Three-Dimensional Architecture of Ancient Lower Delta-Plain Point Bars Using Ground-Penetrating Radar, Cretaceous Ferron Sandstone, Utah

Abstract Accurate three-dimensional description of reservoir architecture using outcrop analogs is hampered by limited exposure of essentially two-dimensional outcrops. This study contains the first fully three-dimensional description of ancient marine-influenced point bar sandstones of lower delta-plain distributary channels and is based on the integration of detailed outcrop and drill-hole data, and two-and three-dimensional ground-penetrating radar data. The studied outcrops are in the Cretaceous Ferron Sandstone of east-central Utah. Point bars deposited in marine-influenced, lower delta-plain channels show complex facies and geometries that resemble both fluvial point bars (upward-fining grain-size distribution and laterally stacked inclined bedsets), and tidally influenced point bars (extensive mud drapes on the inclined bedset surfaces and upstream migration of inclined bedsets). The bankfull width and mean bankfull depth were estimated at 225-150 m (738-492 ft) and at 3.9-5.2 m (12.8-17.1 ft), respectively. The heterogeneities in these point-bar deposits include mudstone drapes on the upper bounding surfaces of the inclined bedsets, and mudstone intraclast conglomerates lying on basal erosional scours of inclined bedsets. The spatial distribution of these heterogeneities is determined by direct mapping in outcrop in conjunction with modeling ground-penetrating radar amplitudes by geostatistical techniques. Mudstone layers are generally 5 m (16 ft) in length in the direction parallel to flow with a small percentage of mudstone layers 15 m (49 ft) in length, and 10 m (33 ft) perpendicular to flow, downdip along the inclined beds. The detailed distribution of heterogeneities inside reservoirs potentially affects flow behaviour.