Three-dimensional characterisation of sedimentary heterogeneity and its impact on subsurface flow behaviour through the braided-to-meandering fluvial deposits of the Castissent Formation (late Ypresian, Tremp-Graus Basin, Spain)

Abstract

Fluvial deposits of the Castissent Formation (late Ypresian) form part of the Eocene infill of the Tremp-Graus piggyback basin. The Castissent Formation has been subdivided into three complexes (A, B, and C). This project focuses on the proximal fluvial-sandstones of Complex A in the Mas de Faro outcrop (NE Iberian Peninsula). There, three amalgamated channel belts are exposed in a 15-24m thick succession that passes upwards from sandy braided-river deposits (A1-A2) to coarse-grained meandering-river deposits (A3). Sedimentary heterogeneities within these deposits are characterised and their impact on oil recovery in a reservoir analogue are estimated using field descriptions, virtual outcrop interpretation, a 3D geocellular facies model and fluid-flow simulations. Petrophysical values of different facies types were derived from well data through analogous fluvial deposits of the Eiriksson Formation, in the Norwegian North Sea. Facies analysis show a waning succession of gravel channel lags, unit-bar and dune deposits in A1; a basal mud-clast channel lag, unit-bars and dune deposits in A2, and a coarse-grained point-bar succession in A3. Flow simulation shows: a) fingering of the waterfront related to the gravel channel lag and unit-bars of A1 and A2, and the gravel bars, scour-and-fill deposits and dunes of A3, which act as thief zones hindering efficient sweeping of the intervals immediately above; b) segregation of the injected fluids towards the base of the channel belts due to the fining-upwards successions; and c) vertical compartmentalization due to a relatively impermeable barrier at mud-clast channel lag beds. Permeability contrasts trapped 35% of the original oil in place after 0.5-0.6 pore volumes were injected.