Sedimentary facies and three-dimensional reconstructions of upper Oligocene meander belts from the Loranca Basin, Spain

In this article, we reconstruct and explore the three-dimensional (3-D) architecture of superimposed meander-belt sediments as an analog of subsurface reservoirs. Two exposures, named the Huete anticline and Garcinarro, were selected from the Loranca Basin, central Spain. In both of them, deposits are dominated by meandering river sediments of Oligocene age. Some point bars show fining-upward sequences of sedimentary structures and grain size typical of the helicoidal flow, but most of them present multiple vertical changes in grain size and are characterized by the occurrence of small-scale cross-stratification filling scours. The origin of these structures is related to oppositely rotating spiral vortices associated with flow separation long the convex bank of the meander bends. The 3-D architecture of superimposed meander-belt sediments allowed the reconstruction of meandering channel orientations and meander wavelength, calculation of the net/gross sandstone content, and the fraction of interconnected sandstone bodies. The 3-D architecture revealed that the interconnection areas are nearly absent when the orientations of the superimposed meander belts were perpendicular and that interconnection areas are more abundant and larger where a coincidence in the orientations of superimposed meander belts is observed. Horizontal projections of the highly interconnected areas allow calculation of the area fraction where one or more sandstone bodies could be intersected by vertical drilling. Differences in sandstone fraction between the two studied exposures were caused by the proximity to the Altomira Sierra thrust belt, which forms the western margin of the Loranca Basin.