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From Conventional Outcrop Datasets to Flow Simulation: the Fluvial Strata in Pont de Montanyana (Ypresian, Southern Pyrenees)

Abstract

The Pont de Montanyana Outcrop is located some 400 m north of the Pont de Montanyana village (Huesca Province, Aragón). The exposed succession is part of the Montllobat Fm., in turn part of the infill of the Tremp-Graus piggy-back basin, on top of the Montsec thrust sheet, one of the main southwards displaced thrust sheets in southern Pyrenees. This outcrop, wellknown from several studies, beautifully exposes point-bar deposits and is one of the classical Pyrenean outcrops used by industry and academia on fieldtrips.

The map dimensions of Pont the Montanyana outcrop are about 160 m × 600 m. It has ideal characteristics to be used as a reservoir analogue, allowing for the 3D characterization of this type of deposits. The main goals of this work are: a) to illustrate the depositional and architectural characteristics of the outcropping point-bar deposits by use of digital outcrop models; b) to build a very detailed 3D facies model of this outcrop; and c) to flow simulate these models in order to understand the role of facies heterogeneity on hydrocarbon recovery. The base dataset integrates conventional outcrop data and digital outcrop models (LiDAR and Multiview 3D).

Two main channel-fill bodies with lateral accretion were distinguished, separated by a mudstone-predominant, overbank interval. Measurements of channel body dimensions report widths in the range of 100 to 200 m, and thicknesses of 5–7 m, which would be related to meandering fluvial systems with sinuosity of 2.62. The 3D facies model focused on capturing the facies heterogeneity in the uppermost channel-fill body. The modelling started by constructing six horizons with architectural meaning. This was followed by the creation of a 3D grid designed to reproduce the bedding characteristic of each channel evolution's stage. A combination of stochastic methods was used to create a facies model which was constrained to 5 field-measured stratigraphic logs and 126 pseudowells derived from the digital outcrop model, in order to accurately capture the actual heterogeneity. The obtained model reproduces a lateral change from coarse and very coarse-grained sandstones at the base of the accretional units to very fine-grained bioturbated sandstones developed at the bar platforms. Afterwards, the 3D grid was populated with petrophysical data from point-bar reservoirs available in the literature and constrained to the facies model. Moreover, several flow simulations, based on water injection strategies, were performed.

The results show that more than 20% of moveable oil is trapped at the upper part of the point-bar deposits, revealing the strong control of the facies distribution on the efficiency of this type of reservoirs. In addition, the results also point out that the location of the injection-production wells exerts an important impact on the recovery efficiencies.