Measuring the Ratio of Storm-Deposited Gutter Casts, Upper Cretaceous Gallup Sandstone, New Mexico, USA

Abstract

Understanding unconventional petroleum reservoirs and HALO plays, areas outside existing production zones which are made up of thin-bedded or heterolithic intervals, has gained in importance in recent years with the continual demand for energy and the increased development of such prospects. Heterolithic units within deltaic successions host vast quantities of oil and gas that often go unexploited due to a lack of understanding of the sandstone shape, morphology and continuity within such reservoirs. The purpose of this work is to examine the thin-bedded units within the Late Cretaceous prodelta successions of the Gallup Sandstone in the Shiprock area of New Mexico in order to quantify the fundamental processes responsible for the deposition of these sands and to perform a correlation for such heterolithic deposits. Rock Ridge and Sanostee show heterolithic deposits, within parasequences 5a and 7a of the Gallup sandstone, and were analyzed, using measured sections and photomosaics, near the Shiprock area in order to compare the heterolithics in different stratigraphic settings. Five measured sections were collected at the centimeter scale in order to observe every facies change at a high resolution. Facies associations, such as tempestites, turbidites, and hyperpycnites, were used to identify dominant processes of deposition. Corresponding net-to-gross calculations yielded the sandstone percentage of each section with the result that storm-dominated beds present at Sanostee yielded the highest percentage of sand content. Two high resolution gigapan photomosaics of each heterolithic exposure were collected in order to perform a correlation of all the sandstones observed within the measured sections, across the entire exposure. Results show that dominant process of deposition varies along strike. Towards Sanostee, in the south, storm-waves are dominant while farther north rivers exert greater influence. However, the presence of large scale storm-deposited gutter casts at Sanostee has significant implications for lateral and vertical continuity of sandstones in comparison to the more tabular sandstone facies found at Rock Ridge. These guttered facies have a major impact on net-to-gross and vertical conductivity due to their high degree of amalgamation and therefore may signify important areas for prospecting in such heterolithic reservoirs. Furthermore, the presence of large scale gutters beneath a sharp-based shoreface is indicative of a Falling Stage Systems Tract, leading to the conclusion that Parasequence 7a-6d were affected by forced regression.