From conventional outcrop datasets and digital outcrop models to flow simulation in the Pont de Montanyana point-bar deposits (Ypresian, Southern Pyrenees)

Abstract

Abstract The Pont de Montanyana outcrop exposes strata of the lower Eocene Montllobat Formation deposited in alluvial plains and in meandering river systems of the Tremp-Graus piggyback basin (Southern Pyrenees, Spain). This article focuses on the characterisation of the outcrop using conventional descriptions (photomosaics, correlated stratigraphic logs and palaeocurrent data) together with 3D interpretations of digital outcrop models. The architectural elements identified include lateral-accretion deposits, channel fills (meandering, chute and distal alluvial-fan types), overbank sheets (wings and crevasse splay deposits) and floodplain fines. The channelised deposits were collectively classified, using the width (W) and thickness (T) ratio, as ribbons (W/T ≤ 15) to narrow sheets (W/T ≤ 100). Variations in sandstone/mudstone ratio (between 20% and more than 90% of sandstone) and in the internal architecture of the meandering channel fills suggest deposition in the abandoned channel at different times of the cut-off process and at different locations in relation to the position of the active channel. A high-resolution 3D facies model of the upper stratigraphic interval was constructed, containing point-bar deposits and an associated channel fill. The model was subsequently populated with petrophysical data representative of point-bar reservoirs. Flow simulations based on a water-injection development strategy were carried out. The flow behaviour and the distribution of the bypassed oil are determined by the high degree of internal heterogeneity in the meandering channelised deposits. This heterogeneity is produced by the facies variability, the inclined bedding of the lateral-accretion deposits, the lithology of the channel fill, the downstream fining along the bend and by the change in the orientation of the lateral-accretion surfaces due to expansion and rotation of the point bar. The moveable oil was effectively drained from the sandstone-dominated channel-fill body. However, more than 27% of the moveable oil remained trapped in the uppermost part of the lateral-accretion deposits.