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Measuring the Ratio of Storm Deposited Gutter Casts in a Shallow Marine Environment of the Cretaceous Gallup Sandstone, Shiprock, New Mexico


Abstract: With current interest in unconventional reservoirs, knowledge of thin-bedded heterolithics has an increased importance in order to exploit bypassed pay in deltaic systems. Heterolithics deposited in shallow marine environments are influenced by storm deposits and these sandstone units can be characterized by either guttered or tabular beds, properties critical in determining the conductivity of potential reservoirs. This study documents the ratio of gutter casted to tabular sandstone beds within the heterolithics of the Gallup sandstone, located in Northwestern New Mexico; in order to evaluate lateral continuity 13 sections were measured at centimeter scale over 5 different locations, from proximal to distal, 5 high resolution photomosaics were used to correlate sandstone beds. Each measured section includes: lithology, grain size, sedimentary structures, bedding thickness, ichnofacies, and trend of the gutters where possible. Lateral variability of gutters changes with stratigraphic position as more proximal deposits have a greater degree of amalgamation making for more connected reservoirs, with a net to gross of 0.60 compared to 0.35 of the distal tabular facies. Results indicate that the depth to width ratio (1:5 – 1:10) of gutters decreases in more distal locations, however the deeper gutters tend to have lesser lateral extent, often pinching out or amalgamating with other beds within 15 m. In more distal locations depth to length ratio decreases with more tabular sandstones and rare gutters. However, these sandstones tend to correlate over the entire exposed panel of heterolithics, indicating the tabular sandstones have a high lateral continuity. Results show that though larger gutter casts are thicker, they have significantly lower lateral extent. The more distal tabular sandstones make for more continuous reservoirs, although loss of connectivity, through scours, results in lower vertical connectivity.