Capturing Reservoir Quality Heterogeneities at the Stage of Sedimentological Coding: an Innovative Depositional Environment Scheme
Authors: Galluccio Laura , Breislin Catherine , Tendil Anthony J.-B. 
Published: October 2022

The interpretation of depositional environments following sedimentological description is one of the most fundamental steps for the understanding of the vertical and lateral lithofacies groups’ distribution. This is directly linked to sequence stratigraphy and reservoir quality heterogeneity.

Despite its importance, many interpretative lithofacies association schemes fail to capture, in a clear, systematic and flexible way, the key environmental features affecting the depositional fabrics’ distribution and their associated reservoir quality. This is especially true in carbonate systems, where the reservoir quality evaluation is complicated by the deep interrelation between physical (i.e. hydrodynamism) and biological processes.

Our innovative depositional environment scheme (DE) comprises a variety of codes relating to key depositional and environmental characteristics in a way that captures their hierarchical organisation. These codes comprise groups of lithofacies interpreted to be genetically related, which provide metre-scale information commonly correlatable with wireline logs, that constitute ‘building blocks’ for static depositional architecture. The carbonate DE scheme is designed to be generic and flexible and therefore can be applied to any carbonate ramp, shelf or platform setting.

The DE nomenclature uses an ‘initials-type’, letter-based abbreviation coding, which provides a straightforward and accessible understanding of the interpreted depositional environments. This comprises a suite of codes organised from a large to a small(er) scale using the formula: ‘1.2’. The first uppercase abbreviation code (1) defines the environmental belt and the energy level (i.e. hydrodynamism) from the continental to the deep offshore realm. Hydrodynamism abbreviations are typically appropriate for inner/proximal settings where a wide range of energy levels may occur.The second lowercase abbreviation code (2) corresponds to qualifiers that are used to highlight subtle variations in environmental conditions favouring the development of specific faunal assemblages (e.g. Lithocodium/Bacinella or rudists) or sedimentological features (e.g. clay content, lamination). These qualifiers allow a direct link to reservoir quality variations at the stage of sedimentological coding.

Where specific geobodies can be interpreted within an environmental belt (e.g. an oolitic shoal in a dominantly high-energy inner/proximal setting) a code referring to the geobody can be added as a suffix in uppercase.

The hierarchical organisation of this scheme is proven to be of great value for reservoir quality analysis. This is because it allows a simple and direct visualisation of the gross environmental settings on a carbonate platform, alongside its hydrodynamism (i.e. high-energy oolitic grainstones vs low-energy argillaceous-rich mudstones). Furthermore, the qualifiers capturing argillaceous content, allochem composition and/or depositional features are extremely effective to record reservoir quality heterogeneity and enhance sweet-spot prediction.