Stratigraphy and static connectivity of braided fluvial deposits of the lower Escanilla Formation, south central Pyrenees, Spain

Abstract

This study addressed the architecture and dimension distributions of fluvial deposits of the Olson Member of the Escanilla Formation through analysis of outcrops in the southern Buil syncline, Ainsa Basin, south central Pyrenees, Spain. The Olson Member consists of wide multistory channels alternating with mud-rich intervals composed of flood-plain deposits and isolated individual channels. Field descriptions and a combination of terrestrial light detection and ranging data and digital orthophotographs were used to map and document the abundance, distribution, and dimensions of sandstone bodies. These data show that channels are distributed according to the local stratigraphic framework and differential subsiding areas. Morphology (width, depth, and sinuosity) of the channels throughout the sequence evolve according to their stratigraphic position. During low-accommodation periods, channels stacked laterally, forming wide multistory channel belts. Under these conditions, single channels have a mean width-to-thickness (W:T) ratio of 49. During high-accommodation periods, single channels have a mean W:T ratio of 29. Moreover, the tendency of narrower and thicker channels having developed during high-accommodation periods is also clearly observable vertically from the base to the top of high-accommodation intervals throughout the sequence. This pattern of deposition results in increasing vertical connectivity, even in mud-rich high-accommodation periods. A geocellular three-dimensional model has been performed using object-based simulations. A succession of stochastic simulations was performed from the simplest one (random simulations) to more elaborated simulations, integrating successively outcrop observations. These simulations serve to image sandstone bodies' connectivity evolution according to outcrop observations and sedimentologic knowledge. Subsurface application of such vertical channel morphology evolutions and distribution could lead to more predictive flow-unit definitions and extensions.