Fractured & Weathered Basement Reservoirs - Best Practices for Exploration and Production: Examples from USA, Venezuela and Brazil
Fractured and weathered basement rocks are important oil and gas reservoirs in various basins in the worldwide. This author has followed this subject very closely for 30 years and hereby shares his knowledge and experience. This paper focuses on relevant fields in the USA including the Kansas fractured quartzite "buried hill" oil fields and basement oil fields in California including the El Segundo and Edison oil-bearing schist reservoirs. Also reviewed are the La Paz and Mara basement oil fields in Venezuela and also basement reservoirs in the Carmopolis oil field, onshore Brazil.
Best practices include the following: production wells should be drilled near-perpendicular to the dominant fracture system. Exploration wells should also be drilled highly deviated rather than vertical in order to optimally intersect the dominant fracture systems. Highly focused 3-D seismic such as CBM (Controlled Beam Migration) is needed to define the fracture systems in basement. Extensive core coverage is necessary to provide critically important information on the litholgies and reservoir parameters. Some of the cores should also be radiometrically age dated in order for the geologists to understand the complexities of the basement reservoirs they are dealing with. Development wells must be sufficiently deep to fully drain the reservoir. Wells should not just "tag" into the top of basement. For example wells in the La Paz field, Venezuela were typically drilled 500 meters into the basement.
In a general sense, fractured granites and quartzites are the optimum reservoirs. Weathered "rotten" granites can also be excellent reservoirs as can be observed in outcrop in tropical areas. Rocks such as schists and gneisses are less attractive since they are ductile and tend to "smear" and not fracture when subjected to tectonic stress. The high mafic content of schists also negates the creation of secondary porosity by weathering. Likewise, granites and quartzites are more likely to provide attractive, highly porous "granite wash" sands whereas eroded schists do not produce such good reservoirs.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013