| Description |
Core and outcrop exposures of the late Cretaceous John Henry Member (JHM) of the Straight Cliffs Formation, located in the Kaiparowits Plateau of southern Utah, offer an excellent opportunity to evaluate reservoir quality of paralic deposits. Paralic deposits are inherently complex due to multiple active depositional processes which impact the reservoir quality on inter- and intra-geobody (depositional element) scales. Facies and rock property data from core and outcrop of the JHM provides insight into the impact of depositional processes on reservoir quality. Five cores from the JHM were described in this study. In these cores, three facies associations: FA-1: shoreface, FA-2: channelized deposits, and FA-3: marine influenced back-barrier and coastal plain were delineated. Porosity, permeability, and thin section analysis from 87 1-inch core plugs are combined with interpretations of inferred facies associations and depositional processes. Outcrop data are used to supplement core data and confirm subsurface interpretations. The results of this study show that middle-upper shoreface and fluvial channel deposits represent higher quality reservoir sandstone within the JHM. Middle-upper shoreface deposits have the most favorable geometric characteristics with thicknesses up to 24.1 m and lateral extents reaching 10s of kilometers. Fluvial channel deposits reach lateral extents of 100s of m and can amalgamate to reach thicknesses greater than 45 m. Tidal channel deposits have a comparable lateral extent but are generally less than 2 m thick, at times amalgamating to exceed 13 m. Rock property analyses show that middle-upper shoreface and fluvial channel deposits have higher average porosity of ∼ 27 % and average permeability values of 411.8 and 367.5 mD, respectively. Tidal deposits have lower reservoir qualities of an average porosity below 20 % and average permeability of 70.69 mD. The lower reservoir quality of these deposits is attributed to slackwater periods during which mud laminae are deposited and the relatively lower energy regime associated with a marine influenced back-barrier environment. |
