Reservoir architecture and heterogeneity distribution in floodplain sandstones: Key features in outcrop, core and wireline logs

Abstract

Exploration and production from formations deposited in low-gradient fluvial systems is typically associated with a high degree of uncertainty; a reflection of the inherent characteristics of these environments, notably the dominance of non-reservoir floodplain fines, rapid lateral facies variations and associated heterogeneities at different scales. However, for a field development to be successful it becomes crucial to know the location, geometry, dimensions and connectivity of the most permeable facies, related to the main channel and the associated proximal overbank deposits (crevasse-splay complexes). With the aim of addressing this problem, a multidisciplinary study is presented, combining outcrop data, high-resolution sedimentological descriptions and advanced visualization techniques based on Digital Outcrop Models. This is compared with subsurface data from behind the outcrop (core, gamma ray and borehole image logs). The Mudstone-Sandstone Unit of the Triassic Red Beds of Iberian Meseta formation in southcentral Spain was selected for the present study. The unit is characterized by the lateral and vertical stacking of four architectural elements: (i) channelized sandstone bodies; (ii) asymmetrical sigmoidal-shaped sandstone bodies; (iii) lobe-shaped to sheet-like sandstone bodies; and (iv) sheet-like mudstones. These elements represent meandering channel, crevasse-channel-splay and floodplain sub-environments, comprising a distal, low-gradient meandering fluvial system. Together with well-documented outcrop and core facies, calibrated log responses are also presented for the channel bodies (bell-shape Gamma Ray profile, random azimuths and low to high dip angles), the crevasse-splay bodies (funnel-shape Gamma Ray profile, unidirectional azimuths and low dip angles) and the floodplain deposits (serrated Gamma Ray profile, unidirectional azimuths and very low dip angles). The full integration of outcrop and subsurface datasets has enabled generation of a robust conceptual model with predictive potential when establishing the three-dimensional stacking of facies, distribution of heterogeneities, and the connectivity between reservoir rock geobodies of primary (channel) and secondary (crevasse complex) interest in this type of fluvial reservoir.

Where humid and arid meet: Sedimentology of coastal siliciclastic successions deposited in apparently contrasting climates

“…During these flood events, ephemeral currents transported large dinosaur bone remains, as similarly reported in other ancient settings (González Riga & Astini, 2007; Vogt et al ., 2016; Coram & Radley, 2017). Repeated flooding episodes produced the progradation of the splay deposits, giving rise to the coarsening and thickening‐upward bodies, which are common in this type of deposit (Farrell, 1987; Bridge, 2006; Yeste et al ., 2020). Moreover, the sigmoidal cross‐strata observed in some bodies is interpreted as the result of progradation of splay lobes into standing water bodies ( cf .…”

“…Bioturbation observed as paired circular openings at the top of sandstone may correspond to U‐shaped or Y‐shaped burrows, which are common in marginal‐marine environments (Buatois & Mangano, 2011), as similarly occurs in the deposits of the fluvial–tidal transition in the Upper Cretaceous Tremp Formation, in the Pyrenees (Díez‐Canseco et al ., 2014, 2016), for example. Splay lobes underwent subaerial exposure, as indicated by the occurrence of edaphic features, as similarly reported in other ancient deposits (Yeste et al ., 2020), as well as by the local occurrence of dinosaur tracks at the top of sandstone. The occurrence of natural track casts at the base of sandstone indicates that dinosaurs passed across the flood plain, producing tracks in the underlying muddy sediment, as interpreted by Campos‐Soto et al .…”

“…Specifically, reddish siliciclastic mudstone displaying carbonate nodules, green mottling and root traces is interpreted as deposited on a flood plain (Miall, 1996; Selley, 2000; Viseras et al ., 2006) subject to subaerial exposure periods and palaeosol development (Freytet & Plaziat, 1982; Alonso‐Zarza & Wright, 2010; Soares et al ., 2020; Yeste et al ., 2020). Non‐channelized sandstone displaying parallel lamination at the base and current or, commonly, climbing ripple strata at the top is interpreted as splay lobe deposits (Burns et al ., 2017, 2019; Yeste et al ., 2020), which developed due to the spreading out of an unconfined flow as a result of the breaking of a levée of a fluvial channel during flood events (Coleman, 1969; Miall, 2010). During these flood events, ephemeral currents transported large dinosaur bone remains, as similarly reported in other ancient settings (González Riga & Astini, 2007; Vogt et al ., 2016; Coram & Radley, 2017).…”

Drainage and environmental evolution across the Permo–Triassic boundary in the south‐east Germanic Basin (north‐east Bavaria)

“…Common root prints and desiccation features are indicative of subaerial exposure and pedogenetic overprint of exposed floodplains (Walker, 1984; Miall, 2006; Galloway & Hobday, 2012). Thin ripple‐laminated sandstones interbedded with massive mudstones may represent crevasse splay and rare levée deposits (Gulliford et al ., 2017; Yeste et al ., 2020). Conglomeratic beds occasionally occurring at the bottom of the fining‐upward cycles may represent either remnant higher energy gravel bedforms or channel lag deposits (Miall, 2006).…”

“…Obvious basal scours overlain by thin gravel lenses, the highly variable cross‐bed dips (Fig. 6H) and pronounced fining‐upward trends point to lateral accretion of side/point‐bar elements of mixed load rivers with increased sinuosity (Miall, 1985; Yeste et al ., 2020). Fines form up to 30% of the sequence thicknesses, reflecting considerable suspended river‐load.…”