Discovering oil deposits in fractured basement rocks is unprecedented in world oil and gas geosciences. Sometimes, the peak output reached 12 million tonnes a year and the oil recovery rate set a record, with some places topping 40 per cent. To date, Vietsovpetro Joint Venture has extracted more than 220 million tonnes of oil from the basement rock layer and more than 15 million tonnes of liquefied petroleum gas (LPG) and condensate, and earned about US$75 billion of revenue. And during that time, more than 30 billion cubic metres of associate gas has been brought ashore to feed many power plants.
Unseen elsewhere
In celebration of the 30 years of starting commercial oil production from the basement rock structure, leaders of the Vietnamese oil and gas industry, said: On September 6, 1988, Vietsovpetro Joint Venture started commercial oil in the fractured basement rock structure in Bach Ho (White Tiger) oilfield where oil reserves measure nearly 4 billion barrels on an area of nearly 60 square kilometres and at a well height of 1,300 metres. This is a unique non-traditional oilfield, unseen elsewhere in the world. Exploiting oil from the basement rock structure laid the foundations for the Vietnamese oil and gas industry.
Discovering oil reserves in Oligocene granitic basement rocks at Bach Ho field is a major contribution to the oil and gas science, marking a turning point in observing and assessing oil and gas potential in the Cuu Long Basin, south of the continental shelf of Vietnam and changing the concept and direction of determining oil production strategy in this area. Finding oil in granitic basement rocks is a coincidence but it came from bold decisions built from the long history of science and years-long practical experience in research, survey and exploration of Vietnamese and Soviet oil and gas scientists.
Vietsovpetro’s oil production results in Bach Ho and Rong (Dragon) fields and other fields show the diversity of geological forms of oilfields and oil bodies in the presence of water edge, the difference in distribution depth and deposit composition (oil and gas), oil bodies in fractured basement rocks found at depths nearly 2,000 - 4,500 metres, and oil physical parameters such as density, viscosity and solubility. The porosity-permeability distribution is potential for complex industrial currents resulting in high risks when the joint venture placed wells to search and develop. Many fields were rediscovered from many dry drilled wells abandoned by other drillers like Hai Su Den (Black Sea Lion) and Ho Xam (Grey Tiger) fields.
Another traditional way to find oil is to drill through soft sediments and stop drilling when touching hard rocks or foundations, Vietsovpetro designed and drilled first exploratory wells at the Bach Ho field, affirmed by core samples.
The discovery and extraction of oil in granitic basement rocks by Vietnam National Oil and Gas Group (PetroVietnam), led by Vietsovpetro, is a great contribution to the theory and practice of the world oil and gas industry and to Vietnam’s economic development. This event changed traditional viewpoints on oil and gas search and exploration, opened a new, very important direction in the Cuu Long Basin in particular and the continental shelf of Vietnam in general, became a magnetising motivation for world oil and gas companies to invest back and accelerate oil and gas exploration and production in Vietnam, and contributed significantly to the world oil and gas industry. As a result, oil and gas reserves in sedimentary basins in Vietnam, contributed by oil reserves in basement rocks, are constantly increasing. Oil reserves in basement rocks account for more than 80 per cent of oil and gas reserves in the country.
Great significance
In fact, Vietsovpetro’s comprehensive and systematic approach, including exploration and appraisal of shallow oil layers for quick commissioning on the one hand and data collection for research in deep layers is totally right and effective. In 1985, at the BH-4 exploratory well in Vom Bac Bach Ho, Vietsovpetro signed Miocene oil layer appraisal contract after its Oligocene oil discoveries. In 1987, when assessing Oligocene oil reserves at BH-6 well, Vietsovpetro encountered a big discovery (well flow of 505 cubic metres a day) in weathered granitoid after launching torpedoes at depth of 3,508 - 3,515 metres. After meeting oil in weathered foundations, Vietsovpetro strived to produce Oligocene oil in the priority area on the one hand while exploring and testing deep in foundations by operational rigs on the other. On May 25, 1988, the joint venture started designing the BH-47R exploratory well to drill into the central arch at a vertical depth of 3,300 metres, expected to drill diagonally from the MSP-1 to the MSP-2 base. While waiting for construction materials, the joint venture carried out recommendations from the Offshore Drilling Bureau on June 24, 1988 - retesting seams in the foundation layer at the BH-1 well which was running out Miocene oil. As a result, on September 5, 1988, oil from the roof of the foundation spouted out with a capacity of 407 tonnes a day and the BH-1 well was immediately put into operation on September 6, 1988 with a drill rod of 89-mm diameter. This is a large oil body in fractured granitic rocks with estimated reserves of over 500 million tonnes covering an area of nearly 60 square kilometres and the body height of 1,300 metres. The peak output exceeds 12 million tonnes a year and the peak well flow reaches 2,000 tonnes a day.
| Professor Tran Van Tri, former Chairman of the State Council for Ho Chi Minh Award 2010-2011, said: In a short time, with moderate equipment and technology, workers of PetroVietnam, pioneered by the staff from Vietsovpetro, worked and learned at the same time to get mature and make a miracle and were awarded Ho Chi Minh Prize in Science and Technology 2010 for the project “Effectively searching, discovering and exploiting oil bodies in Early Oligocene granitic basement rocks in Cuu Long Basin on the Vietnamese continental shelf.” |
From Vietsovpetro’s experience, many oilfields on fractured basements have been discovered and put into operation. Foreign companies also followed to improve oil drilling and extraction technology in base rocks, helping increase oil production in Vietnam. Currently, in Cuu Long Basin, in addition to operational oil and gas fields like Bach Ho and Rong discovered and operated by Vietsovpetro, Nam Rong - Doi Moi field of Vietnam - Russia - Japan Joint Venture; Su Tu Den, Su Tu Vang and Su Tu Nau fields of Cuu Long JOC; Rang Dong field of JVPC; Ruby field of Petronas; Ca Ngu Vang field of Hoang Long - Hoan Vu JOC; and many other fields like Jade, Diamond, Pearl, Hai Su Den, Thang Long, Ho Xam South are ready for commercial production soon.
Right technological application has significantly increased oil recovery from oil bodies in Bach Ho oilfield. As of January 1, 2018, the joint venture produced nearly 190 million tonnes of oil, with a recovery factor of 37 per cent, including more than 100 million tonnes of crude oil and about 15 billion cubic metres of gas.
Nguyen Thi Binh
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Vietnamese
