Secondary oil recovery by gas and water drive is taking place in Maraven's Block V concession in Lake Maracaibo, where the Eocene sandstone reservoirs have produced 531 million bbl of oil. Geologic core studies have been undertaken to explain and quantify reservoir inhomogeneities which affect the injection of fluid and control its distribution. Core interpretations were also used to calibrate subsurface logs. The upper "C" sands were deposited in a river-dominated delta-front environment, and two cores from this unit have been divided into three principal lithofacies, which are further divisible into several subfacies based on grain size, sandstone:shale ratio, and sedimentary structures.

The large variations that occur in reservoir quality in

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the sandstones can be directly correlated with the facies distribution, indicating a strong depositional control on porosity and especially permeability. Diagenetic effects in the sandstones are minimal. The sandstone permeabilities were calibrated against various well-log responses, and it was found that a cross-plot of gamma ray and induction-log values can be used to identify the lithofacies and, to a certain extent, the subfacies in the oil-bearing sequence. Thus, in uncored wells in this reservoir, a cross-plot of these logs can be used to assess the depositional paleoenvironments, and an estimate can be made of the permeability distribution to predict fluid-migration paths during secondary recovery.

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