Predicting Porosity Distribution within Oolitic Tidal Bars
Larry J. Cavallo, Richard Smosna, 1997. "Predicting Porosity Distribution within Oolitic Tidal Bars", Reservoir Quality Prediction in Sandstones and Carbonates, J. A. Kupecz, J. Gluyas, S. Bloch
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The Mississippian Greenbrier Limestone is a major gas reservoir in the Appalachian basin, but its complex porosity patterns often deter active exploration. In southern West Virginia, the reservoir consists of oolitic tidal bars that are composites of smaller shoals. Porosity trends closely follow the ooidgrainstone facies that occupied shoal crests where coarse-grained, well-sorted ooid sand was generated with either unidirectional or bidirectional crossbeds. Nonporous packstone occurred in adjacent tidal channels, and a transitional grainstone/packstone facies of marginal porosity was situated along the flanks of the shoals. The key to drilling successful wells is in understanding the complex internal geometry of Greenbrier ooid shoals. A well penetrating the oolite with good porosity and bimodal cross-beds should be offset perpendicular to the dip directions; that is, parallel to the shoal axis. However, a well penetrating thin, porous limestone with one dominant crossbed azimuth should be offset opposite to that dip direction; that is, up the flank of the ooid shoal. Shaly interbeds characterize the edges of the shoals and mark the limit of productive wells. Schlum berger’s Formation MicroScanner log, which provides data on both lithology and cross-bedding, has proven to be a useful tool in predicting the distribution of oolite porosity.
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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.