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Basement reservoirs in Africa Algeria Saharan plateau basaltic lavas are reported to contain hydrocarbons Egypt The basement oil fields of Egypt are located approximately 196 km southeast of Suez on the west shore of the Gulf of Suez. Oil is produced mainly from the Hurghada and Gemsah fields. The Gemsah oil field is located on a granite buried hill. Oil is found in a coral reef that lies above the granite ridge and is probably of Miocene age. However, it is not a basement reservoir and has not been included in this compilation. The oilfields at the entrance to the Gulf have been described by Salah & Alsharhan (1998). Basement fracture analogues for the offshore oilfields have been described by Younes et al (1998). Hurghada Field The Hurghada field lies southeast of the Gemsah field and close to the shore of the Gulf of Suez. It is a shallow, granite buried hill. Wells drilled through Miocene and Cretaceous strata penetrated a granite core at depths of approximately 1,670 ft to 2,000 ft (510 m to 610 m). Oil was found in the Cretaceous sandy shale, Nubian sandstone and in the weathered surface of the granite (P'An, 1982). The Miocene strata are unconformable with the Cretaceous beds and are less highly folded. The Hurghada field produces heavy oil (no production rates available). Zeit Bay Field The Zeit Bay field is a northwest-southeast trending structure which measures approximately 2.5 km by 4.5 km located in the southwest corner of the Gulf of Suez (Khalil & Pigaht, 1991). The field was discovered in 1981 when well QQ 89-1 found gas. In October of the same year, the appraisal well, QQ 89-2, intersected a 830 ft thick oil leg 2 km south of well QQ 89-1. According to Zahran & Askary (1988), the Zeit Bay field fractured basement contains nearly one-third of the total oil in place for the field and the flow rates per well varied from 700 bbl/day to 10,000 bbl/day. Due to its well established production potential, 60% of the field's development wells were drilled down to the basement (Zahran & Askary, 1988). The Zeit Bay basement consists of granitic rocks of pegmatitic to coarse porphyritic texture. The reservoir in Zeit Bay is an hydraulically communicating sequence of PreCambrian igneous and metamorphic rocks as well as sedimentary reservoirs (Khalil et al, 1993; El Hamalawy et al, 1993). The 830 ft thick oil column covers the total reservoir sequence. The field commenced production in 1984 reaching approximately 80,000 bbl/day (Salah Alsharhan, 1998) and by 1991 it was estimated that 65% of the recoverable oil had been produced (Khalil & Pigaht, 1991). A significant proportion of early production was from basement wells with individual flow rates of up to 10,000 bbl/day being recorded. Pressure maintenance by gas injection was implemented in 1987. Out of 36 production wells drilled in the Zeit Bay field, 24 wells penetrated the fractured basement, 14 of these were completed as openhole basement producers. The Zeit Bay field basement can be classified into five rock types:
Libya Nafoora-Augila Field PreCambrian granite is the host for one of the primary oil producing reservoirs in the Nafoora-Augila field which is one of the main giant fields in the Sirte Basin (Belgasem, 1991). The Nafoora-Augila field is located in the northeast of Libya (southeast of the Amal field) and is at the top of the Rakb High. The Nafoora-Augila area was originally a concession of Oasis Oil Co., the owner of the Amal field. The company drilled two wells near the top of the High but then abandoned the concession as the wells proved to be dry. The concession was then obtained by Occidental Petroleum (UK) Ltd in 1966. The first successful well of Occidental was Dl, which had an initial production of 14,800 bbl/day. Production came from porous fossiliferous limestone perforated throughout the interval from 8,530 ft to 8,563 ft (2,600 m to 2,610 m). This reservoir rock was lower Rakb carbonate and was not a basement reservoir (P'An, 1982). The fractured and weathered basement (a late PreCambrian or early Paleozoic granite) is one of three producing horizons. Some of the oil wells started production from the basement reservoir only while others produced from the basement and/or sedimentary reservoirs. The basement rocks are gradations of granophyre, granophyric granite, granite and rhyolite (Belgasem et al, 1990). The basement reservoir contains a large accumulation of oil in fractures and weathered zones. However, due to the heterogeneous nature of the reservoir, the porosity distribution is not well known. The first basement reservoir encountered was Well D2 drilled on possibly the highest point of the High. Well D2 produced at a rate of 7,627 bbl/day from devitrified rhyolite and highly weathered and fractured granophyre. Well D9 also produced from the basement only with an initial production of 1,500 bbl/day. The reservoir rock was weathered granite. Wells D3, D4, D5 and D6 were all step out tests that became oil wells, the most productive being D5 which produced at a stabilised rate of 14,140 bbl/day from two perforated intervals consisting of 59 ft (18 m) of carbonate rock and 39 ft (12 m) of granite. Well D6 produced from the basement reservoir with an initial flow of 1,200 bbl/day. D8 was an openhole basement completion, testing at 18,000 bbl/day from basement rocks and 36 ft (11 m) of perforated carbonate rocks (P'An, 1982). Morocco Landes et al (1960) state that there are at least eight oil fields in northwestern Morocco which produce from fractures in basement rocks. Lardenois et al (1956) refer to nine named fields in basement. In a couple of the fields, the reservoir rock consists of fractured, pink PreCambrian granite. In seven fields, the reservoir is a fractured chloritic quartzite and shale or slate of Palaeozoic age which is also basement rock. The basement rock accumulations are in upfaulted blocks that are marginal to a Mesozoic sedimentary basin. It is assumed that the fractures in the PreCambrian granite and in the Palaeozoic meta-sediments were filled with oil by lateral migration from the adjacent deeper Mesozoic sediments. Although the source of oil in the basement rock is probably the sediments in the adjacent Mesozoic basin, more oil has been produced from the basement rocks than from the sediments. By the end of 1957, the basement reservoirs had produced more than 3.75 million barrels of oil since discovery in 1947 (P'An, 1982). Navigation Back to main textEurope North America South America Asia CIS and Russia Middle East Oceania |