Global Accumulations of Oil & Gas in Fractured and Weathered Basement: Best Practices for Exploration and Production
Fractured and weathered basement rocks are important oil and gas reservoirs in various basins world-wide. This author has followed this subject very closely for over thirty years and hereby shares his knowledge and experience. This paper focuses on important oil and gas fields in Indonesia, Viet Nam, China, and Venezuela and explains how these fields were eventually discovered despite their complicated geology. Also reviewed is how the operators of these fields are able to efficiently and economically produce oil and gas from the basement reservoirs. 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 3D 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 and reservoir engineers 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 which produces from granitic basement were typically drilled 500 meters into the basement. Similarly exploration wells should penetrate at least 100 meters into basement rather than just barely penetrate the top of 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 to not produce such good reservoirs.
AAPG Datapages/Search and Discovery Article #90194 © 2014 International Conference & Exhibition, Istanbul, Turkey, September 14-17, 2014