Abstract
An investigation of the depositional architecture of the Curtis Formation and an evaluation of how the observed facies distribution affects overall reservoir quality is conducted. This tidally influenced marginal marine succession is well exposed in southeastern Utah (Humbug Flats, northern San Rafael Swell), and provides a high-resolution analogue to subsurface reservoirs. In order to determine the distribution of reservoir-grade sandstone bodies and characterize their heterogeneities, traditional sedimentological field methods were applied and LIDAR-scan data were collected. Mineralogical and reservoir quality parameters were evaluated from thin sections and were used to constrain the model. The Middle Jurassic (Callovian) Curtis and Summerville formations represent a complex set of vertical and lateral facies transitions from tidal shallow marine to supratidal sabkha deposits, respectively. The Curtis Formation developed in a transgressive to regressive, low-gradient epicontinental setting where sandstone body stacking patterns were determined by changes in accommodation and sedimentary supply. Determining reservoir quality in tidal sediments is extremely challenging due to inherently complex and heterogeneous sand distribution. Sub-seismic scale reservoir heterogeneities are the main focus in this study. Sedimentological characterization is used for classification of significant structures and layers, which form potential baffles to fluid flow within genetically related units. Excellent outcrop quality allowed lateral correlation between log sites, supplemented by conventional photographs and LIDAR imagery. The lower succession, interpreted as subtidal shelf deposits, contains channelized sandstone and conglomerate bodies in a heterolithic matrix. Even though the conglomerates have excellent reservoir quality, their connectivity is limited. A cleaning upwards trend towards the middle part corresponds with a transition to a subtidal to lowermost intertidal environment where wave and tide energy was at its highest, which in turn represents improved reservoir quality with laterally extensive, relatively homogeneous sandstones. The uppermost part displays a dirtying-upwards trend with increasingly heterolithic strata. In general, the reservoir quality of the Curtis Formation shows a general water depth-related trend: sand-to-mud ratio is low during the transgressive phase, becomes highest during the early highstand and decreases again during the regressive phase. Lateral connectivity is generally high, whereas siltstone interlayers provide frequent vertical flow baffles. Locally, mud-drapes and shifts in depositional transport directions cause variation in directional permeability. This depositional model may serve as an inter-well scale analogue with respect to reservoir property distributions in tidally deposited heterogeneous reservoir rocks.