.
The nine contiguous blocks 122-130, with a total area of more than 60,000 sq km, have now been geophysically surveyed with a new seismic data package of 6,900 km in an area previously having very poor seismic coverage (Fig. 1, Table 3). Water depths range from near shore to more than 2,000 m, while the bulk of the new seismic data are from water less than 500 m deep. Some of the open acreage is in shallow water of less than 200 m.
Interpretation of the new seismic data package proves the existence of sedimentary basins with a thick sedimentary succession in the open blocks, and a variety of play types can be recognized in a diversified structural setting. The petroleum bearing Cuu Long and Nam Con Son basins can be demonstrated to reach into the southern half of the area to be licensed, while the northern half is dominated by the unexplored Phu Khanh basin and the important transitional shear zone transecting northwest-southeast.
EXPLORATION HISTORY
The history of the petroleum activities in Viet Nam can be examined in different phases corresponding to different legal regimes.
PRE-UNIFICATION
Exploratory work until 1975 was carried out in the north in the Red River delta by the Vietnamese General Department of Geology with the financial and technical assistance of the Soviet Union.
The first deep well was drilled in 1969, and the first gas discovery was made in 1975. Tien Hai gas field is still producing small quantities of gas from about 1,000 m. Petrovietnam is bottling formation water from 450 m for its own use as drinking water.
In the south, the General Department of Petroleum and Minerals of the former Saigon administration opened more than 30 blocks in the early 1970s. Thirteen concessions were signed with international oil companies. Several wells drilled in the Nam Con Son and Cuu Long basins gave encouraging results, including the first well in White Tiger field drilled by Mobil. The well flowed 2,400 b/d of oil from the Lower Miocene.
All the concessionaires terminated their activities after the country's unification.
POST-UNIFICATION
The next legal regime extends from post-unification until promulgation of the Foreign Investment Law in Viet Nam (1975-1987).
In September 1975, the General Department of Off & Gas was established with the function of administering and managing petroleum activities from exploration to production and refining countrywide.
After the formal unification of North and South Viet Nam on July 2, 1976, all South Vietnamese laws were rescinded and all the concessions granted thereunder were abrogated and declared illegal. U.S. oil companies have since been barred by a trade embargo from exploring in Viet Nam, albeit an easing of the embargo in 1993 changed this.
In September 1977 the Vietnam Oil & Gas Co. (Petrovietnam) was set up as the legal entity responsible for the management of the negotiation and execution of petroleum contracts with foreign oil companies. Until 1978 Petrovietnam signed five contracts for exploration and production of oil and gas on the continental shelf of southern Viet Nam. Eleven structures were tested that resulted in two small oil discoveries.
All operators, however, ceased their operations for technical, economic, and political reasons upon the expiration of the initial term.
In June 1981, Vietsovpetro - a joint venture of the Soviet Oil Industry Ministry and the Vietnam General Department of Oil & Gas - was set up after the agreement between the two governments for cooperation in the field of oil and gas.
The first commercial production from White Tiger (Bach Ho) field started in 1986, and two additional discoveries were made: Dragon (Rong) and Big Bear (Dai Hung). The latter is now being developed by a consortium of whom the Australian company BHP Petroleum is operator.
During this period regional geophysical surveys (seismic, magnetic, gravity) were conducted in most of the area from the north to the south of the Vietnamese continental shelf.
1987 AND BEYOND
This period commenced with introduction of the Foreign Investment Law and the open-door policy.
This policy and the law designed to encourage foreign oil companies to explore offshore Viet Nam have led to an exploration surge.
Vietsovpetro surrendered a large portion of its acreage in blocks 4, 5, 10, 11, and 15, which are all believed to be very prospective. For the purpose of promoting this acreage the blocks were all divided into smaller blocks, namely 4-1, 4-2, 4-3, 5-2, 5-3, 11-1, 11-2, 15-1, and 15-2.
Petrovietnam called for selective tenders, and as a result 27 different PSCs have been entered.
