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Reservoir description of low resistivity sandstones in the Mugrosa Formation (Oligocene) of Gala-Llanito Fields, Colombia, South America
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The Gala-Llanito oil fields are located in the Middle Magdalena Valley Basin in northern Colombia, South America. These fields are operated by Empresa Colombiana de Petroleos (ECOPETROL) which is the oil national company. They are in the primary production phase and ECOPETROL is planning to develop new blocks and implement some type of secondary or EOR process in the depleted blocks. The C and B zones of Mugrosa Formation are the productive horizons in these fields. Usually formations showing high resistivity log response produce oil. However, some of these reservoirs which present a low-resistivity response on well logs still produce water-free oil irrespective of the calculated water saturation value. Identification of the fluid saturations from well logs is very difficult A reservoir description study was performed on the Gala Block of these fields to solve the problem of low resistivity response in oil bearing sandstones of the C zone. Clay mineral and capillary pressure effects, and non-traditional concepts in the formation evaluation, such as depositional environment were of primary concern in this reservoir description study. The C zone reservoirs were characterized stratigraphically as layered reservoirs composed of both finning upward and coarsening upward sequences. In the upper C zone four subzones were identified and within these subzones seven flow units were defined. Multiple water/oil contacts (WOCS) controlled by the sealing character of the faults and by the existing blocks were identified for each flow unit. Within a specific block of the Gala Block, each flow unit exhibits an independent WOC which is caused by the variation in the reservoir quality and by capillary pressure effects. The solution of the problem of low-resistivity response was based on the integration of the geological and petrophysical reservoir characteristics. This integration showed that the resistivity response in these reservoirs is strongly controlled by the environment of deposition and the position of the reservoir with respect to the water/oil contact. Simandoux's equation combined with generated capillary pressure curves appears to be the best model to estimate water saturation in a prospective sandstone in C Zone of Mugrosa Formation in Gala Block.
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Includes bibliographical references.
Bernal Guerrero, Maria Cristina (1993). Reservoir description of low resistivity sandstones in the Mugrosa Formation (Oligocene) of Gala-Llanito Fields, Colombia, South America. Master's thesis, Texas A&M University. Available electronically from
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