Abstract

Oil and gas occur in basement reservoirs in many parts of the world. The reserves of basement fields are as small as one or two million barrels of oil or gas-equivalent to over 2.3 billion barrels of oil as in China's Reniqiu oil field. Exploration for oil and gas in basement has been remarkably successful in the past decade with important discoveries in basement in China, Indonesia, United Kingdom, Norway, Chad, and Argentina. To successfully develop basement oil and gas fields and to avoid costly mistakes, all available geological, geophysical and reservoir engineering must be closely studied. Also, it is very important to study analogues of basement oil and gas fields worldwide to understand why some fields are very successful and others turn out to be failures. The author has followed this subject closely for over forty years, since being involved in 1982 with the development of the Beruk Northeast basement oil pool in Indonesia. He has also been involved with evaluating basement oil discoveries in Angola, Uganda, and the interior of the USA. He hereby shares his knowledge and experience. This paper provides a technical review of select fields in Asia, Africa, and the Americas. Also reviewed are "best practices" for exploring and developing basement fields. Best practices include drilling at least 100 meters through the top of the basement. Exploration and development well locations must be based on state-of-the-art 3D seismic data. Depending on the orientation of the dominant fractures, all wells must be drilled almost perpendicular to the fractures. The success of optimally placed development wells has been documented in prolific fields such as the Bach Ho (White Tiger) and CNV oil fields in Vietnam and the giant Suban gas field in South Sumatra, Indonesia. Although this paper reviews mostly "success stories", there are also failures, since basement reservoirs can be very complicated and unpredictable. Accordingly, two basement fields which proved to be economic failures, Dai Hung (Big Bear) in Vietnam and Beruk Northeast in Indonesia, are also reviewed. Reviewing such situations is important due to the saying, "We learn from our successes, but we also learn from our failures". The world's largest oil and gas basement fields occur within basement which is heavily naturally fractured. The opinion of this author is that the best rock types are fractured quartzites or granites, since they are brittle and thus fracture optimally. Weathered granitic basement can also be an excellent reservoir, as in the giant Augila-Naafora oil field in Libya. Fractured granitic gneisses are moderately good basement reservoirs whereas phyllites, schists and slates are the least preferrable basement reservoirs. My ongoing global review of basement oil and gas fields indicates that the best place to explore for oil and gas in basement is beneath oil or gas fields which produce from the sediments overlying basement. Prime examples are the Indonesia's Suban gas field as wells as the La Paz and Mara oil fields in Venezuela and the Octogono oil field in Argentina.

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