ABSTRACT: Depositional Environment of the Caballos Formation, San Francisco Field, Upper Magdalena Valley, Colombia

John S. Sneider

Early Cretaceous Caballos sandstones in the San Francisco field, Upper Magdalena Valley, Colombia, contain approximately 500 million bbl of oil in place. Most of the oil is in the upper Caballos. The upper and lower Caballos average 85 and 45 ft of sandstone, respectively. The Caballos is at depths of 2080 to 3680 ft.

Distribution of reservoir and nonreservoir facies in the Caballos Formation is controlled by depositional environment and diagenesis. Understanding the facies distribution is crucial to the implementation of the proposed waterflood and for predicting reservoir distribution in wildcat areas.

The Caballos Formation was deposited during a world wide transgression and rests nonconformably on Jurassic volcanics. The lower Caballos is composed of braided stream deposits with individual reservoir sandstones up to 20 ft thick. The lower Caballos sediment was derived from quartzarenites of the Guayana shield and from local erosion of the basement. The middle Caballos consists of shale and sandy shale deposited in restricted to open-marine and bay environments. The upper Caballos was deposited in a fluviodeltaic environment, and individual sandstone units, which are separated by shale, have a lobate, elongate, or ovate shape. Sediments were derived from the Guayana shield and reworked lower Caballos sandstones.

The lower Caballos litharenite sandstones have a mean grain size of 0.63 mm (coarse grained); individual bed sets fine upward. Clean sandstones have porosities from 15 to 20% and permeabilities from 10 to 1000 md. The upper Caballos quartzarenites have a mean grain size of 0.62 mm (coarse grained). Individual bed sets of the delta bars coarsen upward; individual bed sets in the distributary channels fine upward. Clean sandstones have porosities from 12 to 26% and permeabilities from 70 to 6,000 md. Dissolution of volcanic rock fragments, feldspar, and calcite cement contribute significantly to sandstone porosity and permeability.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990