Basement reservoirs have been exploited for decades. However, since the mid-1990s, there has been increasing interest following major discoveries in Vietnam and Yemen, combined with the creation of new downhole tools, seismic technics and improved drilling technologies; and the higher oil price which has made basement projects more economically viable.
The best known basement reservoir examples are offshore Vietnam, where the Cuu Long Basin (pictured above) comprises 95% of the country’s hydrocarbon production and 85% of this comes from the fracture granitic basement. Other significant discoveries have been made in the Yemen and Argentina’s Cuyo and Neuquen fields. Beyond this, producing basement reservoirs have been explored in around 30 countries worldwide including Alaska, Canada, Chile, China, Egypt, former Soviet Union countries, India, and Greenland.
Basement reservoirs are a subset of naturally fractured reservoirs and owe their hydrocarbon storage capacity and productivity to the presence of naturally permeable fractures. The fractures have developed through a variety of geological processes and are so distributed that they provide a connected network of void space. The rocks hosting such fractures are typically igneous and metamorphic rock such as granite, basalt and gneiss.
Fractured basement reservoirs are typically associated with structures which, over time, have been uplifted in relation to the source (sedimentary) rock such that hydrocarbons are able to migrate into the basement fractures. The oil is not found in the rock itself, but in the faults and fractures between.