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Geometry and facies heterogeneities of an Upper Jurassic oolitic-siliciclastic shoal (Ricla, Spain): a 3D outcrop reconstruction at reservoir scale

Abstract

The Kimmeridgian (Upper Jurassic) outcrops of Ricla (Iberian Ranges, NE Spain) expose the shallow areas of a mixed siliciclastic-carbonate (i.e., oolitic) platform in different dip to strike-oriented sections. Extensive fieldwork on these outcrops has resulted in a precise facies reconstruction of an oolitic-siliciclastic sand-shoal complex, across a 4 × 1 km (downdip x strike) square area. Lithofacies and geometries were mapped using continuous photomosaics, including paleocurrent measurements and logging and sampling for detailed facies reconstruction. The studied sedimentary body is up to 22 m-thick in proximal (Northern) areas and phinches out basinward (to the South), down to 5 m-thick in distal localities. A number of facies types dominated by different cross-bedded structures have been mapped across the down-dip and strike sections. In the more proximal localities the succession consists of two vertically stacked shallowing-upward sequences. Planar cross-bedded units, with sets from 0.5 to 3 m-thick, form the larger volume of these two sequences. Southeast to southwest oriented paleocurrents measured on these planar cross-bedded units, indicates a dominant offshore migration of large-scale bedforms due to unidirectional storm-induced return currents. The upper (shallower) part of the sequences show more dispersed palaeocurrent measures, indicating a significant contribution of the inshore and alongshore currents in the origin of the cross-bedded units. The composition of the cross-bedded units (either dominated by sand-size quartz grains or by ooids) has been also mapped in detail. The observed facies architecture was the result of a rapid sand-shoal progradation, which has been related to the stage of stillstand of sea level observed across the Iberian basin at the onset of the Late Kimmeridgian (i.e., late regressive stage of the third-order Kim-1 Sequence). The studied outcrop has potential use as a reference analogue for sand-shoal hydrocarbon reservoirs such as the Upper Jurassic reservoirs found in the Jurassic Smackover and Arab formations.