MAIN ELECTROFACIES IN CARBONATES OF THE HEAVY CRUDES NORTHERN BELT OF CUBA

摘要

The rocks of the Heavy Crude Northern Belt have similar geologic history, composition and reservoir properties. In their oil fields, five fundamental groups of electrofacies are distinguished. Three of them are related with carbonatic rocks, important for production, one is the regional seal, and in the other predominates the siliceous rocks. The electrofacies correspond with identified lithotypes and they correspond to a defined age. Their descriptions are based on the well logs which are accompanied by typical image logs for each one, which respond to patterns based on the studies. Further, the calcimetry profile was used as auxiliary way. The first group of electrofacies, basically in the wells located toward the south, is identified as a smaller porosity reservoir, less than 5 percent, with high values of resistivity and scarce volume of clay content. Its production is not appreciable. The second one is a carbonatic rock with good porosity, higher than 30 percent, very high resistivity and small values of clay volume. This one is developed toward the north, in inclined and horizontal wells. In its oil production, the fracturation degree plays a fundamental role in the total permeability, because of the existence of a high percent of micropores included in the total porosity. A third group of electrofacies, the most extended toward the north, has irregular values in all their properties, but in average can reach up to 20 percent porosity, for different resistivity and different degree of clay volumes. This one is related with the best productions in the area, with great influence of the fracture degree. There is a fourth group of electrofacies with similar characteristics to the previous one, but it exhibits very low content of carbonates, however, cherts are abundant. The seals are typical argillaceous or clayey rocks with low resistivity and porosity higher than 30 percent. These groups can be subdivided according to the variations that affect some of their properties.