ABSTRACT

Completely dolomitized Upper Devonian Leduc buildups at depths >4000 m have higher porosities and permeabilities than adjacent limestone buildups; dolostones are more resistant to pressure solution and tend to retain their porosity during burial. Distribution of pore types is controlled by depositional facies, whereas distribution of permeability is largely controlled by diagenetic processes, especially dolomitization.

In pool D3A of the Strachan reservoir, porosities and permeabilities are highest in the interior of the buildup where the strata are completely dolomitized. In the reef margin, porous and permeable dolomitized zones are interbedded with nonporous and nonpermeable limestone units. The presence of porous and permeable zones is closely related to the degree of dolomitization, with the greatest porosity and permeability occurring in completely dolomitized rocks.

The reservoir character in the Ricinus West buildup closely follows depositional units, despite complete dolomitization. At the reservoir scale, porosity and permeability have relatively similar values throughout the buildup. At the meter to tens of meters scale, the upper buildup interior is characterized by 1- to 2-m-thick, permeable and laterally continuous lagoonal strata. The lower reef interior consists of laterally discontinuous permeable zones. In the reef margin, permeability is controlled by fractures and interconnected vugs. At the millimeter scale, porosity and permeability are controlled by diagenetic processes.

Late cementation and dissolution processes have slightly decreased and increased porosity and permeability, mainly in the lower part of the reservoirs. Bitumen plugging decreased porosity and permeability in the upper part of the reservoirs. Although it is difficult to predict reservoir porosity and permeability trends, the secondary porosities in these deeply buried

1 Department de Exploracion, Maraven S.A., Apartado 829, Caracas 1010-A Venezuela.

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dolomites are mainly controlled by the primary porosity distribution and the depositional facies. The permeability is mainly controlled by diagenetic processes, especially dolomitization and various phases of cementation and bitumen plugging in the upper part of the reservoirs. Available data from the deep basin and the adjacent Rocky Mountains suggest that these porous dolomites are regionally extensive, and dolomite buildups elsewhere should have porosity and permeability variations similar to the Strachan and Ricinus West reservoirs. However, late-stage dolomite, anhydrite, and bitumen can locally partially to completely fill the pore spaces.