Predicting Reservoir Properties in Dolomites: Upper Devonian Leduc Buildups, Deep Alberta Basin
Published:January 01, 1997
Eric W. Mountjoy, Xiomara M. Marquez, 1997. "Predicting Reservoir Properties in Dolomites: Upper Devonian Leduc Buildups, Deep Alberta Basin", Reservoir Quality Prediction in Sandstones and Carbonates, J. A. Kupecz, J. Gluyas, S. Bloch
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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 depo-sitional 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 reser-voirs. 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 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.
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Reservoir Quality Prediction in Sandstones and Carbonates
The accurate prediction of reservoir quality is, and will continue to be, a key challenge for hydrocarbon exploration and development. Prediction is a logical and critically important extension of the description and interpretation of geological processes. However, in spite of the profusion of publications on sandstone and carbonate diagenesis, relatively few articles illustrate the application of such studies to reservoir quality prediction. This Memoir represents the first attempt to compile worldwide case studies covering some predictive aspects of both siliciclastic and carbonate reservoir characteristics. We have attempted here to focus on the variability due to diagenetic effects in sandstones and carbonates, rather than on sedimentological effects, i.e., the presence or absence of a given reservoir. The chapters cover the spectrum of stages in the explorationexploitation cycle (Table 1).
The importance of reservoir quality in pay evaluation has been illustrated by Rose (1987), who analyzed an unnamed company's exploration results over a 1-year period. Of 87 wildcat wells drilled, 27 were discoveries (31 % success rate); incorrect predictions of the presence of adequate reservoir rocks were made in 40% of the dry holes. Importantly, the geologists believed that reservoir quality was the primary uncertainty in 79% of the unsuccessful wells. Similarly, a comparison of predrill predictions with postdrill results by Shell (Sluijk and Parker, 1984) indicated that reservoir quality was seriously overestimated, whereas hydrocarbon charge and retention predictions were more accurate. Although these statistics do not clearly separate drilling failure due to lack of potential reservoir from the lack of adequate reservoir quality, it seems that although explorers are aware of the significance of reservoir quality prediction, generation of predictive models continues to be a formidable task.