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A Lithological Core Evaluation of the Calcarenite and Breccia Formations Within the Ku-Maloob-Zaap Fields, Offshore Campeche, Mexico
An integrated reservoir study was jointly carried out by CanPetro International and Petroleos Mexicanos (PEMEX) on the Ku-Maloob-Zaap oil fields, offshore Campeche, Mexico. The primary objective of the study was to characterize three (3) producing formations with respect to stratigraphy, structure, petrophysical characteristics and engineering properties. Though analysis of the core material had several objectives, the lithological and porosity description of two (2) units (Calcarenite, Breccia) are the focus of this discussion.
The main reservoir unit (Breccia – Upper Cretaceous) has a macroscopic texture which ranges from dolomitic rudstones (pack breccia) with clasts in contact to dolomitic floatstones (float breccia) with clasts floating in a finer-grained groundmass. Porosity is generally non-fabric selective as the pore geometry is independent of textural and depositional features. Fractures, vugs and solution-enlarged fractures are the main pore types within this unit. It is suggested that the breccia texture has resulted from the flow of detritus down the Yucatan (Carbonate) Platform margin. Further evidence also indicates that karst development may also have an influence on dolomite texture.
The second oil-bearing unit (Calcarenite – Middle Eocene) is a grainstone (limestone) which consists of uniform, medium to coarse skeletal and intraclast grains. Thin-section petrography shows that the main grainstone framework is comprised of micritic peloids, intraclasts, algae and benthonic foraminifera. Porosity is dominated by fabric selective interparticle porosity between allochemical constituents.
The lack of significant secondary cements (calcite spar) allows for excellent preservation of porosity. Reservoir quality is predominantly a function of depositional rather than diagenetic influences. Generally, those rocks which contain larger allochems and less micrite (i.e., higher depositional energy) have the highest reservoir quality. The allochem assemblage along with the overall texture indicates a shallow platform/shoal depositional setting.
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