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Morantes, Julymar M.1 
(1) PDVSA-Intevep, Los Teques, Venezuela

ABSTRACT: Mechanisms of Porosity Reduction of the Upper Cretaceous Sandstones in the Eastern Venezuela Basin: Carito Oil Field

Cores and samples of sandstones from four third-order sequences were studied in order to reconstruct: (1) the diagenetic evolution, including porosity evolution of the sandstones and, (2) quantify the relative importance of porosity loss by compaction versus cementation by quartz. Sandstones were deposited mainly in deltaic environments with variable tidal influences, although continental and shallow-marine depositional environments are also represented. 
Point counts of 92 thin sections were made to obtain petrographic data to aid in reconstructing the diagenetic history. Sandstone composition ranges from Q64F1.6R34.4 to Q100R0F0 (average Q90R3F7), where sublitharenites and quartzarenites are the most abundant rock types. The most important diagenetic processes were mechanical compaction, quartz cementation, partial or complete dissolution of feldspar grains and felsitic volcanic rock fragments, and kaolinite precipitation. Average values of porosity loss by compaction (COPL) range from 32.5% to 36.5% are larger than values of porosity loss by cementation (CEPL), which ranges from 5.9% to 11.1%. Petrographic evidence and COPL and CEPL values indicate that porosity loss is chiefly by grain rearrangement; however some porosity was also lost by brittle fracture and pressure solution. 
The best reservoir quality is present in medium- to coarse-grained sandstones from sequences K3, K1, and T1 at burial depths from 4,200 to 4,800 km and coarser. Sandstones buried more than 4,800 km have total porosity values lower than 4%, the lowest IGV values, and the highest amount of cement and replaced grains.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.