Of these 27 PSCs signed since 1987, 25 are on the continental shelf of Viet Nam, one is in the Viet Nam-Malaysia overlapping area, and one is onshore (Red River delta). To date, 34 prospects have been drilled-10 in the Gulf of Tonkin (Song Hong basin), two in the Phu Khanh basin, seven in the Cuu Long basin, 12 in the Nam Con Son basin, and three in the Viet Nam-Malaysia overlapping area. Six oil and six gas discoveries have been reported, with many other wells having good oil and gas shows.
At the end of 1993 Mobil, as part of a consortium with three Japanese companies, was awarded an interest in a highly prospective block known as the Blue Dragon (Thanh Long) prospect situated near Dai Hung oil field.
OPENING NEW BLOCKS
Petrovietnam in 1993 formally announced a licensing round including blocks 122130 off central Viet Nam.
Petrovietnam and Nopec held promotion seminars earlier this month in London and Houston. Oil companies will have until Oct. 1, 1994, to interpret the available data, study the multiclient report, and submit applications. Petrovietnam will start further negotiations with individual companies after this Oct. 1.
In preparation for the licensing round a new seismic data package has been put together through a co-operation between Petrovietnam and Nopec. A survey acquired in 1984 by a Russian vessel has been reprocessed (VRP-93) and a new survey has been acquired in 1993 (VOR-93) using the vessel GECO Delta (Fig. 1). VRP-93 constitutes 3,522 km and VOR 3,355 km. Gravity data and well data supplement the seismic data and form the basis for an integrated interpretation outlining play types and prospectivity.
STRUCTURAL DEVELOPMENT
The VOR-93 and VRP-93 seismic data packages cover the junction between the Cuu Long, Nam Con Son, and Phu Khanh basins. These basins were originally formed as rift basins. The rifting commenced in the Paleogene and was related to the opening of the Eastern Sea, following the collision of the Indian and Eurasian plates.
The Cenozoic tectonic evolution of the basins can be divided into three phases (Fig. 2). Phase 1 occurred in the Oligocene to early Miocene and is considered to be the main rifting phase which initiated the development of grabens parallel to the opening of the deepwater Eastern Sea basin. The dominant style of deformation was extensional and transtensional. Transpressional deformation is also observed along strike-slip faults.
Phase 2 occurred in the middle Miocene and is associated with a regional subsidence and an eastward tilting of the entire area. The dominating tectonic style in the middle Miocene was transpressional, forming large size flower structures in the sedimentary succession and inversion of the basement blocks.
Phase 3 took place in the late Miocene to Quaternary. In the Phu Khanh and Nam Con Son basins there was a third rifting phase while in the Cuu Long basin a low-amplitude inversion is recognized. The final subsidence along the inherited fault zones at the shelf margin and a relatively low-amplitude uplift of the western part of the inner shelf gave the basins their present day morphological structures.
SEDIMENTARY BASINS
The area to be licensed by Petrovietnam covers several known structural elements, such as Phu Khanh basin, the north-eastern part of the Cuu Long basin, the northernmost part of the Nam Con Son basin, the Phan Rang shelf, and the Da Nang shelf.
In addition, two new names are introduced here: Horst "A" and the Tuy Hoa shear zone.
The thickness of the sedimentary basins varies from more than 10,000 m in the main depocenters to less than 500 m along the western margins of the basins.
The Phu Khanh basin is a deepwater basin, bounded to the east by a steep continental slope and separated from the Cuu Long basin to the south by the Tuy Hoa shear zone. During the main rifting phase the basin was controlled by a north trending fault system along the outer margin of the Da Nang shelf.
The northeastern part of the Cuu Long basin is bounded by the Phan Rang shelf to the north, Horst "A" to the south, the Viet Nam mainland to the north-west, and the Tuy Hoa shear zone to the northeast. The Cuu Long basin has a complex structure that is best described as "basin and range" style.
The northernmost part of the Nam Con Son basin has a complex structure with many linear horsts and grabens, formed during the main rift phase and controlled by normal faults detached from the basement.
Unlike the Phu Khanh and Cuu Long basins the middle Miocene subsidence in the Nam Con Son basin represents the second rift phase. Some of the faults developed during this phase show evidence of lateral movements, even producing local thrusting. The subsidence of the third rift phase accompanied by lateral movements created the present day basin.
