Best Practices for Exploring and Producing Oil & Gas from Fractured & Weathered Basement: Examples from Asia
Business Development, Gaffney, Cline & Associates, Bellaire, Texas, TX.
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 Viet Nam (Ca Ngu Vang), China (Dongshenpu and Yaerxie) and Indonesia (Beruk Northeast, Sumatra and Tanjung, Kalimantan).
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 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 needed 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 China's Dongshenpu "buried hill" basement field the oil column is 400 meters thick thus necessitating the drilling of development wells through most of the reservoir.
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 Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012