Hydrocarbons in Fractured and Weathered Basement — From Overlooked Exploration Play in West Africa
Social Projects, Tullow Oil Angola, Luanda, Angola
Basement rocks are important oil and gas reservoirs in a number of countries and serve as a reminder that in areas where basement is not too deep, basment should be considered a valid exploration objective.
Basement reservoirs included fractured and weathered granites, fractured quartzites, and metamorphic rocks such as fractured schists and argillites.
In South America, basement reservoirs produce oil in Venezuela and Brazil. Oil is produced in the USA from basement in California, Kansas and Texas. In North Africa, basement oil and gas production occurs in Libya, Algeria and Egypt. In the Middle East, in the last decade in Yemen, very important oil reservoirs have been discovered in granite basement. Significant basement reservoirs occur in Russia's West Siberia Basin and also in China in "buried hill" basement structures. In Southeast Asia, very prolific basement reservoirs are the main contributor of oil in Viet Nam. Giant size gas fields(up to 5 TCF) are major gas producing reservoirs in South Sumatra, Indonesia.
In all of West Africa, oil or gas has never been commericially produced from basement reservoirs. Only in the onshore area of Cabinda, northern Angola has oil been tested from one or two wells. Very little information is known about these basement reservoirs in Cabinda.
The tendency in West Africa has been to terminate drilling prior to penetrating basement. There has never been a deliberate search for oil or gas in basement. Therefore it is possible that there are oil and gas fields "left behind" in areas where basement should have been evaluated by drilling because regional data indicated that the basement was likely fractured or weathered and where seismic defines structural closure. Also, for such oil or gas fields to occur, mature source rocks must be close to basement and cap rocks must overly the possible basement reservoir.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery