Depositional Controls Over Porosity Development in Lithic Sandstones of the Appalachian Basin: Reducing Exploration Risk
Richard Smosna, Kathy R. Bruner, 1997. "Depositional Controls Over Porosity Development in Lithic Sandstones of the Appalachian Basin: Reducing Exploration Risk", Reservoir Quality Prediction in Sandstones and Carbonates, J. A. Kupecz, J. Gluyas, S. Bloch
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Litharenites and sublitharenites of the Devonian Lock Haven Formation contain abundant rock fragments of shale and phyllite. These labile grains suffered varying degrees of destruction in several depositional environments; hence, sedimentary processes largely controlled the sandstones’ mineral composition. Fluvial sandstones have a high lithic content, distributary mouth-bar and offshore-shelf sandstones have an intermediate content, and barrier-island sandstones have a low content.
Primary porosity relates inversely to compaction of the lithic grains, decreasing from a maximum minus-cement porosity of φmc = 33% down to zero as lithics increase. The majority of primary porosity, however, has been occluded by cementation. Secondary porosity, created chiefly by dissolution of the chemically unstable rock fragments, is greatest (φrf = 13%) for sand-stones of a moderate lithic content.
Because of these relationships among depositional processes, lithology, and porosity, we predict that sandstones of different sedimentary environments should exhibit distinct porosity volumes and vary in their reservoir potential. Mouth-bar sandstones will have good total porosity, good secondary porosity, and offer the best reservoir quality. Shelf sandstones will have fair total porosity, most of which is secondary, whereas beach sand-stones will have low total porosity, most of which is primary. Fluvial sandstones will be the poorest reservoirs.
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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.