Abstract

Tidal sandstone reservoirs contain significant intervals of hydrocarbon-bearing heterolithic facies, characterized by the presence of tide-generated sedimentary structures such as flaser, wavy, and lenticular bedding (millimeter to centimeter sand-mud alternations). We have characterized the reservoir properties (sandstone connectivity, effective permeability, and displacement efficiency) of these facies using three-dimensional (3-D) models reconstructed directly from large rock specimens. The models are significantly larger than a core plug, but smaller than a typical reservoir model grid block. We find that the key control on reservoir quality is the connectivity and continuity of the sandstone and mudstone layers. If the sandstone layers form a connected network, they are likely to be productive even at low values of net-to-gross (about 0.3–0.5). This may explain why the productivity of low net-to-gross, heterolithic tidal sandstones is commonly underestimated or overlooked. Connectivity is the dominant control on the transition between productive (pay) and nonproductive (nonpay) heterolithic facies. However, connectivity is difficult to characterize because core plugs sampled from the subsurface are too small to capture connectivity, whereas two-dimensional outcrop measurements can significantly underestimate the true 3-D value. Our results suggest that core-plug measurements of permeability and displacement efficiency are unlikely to yield representative values at the scale of a reservoir model grid block because the connectivity and continuity of sandstone and mudstone layers varies significantly with length scale.

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