The Da Nang and Phan Rang shelves have been relatively rigid blocks during the Cenozoic evolution of the basins. Throughout the main rifting phase these shelves remained relatively elevated intracratonic areas, only weakly altered by block fault movements forming shallow half-grabens. Parts of the grabens were uplifted by Pliocene-Pleistocene movements coupled with weak wrench faulting.
Horst "A" separates the Nam Con Son basin from the Cuu Long basin and extends about 190 km to the northeast, parallel to the general structural trend of these basins. The horst has an en echelon fault block structure and can be subdivided into three second order highs. The uplift of the central part of Horst "A" was synchronous to the subsidence of the adjacent parts of the Cuu Long and Nam Con Son basins during both the main and second rift phases. The northeastern part of the horst was submerged and reworked by block faulting during the second and third rifting phases.
The Tuy Hoa shear zone cuts off the Phu Khanh basin and appears as an area with high-amplitude listric faults, some of them detached from basement. The northwest trend and structural style of the Tuy Hoa shear zone are similar to those of the Red River fault system extending onshore Viet Nam. This depression was formed before the main rift phase, and sediments of Lower Oligocene and probably also Eocene age may exist here.
The Tuy Hoa shear zone was developed as a transtensional fault system during the main rift phase. During the Middle Miocene the tectonic style changed from one of transtensional movements to one of transpressional movements which formed large-scale flower structures and altered the vertical component of the faults from normal to reverse.
PLAY TYPES
The play concepts within blocks 122-130 have been divided into four types: structural, carbonate, stratigraphic, and fractured/,.weathered basement.
STRUCTURAL PLAYS
The reflector named Top Middle Miocene, which is a pronounced event in the entire area, post-dates the main structuring and all structural traps recognized and mapped are within the underlying sequences (Figs. 3, 4).
In spite of a relatively coarse seismic grid, numerous potential structural trapping possibilities have been outlined in the Phu Khanh, Cuu Long, and Nam Con Son basins as well as in the Tuy Hoa shear zone. The plays identified are related to horst blocks, rotated fault blocks, elongated and faulted anticlines formed by both normal and reverse faults, drape features, and positive flower structures. Infill seismic data are necessary to define prospects in some detail.
The oil and gas in White Tiger field, located in the western part of the Cuu Long basin, are trapped in a basement block and in overlying Lower Miocene sandstones. The Lower Miocene sandstone beds form a simple anticline which is partly fault-bounded. In addition, Oligocene sandstones in a downfaulted position may be oil-bearing.
CARBONATE PLAYS
Seismic facies analysis and seismic velocity data have been applied to identify carbonate facies.
Combined with regional geological considerations it is concluded that carbonate deposition took place from latest Oligocene through Pliocene, with a peak in carbonate production during the Middle Miocene. From the beginning of the Middle Miocene the study area was separated into a northern and a southern province. The Phu Khanh basin carbonate platform in the north developed into a very narrow zone, while to the south a very thick, extensive carbonate platform developed.
Carbonate plays include carbonate complexes within which barrier reefs, buildups in general and bioclastic aprons can be identified. Walled reef complexes along the coastline of Viet Nam are judged to have a high potential (OGJ, Dec. 20, 1993, p. 107).
An attractive possible Natuna Sea field structural equivalent has been recognized in the Cuu Long basin in shallow water (Fig. 4).
STRATIGRAPHIC PLAYS
Three main types of stratigraphic plays are identified within the study area: pinch-out zones, traps in submarine fan complexes, and traps related to slope turbidities.
These play types are confined to five different stratigraphic levels-Paleogene, Lower Miocene, lowermost Middle Miocene, uppermost Middle Miocene, and Upper Miocene-Lower Pliocene. Their depositional environments have further been investigated mainly based on seismic facies analyses, In general, the northern part of the study area, the Phu Khanh basin, has a greater diversity of stratigraphic traps at different levels compared to the southern part. Here in the Cuu Long basin the main stratigraphic play type consists of westward pinch-outs within the Paleogene succession.
