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Basement reservoirs in South America Argentina Hydrocarbons are hosted in andesitic sills. Brazil Fractured volcanics (basalt) of the Badejo and Linguado fields (discovered in 1975) produce hydrocarbons (Nelson, 2001). These fields are located in the Campos Basin, offshore Brazil. Chile Lago Mercedes Field Lago Mercedes Well 1 was spudded on 17th January 1991. It was located to test a seismically defined structural culmination located along a blind thrust near the deep foreland axis of the western Magallanes basin. The fault is responsible for a trap geometry that is genetically related to, but fundamentally different from, the numerous un-rooted Tertiary folds in the area (Dean et al, 1993). Although the Lower Cretaceous Springhill Formation comprised the primary target, Dean et al anticipated that the geometry of the fold allowed for the possibility of several fractured intervals, including volcaniclastic rocks of the underlying Jurassic Tobifera basement sequence. The sequence was found to be productive elsewhere along the eastern platform of the basin (Dean et al, op cit). During the drilling of the well, promising gas and condensate shows were observed in several horizons. The most surprising of these shows later proved to be a Permo-Triassic grano-diorite underlying the Tobifera. All the hydrocarbon bearing intervals exhibited minimal matrix porosity but varying degrees of fracturing. Testing of Well 1 yielded combined flow rates in excess of 12 MMCFD of rich gas and 1,140 bbl/day of condensate. The most abundant zone corresponded to an intensely fractured and partially weathered interval. Additional testing was planned prior to any estimate of recoverable reserves. Mexico Many thousands of oil seepages in Mexico are associated with various forms of ferromagnesian igneous intrusions. Most of them occur in the Tampico embayment. The Furbero oil field produces from a laccolith of gabbro and from the overlying shales. The Fubero oil field is located in the canton of Papantla, State of Vera Cruz. A thick sill of gabbro has been intruded into Tertiary shales. The sill is folded and follows bedding. Metamorphism of the overlying shales has occured. Well No. 27 at 30 ft into the gabbro came in with an initial production of 893 barrels and had produced over 200,000 barrels at a rate of 1000 barrels a day. Similarly, Well No. 28 initially produced oil from 157 ft into the intrusion (DeGolyer, 1932). Venezuela Reports from Le Vela (June, 1999) say that Phillips have tested both the Cauderalito Limestone and the fractured basement in well LVC-29 at a depth of 2826 m. Three Drill Stem Tests were completed between 2363-2444 m producing a combined flow of 15.5 MM cf/day of gas and 83 b/day condensate. A second sidetrack was underway. No details of the flows from the basement were easily available.. La Paz Oil Field Discovered in 1922, the La Paz oil field lies approximately 40 km west of Maracaibo City. Oil was first produced from Guasare (Paleocene) sandstone, limestone and Eocene sandstone. Initial production averaged about 500 bbl/day. As a result of deeper drilling, higher production was obtained from Cretaceous limestone from which initial production averaged 5,000 bbl/day (Aguilera & van Poollen, 1979). The reservoir rock was more than 1,650 ft (500 m) thick, the oil mainly stored in secondary voids, cracks, and fissures (Smith, 1956; Landes et al, 1960).
Since oil was stored in Cretaceous limestone fissures, it was felt that if the underlying strongly folded and faulted basement rocks were similarly fissured, they might also contain oil. The first two wells purposely drilled into the basement were unsuccessful. The third well (Well P-86) was completed in April 1953 to a total depth of 8,889 ft (2,709 m) which was 1,089 ft (332 m) below the top of the basement complex. Since then, at least 12 wells have been drilled into the basement with an average penetration of 1,650 ft (503 m), the maximum being 3,087 ft (941m). Average initial production, purely from the basement, was 3,600 bbl/day but one well had an initial yield of 11,500 bbl/day (Landes et al, 1960).
La Paz produces from granite in the cores of anticlines and the total reserve has been calculated at 905 MM bbls (P'An, 1982).
Mara Oil Field Considered as the 'champion' of basement producers (Landes, 1959), the Mara field lies northeast of the La Paz field. The complex basement is composed of metamorphic and igneous rocks. The metamorphic rocks are in the main slightly metamorphosed sediments with hydrocarbon storage exclusively in fractures (Landes, 1959). Their age is uncertain, possibly Silurian-Devonian. The igneous rocks are granitic and are believed to be Hercynian in age. Cores of the basement show intense vertical fracturing and core recovery is often poor. By 1956, the Mara field was producing from 29 wells in the basement reservoir at an average depth of 1,190 ft (363 m). Initial production was about 2,700 bbl/day but one well produced 17,000 bbl/day from the basement (Landes et al, 1960).
Although the oil in both the Cretaceous and basement is practically identical, bottomhole pressure measurements in the Mara field indicate separate reservoirs. It is possible that the well cemented basal sandstone or a 90 ft thick (27 m) marl and shale horizon, which occurs about 76 m above the basement, may have acted as a partial seal.
In July 1955, the Mara and La Paz basement reservoirs together were producing 77,000 bbl/day. The cumulative production for both the La Paz and Mara field by the end of July 1955 was over 31 million barrels.
La Vela Offshore Field The La Vela Offshore Field is on the Northeast coast of the Paraguana Peninsula in Falcon State. A total of 28 wells were drilled between 1972 and 1994, proving reservoirs in fractured basement and overlying Tertiaries at depths up to 10,000 ft. Navigation Back to main textEurope North America Asia Africa CIS and Russia Middle East Oceania |