Best Practices for Exploring and Producing Oil and Gas from Basement Reservoirs (With Applications to West Africa)

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1 Best Practices for Exploring and Producing Oil and Gas from Basement Reservoirs (With Applications to West Africa) Tako Koning, Advisor, 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, basement should be considered as a valid exploration objective. Basement reservoirs include fractured or weathered granites, fractured quartzites, and metamorphic rocks such as fractured schists or argillites. This poster paper is based entirely on public domain information including various geological and geophysical publications, oil & gas journals, and websites. The following is a quotation from the classic paper on oil in basement reservoirs by K.K. Landes et al (1960 AAPG) titled Petroleum Resources in Basement Rocks which stated Commercial oil deposits in basement rocks are not geological accidents but are oil accumulations which obey all the rules of oil sourcing, migration and entrapment; therefore in areas of not too deep basement, oil deposits within basement should be explored with the same professional skill and zeal as accumulations in the overlying sediments. Although this paper was published almost 50 years ago, the advice of Landes and his co-workers still applies to today. In South America, basement reservoirs produce oil in Venezuela (La Paz field) & Brazil (Carmopolis field). Oil is produced in the USA from basement in California, Kansas and Texas. In North Africa, basement oil and gas production occurs in Libya (e.g. Nafoura field), Algeria (e.g. Hassi Mesaoud field) and Egypt (fields in the Gulf of Suez). 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. Oil and gas production is also obtained from basement rocks in the Clair field in the UK part of the North Sea. In Southeast Asia, very prolific basement reservoirs are the main contributor of oil production in Viet Nam. Giant size gas fields (up to 5 TCF) are major gas producing reservoirs in South Sumatra. In all of West Africa, oil or gas has never been produced commercially from basement reservoirs. Only in the onshore area of Cabinda, Angola has oil been tested from basement in one or two wells drilled in the early 1970 s. The followup was minimal. Very little information is known about the basement reservoirs in Cabinda. 1

2 The tendency in West Africa has been to terminate drilling prior to penetrating basement. Therefore it is possible that there are oil and gas fields left behind in areas in West Africa where basement was not evaluated by drilling but where basement is likely fractured or weathered and occurs within structural closure. Also for such oil fields to occur, mature oil or gas source rocks must be close to the basement and cap rocks must overlay the possible basement reservoir. Best practices for producing oil and gas from basement include: Necessity to drill production wells near-perpendicular to the dominant fracture system. Need for focused 3D seismic coverage to highlight fracture patterns in basement. Extensive core coverage is needed to provide information on rock types and reservoir parameters. Development wells must be sufficiently deep to fully drain the reservoir (e.g. wells in the La Paz field in Venezuela typically are drilled 500 meters into basement). There are a number of cases, such as the La Paz field in Venezuela where the basement oil field was discovered much later (30 years) in the life of the field, with the attention initially focused on the producing of the shallow reservoirs. Accordingly, in the case where an operator is producing oil or gas from reservoirs in sediments relatively close to basement, a second look at the basement may be warranted, especially using today s leading edge seismic and drilling technology. In a general sense, fractured granites and quartzites are the optimum basement reservoirs. Weathered granites can also be excellent reservoirs. Rocks such as schists are less attractive since they are ductile and tend to smear 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 schists do not erode and produce such attractive reservoirs. The author of this poster paper has been closely following the subject of oil & gas in basement reservoirs since he was involved with the development of such a field in Sumatra, Indonesia in 1982 while working for PT Caltex Pacific Indonesia (joint subsidiary of Texaco and Chevron). The following are brief summaries of the types of basement reservoirs in a select number of fields worldwide and comments are provided regarding best practices associated with production from such fields. La Paz Oil Field, Venezuela This field has produced over 300 million barrels of oil from fractured granite basement as well as over 800 million barrels of oil from the overlying Cretaceous limestones. Maximum initial production from wells has been as high as 11,500 BOPD with an average initial production of 3,600 BOPD. Average penetration 2

3 into basement in the La Paz field is 500 meters (1,700 feet). This illustrates the case with many basement oil fields worldwide, which is that it is important to drill deep into the reservoir rather than terminating the well soon after penetrating basement. Chinese Buried Hill Basement Oil Fields A case study was published in a 1991 issue of the AAPG Bulletin (Tong XiaoGuang & Huang Zuan) on the Dongshenpu buried hill oil field in the North China basin. This field is located about 500 km northeast of Beijing. Discovery of the field occurred in 1983 with the drilling of Sheng-3 which flowed at 1,570 BOPD and 0.5 million cubic feet of gas per day. The basement reservoirs are Precambrian (Archean) in age and consist of biotite-plagioclase granulites and granitic migmatites. The reservoirs have no primary porosity, rather all of the reservoir consists of secondary porosity and fracture porosity. As with the La Paz oil field mentioned above, wells in this oil field are drilled deep since the oil column is about 400 meters thick. Tanjung Oil Field, Kalimantan Approximately 21 million barrels of oil has been produced from Pre-Tertiary basement rocks. The lithologies are a mixed bag of fractured and locally weathered volcanic rocks, pyroclastics and metasediments. About 74 million barrels of oil has been produced from the overlying Eocene sandstones and conglomerates. Basement Oil Fields, Viet Nam The majority of Viet Nam s production of about 300,000 BOPD is from basement oil fields. The best known is the Bach Ho (White Tiger) oil field in the Cuu Long Basin with estimated reserves of 1.9 billion barrels. Bach Ho has produced over 1 billion barrels of oil to date and is currently in decline. The Rang Dong field has an estimated 320 million barrels of oil and 300 billion cubic feet of gas. SOCO International has had significant success in the discovery and development of the CNV (Ca Ngu Vang) basement oil field in Viet Nam. Information from the SOCO website described their approach to using new and improved seismic processing for detailed mapping of the internal structure of the basement. This led to remapping and reinterpretation of fracture lineaments and emplacement of development wells to optimize perpendicular intersection of the well bore with the lineaments. In 2005 SOCO drilled the CNV-3X well to a total depth of 6,123 meters which included over 2,240 meters into granite basement. This was the longest ever basement well in Viet Nam (reference Upstream magazine, ). The basement section was near-horizontal. The well tested at 13,040 barrels of oil equivalent per day. SOCO published in its 2008 annual report that 4 development wells have already been drilled in the CNV field and the field is expected to be in production for the next 20 years with oil production expected to be between 10,000 to 20,000 BOPD and wet gas production to be between 25 to 50 million cubic feet of gas per day. Production 3

