Reservoir Heterogeneity and Hydrocarbon Production in Mixed Dolomitic-Clastic Sequence: Escandalosa Formation, Barinas-Apure Basin, Southwestern Venezuela

Nicolas Escalona, Juan Abud

Widespread dedolomitization and differential leaching occur in the Turonian "O" Member of the Escandalosa Formation, Barinas-Apure basin. Within this dolostone-dominated succession, calcite was developed through a dedolomitization process occurring in deeply buried dolomitized lime sediments previously deposited on a carbonate platform as well as dedolomitization on the associated glauconitic-quartzose sandstones of small-scale channels that scoured the platform. The dolomitized intervals have a strata-bound nature, and their original fabric is totally obliterated.

The dolomitization process generated a sucrose-textured mosaic of saddle dolomite. Initial dolomite was of the scattered type, but progressive replacement of the host produced a mosaic dolostone with both idiotopic and xenotopic textures. A general increase occurred in the iron and manganese content, and goethite was exsolved from the curved rhombs of saddle dolomite. Calcite usually postdates dolomitization, except in the highly fossiliferous packstones; calcite veins develop in both dolostones and limestones. Leaching is restricted essentially to glauconitic sandstones where calcite and some clay have been leached. This has produced very low intercrystalline porosity within the dolostones and partially dissolved, corroded, and floating grains with oversized pores in the sandstones. hese sandy intervals exhibit maximum potential for hydrocarbon storage, due to contrasting diagenetic influence associated with reservoir heterogeneity.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.