In the Phu Khanh basin the Miocene plays are characterized by submarine fan complexes, basin floor fan deposits and turbidities, and zones that pinch out toward the west. One recent well in the Quang Ngai graben to the north of the study area had good shows of oil and gas in such a submarine fan complex.
The lowermost Middle Miocene is also characterized by onlapping pinchout zones towards the west. The uppermost Middle Miocene and Upper Miocene-Lower Pliocene have prograding shelf wedge systems.
In the Cuu Long basin the most important type of stratigraphic play is the Paleogene onlapping system, while in the downflank southeastern part (e.g. northeastern part of the Nam Con Son basin) onlapping systems are present from Paleogene to Upper Miocene-Pliocene.
FRACTURED/WEATHERED BASEMENT
Fractured and/or weathered basement is well-known as reservoir rock from nearby White Tiger field, further west in the Cuu Long basin.
Oil shows are also encountered in the basement in Dragon field in the same basin and Big Bear field in the Nam Con Son basin. The age of these basement rocks are Mesozoic (jurassic/Cretaceous) and described as crystalline.
Wells have penetrated up to 1,000 m into basement rocks, with oil bearing fractured intervals still present.
All plays mapped are related to the uppermost part of the basement, and all the structural closures are identified on more than one seismic line. The mapped plays are also in direct contact with an Oligocene source rock unit, and appropriate seals are identified.
SOURCE ROCKS
In the absence of well control in the study area it has been necessary to base our predictions on nearby well data, published reports, and seismic facies analysis.
The main source rocks are believed to be black shales within the Oligocene succession, with the organic matter being of lacustrine and lagoonal origin. Eocene lacustrine source rocks are proven in the Nam Con Son basin and need to be taken into consideration in areas of Eocene deposits. The possibility of marine (?) Lower Miocene source rocks should not be overlooked.
A one-dimensional basin modeling exercise based on Yuklerpc software has been used to quantify the geological evolution and the pressure, compaction, temperature and hydrocarbon generation, migration, and accumulation histories at selected intervals along the seismic lines. These pseudo wells were chosen in order to model structurally low areas (kitchens) and the highs into which the hydrocarbons could have migrated.
A series of time sheets has been produced which all point toward the overall conclusion that the southern part of the study area, i.e. the Nam Con Son and the Cuu Long basins, tends to be oil prone while the northern part, i.e. the Phu Khanh basin, tends to be gas prone (Fig. 5). Furthermore the study indicates that oil may exist down to 5,000 m and locally even deeper.
TERMS, CONDITIONS
The Petroleum Law concerning exploration and production of oil and gas was adopted by the Vietnamese Parliament in 1993.
Together with the accompanying Petroleum Regulations now in drafting and the Foreign Investment Law, it forms the backbone of the relevant legislation for petroleum sector investments.
The Vietnamese base their licensing on Production Sharing Contracts (PSC), with terms to be negotiated between the licensee and Petrovietnam. As much of the new acreage is located in deep water (more than 200 m) and frontier acreage, Petrovietnam is carefully evaluating its negotiating position in light of recent deepwater licenses awarded in neighboring countries.
Revised deep water PSC terms will neither be announced in conjunction with the planned promotion seminars nor before any negotiations with individual companies which are planned to be initiated after Oct. 1, 1994. However, Petrovietnam would like to stress the flexibility in their negotiating position. Cost oil allowance, profit sharing, and work programs are all negotiable items.
Royalty rates will according to the Petroleum Law be established depending on geographical, economic, and technical field conditions and the level of production of crude oil or natural gas. For deepwater areas, up to four blocks can be included in a license with an extended exploration period of 7 years. Terms and conditions will be further elaborated on in the promotional seminars for the license round.
ACKNOWLEDGMENTS
Thanks are due Vladimir Kudelkin and Vladimir Kuznetsov of Dalmorneftegeofizika (DMNG) of Yuzhno-Sakhalinsk for their contribution in interpreting the seismic data, and Petrovietnam for permission to submit this article.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