4 will be pipelined to the Bach Ho s production facilities which now have excess productive capacity. CGGVeritas has published on its success in seismically defining basement reservoirs in Viet Nam. The challenge for CGGVeritas was to image the oilbearing fracture zones in complex granite basement reservoirs offshore Viet Nam where conventional methods failed to produce convincing results. They proceeded to reprocess data using their Controlled Beam Migration (CBM) algorithm for the velocity model building and the final migration. This led to one of the operators in Viet Nam to carry out a successful drilling campaign to develop the basement reservoirs. Basement Oil Fields Kansas, USA A number of fields in Kansas produce oil from Precambrian basement, most commonly fractured quartzites. The structures are described as Precambrian buried hills. The oil is sourced from the flanking Cambro-Ordivician or overlying Pennsylvanian sediments. The cap rocks are post-cambro-ordivician limestones and shales. Basement Schist Oil Fields California, USA In California, oil is produced from basement consisting of fractured Jurassic schists. Fields which contain basement reservoirs include the Playa del Rey, El Segundo, Santa Maria, Wilmington and Edison fields. Relatively few fields produce from basement rocks alone; most are multiple completions in both the basement schist and overlying schist conglomerate and Tertiary sandstones. The majority of the oil-producing schists are in a relatively high position and have usually undergone weathering and erosion thus increasing the porosity (Landes et al, 1960). These fields have typically produced in the order of 20 million barrels (Edison field) and 22 million barrels (Wilmington field). These reserve numbers could ultimately be much higher due to ongoing secondary recovery programs and possible application of horizontal drilling programs. In the Edison field, the average production rate is about 1,000 BOPD. Beruk Northeast Oil Field, Central Sumatra, Indonesia A paper was published on this oil field in 1984 by T. Koning and F.X. Darmono (paper referenced below) for two purposes. The primary purpose was to educate the explorationists that it was possible to find and produce oil from basement in SE Asia. This field was discovered before the major basement oil field discoveries in Viet Nam and the giant basement gas fields in South Sumatra, thus at that time SE Asian was almost devoid of oil and gas production from basement reservoirs. The secondary purpose was to point out to the geoscientists and reservoir engineers that basement fields, like Beruk Northeast can, in some cases, be highly unpredictable and difficult to develop and that money can be lost in the process. 4

5 The Beruk Northeast field was discovered in 1976 by Beruk NE-1 which flowed 1,680 BOPD from fractured Pre-Tertiary basement quartzite. The first development well, Beruk NE-2 was drilled off-structure, encountered tight granite and was a dry hole. Two subsequent development wells (Beruk NE-3 & 4) were drilled into poor reservoirs consisting of argillite and hornsfelsic basement which tested at some BOPD. The last well in the field was the Beruk NE-5 which tested 2,252 barrels fluid per day (34% water cut). Rather than carrying out more production testing, the wells were tied-in and placed on production. Rapid water influx was experienced and production of only 2 million barrels was achieved. In hindsight it was realized that the basement lithologies were very complicated and variable and marginally productive in the case of the argillite and hornsfelsic basement reservoirs. Also, the geoscientists and engineers did not recognize the existence of multiple fracture patterns which can result in water bypassing the oil and leading to unexpected rapid water influx. The lesson learned from this field was that such fields require detailed geological and engineering work to accurately determine the commerciality of any basement oil discovery. Concluding Remarks The conventional way of thinking, certainly in the past, has been that the top of basement underlying the sediments is most likely tight and, in oil patch folk lore was generally viewed as tombstone. Even the sands near the top of basement were often described tongue-in-cheek as the suitcase sands since that signified that the well was near to total depth and likely a dry hole and so it was time for the geologist to pack up his suitcase and depart from the wellsite. However, the author believes that significant oil and gas fields in basement rocks remain to be discovered worldwide. Unconventional geological thinking and risktaking has led to many of the world s major oil and gas discoveries and such strategies will also reward the explorers searching for oil and gas in basement. Select References on Oil & Gas in Basement Reservoirs Special Publication Book published in 2003 Hydrocarbons in Crystalline Rocks, Geological Society, London. Koning, T. & Darmono, F.X., 1984, The Geology of the Beruk Northeast Field, Central Sumatra: Oil Production From Pre-Tertiary Basement Rocks, proceedings of the 13 th Annual Convention, Indonesia Petroleum Association, Jakarta. Koning, T. 2003, Oil and Gas Production From Basement Reservoirs Examples from Indonesia, USA and Venezuela, Geological Society Special Publication 214 on Hydrocarbons in Crystalline Rocks. 5

6 Koning, T. 2000, Oil Production From Basement Reservoirs Examples from Indonesia, USA and Venezuela, Proceedings of the 16 th World Petroleum Congress, Calgary. Landes, K.K. et al, 1960, Petroleum Resources in Basement Rocks, AAPG Bulletin, 44, pp