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Abstract: Reservoir Potential of Dolomites in the Western Part of the Maracaibo Basin

Xiomara M. Marquez, Carmona Romulo

Recent exploration activities in the Western part of the Maracaibo basin, has increased interest in the hydrocarbon potential of dolomitized intervals within usually non-porous Cretaceous carbonate sequence. Sedimentological and geochemical data from this carbonate sequence reveals two major dolomitized intervals. The upper dolomitized zone occurs in the Middle Cretaceous Lisure Formation. The lower (and thicker) interval is to be found in the Lower Cretaceous Apon Formation, which is the subject of this study.

In the Apon Formation the rocks am partially (< 75% dolomite) to completely dolomitized. In partially dolomitized rocks, selective matrix replacement is represented by planar, unimodal, euhedral, medium crystalline dolomite rhombs. Porosity is low (<2%) and consists of small irregular vugs. Complete dolomitization (dolomites as such) is characterized by planar, unimodal euhedral to subhedral, medium crystalline dolomite mosaics. Dolomitization occurs during burial as it is related to pressure solution and late diagenetic processes. Polihedral intercrystalline pores, up to 500 micrometers, irregular vugs and minor moldic pores contribute towards an effective porosity value of up to 20%. Permeabilities range from 7 to 2000 md. Hydrocarbon saturation is about 75%. Those dolomites r nge in thickness from 10 to 100 ft and extend for more than 50 km. In some areas (e.g. La Villa Field) the dolomitized interval is the main contributor to production (1500-2200 bopd) of light gravity oil.

This study shows that delineation of the variability and distribution of dolomites will reduce the exploratory risk for this Cretaceous carbonate sequence. In addition, the recovery of oil from dolomites is improved by understanding the relationships between dolomite types and pore system (e.g. relationship to fractures).

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela