Luca Colombera

A database approach for constraining stochastic simulations of the sedimentary heterogeneity of fluvial reservoirs

Quantitative databases storing analog data describing the geometry of sedimentologic features are commonly used to derive input for geostatistical simulations of reservoir sedimentary architecture; however, geometrical information alone is inadequate for the detailed characterization of sedimentary heterogeneity. A relational database storing fluvial architecture data has been developed and populated with literature- and field-derived data from modern rivers and ancient successions. The database scheme characterizes fluvial architecture at three different scales of observation—recording style of internal organization, geometries, and spatial relationships of genetic units—classifying data sets according to controlling factors (e.g., climate type) and context-descriptive characteristics (e.g., river pattern). The database can therefore be filtered on both architectural features and boundary conditions to yield outputs tailored on the system being modeled to generate input to object- and pixel-based stochastic simulations of reservoir architecture. When modeling heterogeneity with stochastic simulations, the choice of input parameters quantifying spatial variation is problematic because of the paucity of primary data and the partial characterization of supposed analogs. This database-driven approach permits the definition of various constraints referring to either genetic units (e.g., architectural elements) or material units (i.e., contiguous volumes of sediment characterized by the same value of a given categorical or discretized variable; e.g., same lithofacies type, clay and silt content, and others), which permit the realistic description of fluvial architecture heterogeneity. Applications of this database approach include the computation of relative dimensional parameters and the generation of auto- and cross-variograms and transition-probability matrices, which are necessary to effectively model spatial complexity.

A relational database for the digitization of fluvial architecture concepts and example applications

Depositional (facies) models of fluvial architecture permit straightforward categorization of deposits, but are necessarily simplistic. Here we describe a complementary database methodology which is designed to encapsulate the inherent complexity of fluvial systems and their preserved deposits. The database is implemented as a series of tables (characterizing qualitative and quantitative architectural and geomorphological properties and system attributes) populated with data derived from peer-reviewed studies of both modern rivers and ancient fluvial successions, and from other reliable sources. Architectural properties (geometries, internal organization, spatial distribution and reciprocal relationships of lithosomes) are assigned to three different orders of genetic bodies organized in a hierarchical framework, ultimately belonging to stratigraphic volumes that are homogeneous in terms of their controlling factors and internal parameters. Interrogation of the database generates a varied suite of quantitative information, whose principal applications include: (i) the quantitative comparison of fluvial architecture to evaluate the relative importance of intrinsic and extrinsic controls; (ii) development of quantitatively justified fluvial depositional models through the integration of data from multiple sources; (iii) development of better constraints on the workflows used to infer borehole correlations and to condition stochastic models of subsurface architecture; (iv) identification of appropriate modern and ancient analogues for hydrocarbon reservoirs.

A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: Implications for sequence stratigraphy

A quantitative comparison of 20 literature case studies of fluvial sedimentary successions tests common assumptions made in published models of alluvial architecture concerning (1) inverse proportionality between channel-deposit density and floodplain aggradation rates, and (2) resulting characteristics of channel-body geometries and connectedness. Our results do not support the relationships predicted by established stratigraphy models: the data suggest that channel-body density, geometry, and stacking pattern are not reliable diagnostic indicators of rates of accommodation creation. Hence, these architectural characteristics alone do not permit the definition of accommodation-based “systems tracts” and “settings”, and this calls into question current sequence stratigraphic practice in application to fluvial successions.

Models for guiding and ranking well-to-well correlations of channel bodies in fluvial reservoirs

A probabilistic method has been devised to assess the geologic realism of subsurface well-to-well correlations that entail the lateral tracing of geologic bodies across well arrays with constant spacing. Models of geo-body correlability (based on the ratio between correlatable and penetrated geo-bodies) are obtained from total probabilities of penetration and correlation, which are themselves dependent on the distribution of lateral extent of the geo-body type. Employing outcrop-analog data to constrain the width distribution of the geo-bodies, it is possible to generate a model that describes realistic well-to-well correlation patterns for given types of depositional systems. This type of correlability model can be applied for checking the quality of correlation-based subsurface interpretations by assessing their geologic realism as compared with one or more suitable outcrop analogs. The approach is illustrated by generating total-probability curves that refer to fluvial channel complexes and that are categorized on the basis of outcrop-analog classifications (e.g., braided system, system with 20% net-to-gross), employing information from a large fluvial geo-body database, Fluvial Architecture Knowledge Transfer System (FAKTS), which stores information relating to fluvial architecture. From these total-probability functions, values can be drawn to adapt the correlability models to any well-array spacing. The method has been specifically applied to rank three published alternative interpretations of a stratigraphic interval of the Travis Peak Formation (Texas), previously interpreted as a braided fluvial depositional system, in terms of realism of correlation patterns as compared to (1) all analogs recorded in FAKTS and considered suitable for large-scale architectural characterization, and (2) a subset of them including only systems interpreted as braided.

A 3D forward stratigraphic model of fluvial meander-bend evolution for prediction of point-bar lithofacies architecture

Although fundamental types of fluvial meander-bend transformations – expansion, translation, rotation, and combinations thereof – are widely recognised, the relationship between the migratory behaviour of a meander bend, and its resultant accumulated sedimentary architecture and lithofacies distribution remains relatively poorly understood. Three-dimensional data from both currently active fluvial systems and from ancient preserved successions known from outcrop and subsurface settings are limited. To tackle this problem, a 3D numerical forward stratigraphic model – the Point-Bar Sedimentary Architecture Numerical Deduction (PB-SAND) – has been devised as a tool for the reconstruction and prediction of the complex spatio-temporal migratory evolution of fluvial meanders, their generated bar forms and the associated lithofacies distributions that accumulate as heterogeneous fluvial successions. PB-SAND uses a dominantly geometric modelling approach supplemented by process-based and stochastic model components, and is constrained by quantified sedimentological data derived from modern point bars or ancient successions that represent suitable analogues. The model predicts the internal architecture and geometry of fluvial point-bar elements in three dimensions. The model is applied to predict the sedimentary lithofacies architecture of ancient preserved point-bar and counter-point-bar deposits of the middle Jurassic Scalby Formation (North Yorkshire, UK) to demonstrate the predictive capabilities of PB-SAND in modelling 3D architectures of different types of meander-bend transformations. PB-SAND serves as a practical tool with which to predict heterogeneity in subsurface hydrocarbon reservoirs and water aquifers.

A test of analog-based tools for quantitative prediction of large-scale fluvial architecture

Outcrop analogs are routinely used to constrain models of subsurface fluvial sedimentary architecture built through stochastic modeling or interwell sand-body correlations. Correlability models are analog-based quantitative templates for guiding the well-to-well correlation of sand bodies, whereas indicator variograms used as input to reservoir models can be parameterized from data collected from analogs, using existing empirical relationships. This study tests the value and limitations of adopting analog-informed correlability models and indicator variogram models and assesses the effect and significance of analog choice in subsurface workflows for characterizing fluvial reservoirs. A 3.2-km (2-mi)-long architectural panel based on a virtual outcrop from the Cretaceous Blackhawk Formation (Wasatch Plateau, Utah) has been used to test the methodologies. Vertical dummy wells have been constructed across the panel, and the intervening fluvial architecture has been predicted using correlability models and sequential indicator simulations. The correlability and indicator variogram models employed to predict the outcrop architecture have been compiled using information drawn from an architectural database. These models relate to (1) analogs that partially match with the Blackhawk Formation in terms of depositional setting and (2) empirical relationships relating statistics on depositional element geometries and spatial relations to net-to-gross ratio, based on data from multiple fluvial systems of a variety of forms. The forecasting methods are assessed by quantifying the mismatch between predicted architecture and outcrop observations in terms of the correlability of channel complexes and static connectivity of channel deposits. Results highlight the effectiveness of correlability models as a check for the geologic realism of correlation panels and the value of analog-informed indicator variograms as a valid alternative to variogram model parameterization through geostatistical analysis of well data. This work has application in the definition of best-practice use of analogs in subsurface workflows; it provides insight into the typical degree of realism of analog-based predictions of reservoir architecture, as well as the effect of analog choice, and draws attention to associated pitfalls.

Application of Quantitative Analysis of Fluvial Sedimentary Architecture to Improved Facies and Reservoir Modelling Workflows

This research aims to develop new workflows that enable the generation of model outputs with improved geological realism compared to outputs commonly obtained through conventional methods. The workflows are applicable to model reservoirs that comprise fluvial meander-belt deposits. Simulation techniques based on multi-point statistics (MPS) are used to integrate complex geological patterns and to honour both soft and hard data. A library of training images – from which MPS modelling algorithms replicate geological patterns – has been developed by using a forward stratigraphic modelling tool, PB-SAND (Yan et al. in revision). The training images are expressed as 3D stratigraphic models generated through mixed process-, geometric- and stochastic- based numerical modelling techniques that are themselves informed by quantitative information drawn from a database of geological analogues (the Fluvial Architecture Knowledge Transfer System, FAKTS) (Colombera et al. 2012, 2013). The application of training images is optimized to different MPS algorithms: SNESIM (Strebelle 2002); DEESSE (Mariethoz et al., 2010) and FILTERSIM (Wu et al., 2006). In this study, workflows for the application of a training image library to SNESIM, DEESSE and FILTERSIM modelling codes have been devised to simulate the sedimentary geology of channel-belt fluvial successions.

A Novel Approach for Directly Informing Facies Modelling Algorithms with High Quality Ancient and Modern Analogue Databases

Summary We have created a novel approach to interrogate and select relevant ancient and modern analogue data from world class relational databases and then directly inform facies modelling algorithms used in reservoir modelling. We present a case study where the direct application of parameterised analogue information has been combined with well data and a pre-existing structural model, to generate more informed hydrocarbon volume ranges and lateral connectivity estimates based upon more realistic geological architectures. The case study shown leverages the capabilities of a fluvial architectural database, however our objective it to extend this approach to cover other siliciclastic environments.

Metadata Approaches and Their Effects on Deep-Marine System Analysis, Analogue Selection and Reservoir Characterisation

A metadata approach is described that enhances the ability to characterise and model siliciclastic deep-marine hydrocarbon reservoirs. This is achieved through a relational database approach - the Deep-Marine Architectural Knowledge Store (DMAKS) – which entails a consistent method of data entry, ensuring data from ancient, modern and sub-surface investigations are stored to a common standard. The DMAKS stores information relating to the depositional systems’ setting and environmental controls, the architectural geometry and facies characteristics, along with hierarchical and spatial relationships. This deep-marine data repository thus enables quantitative meta-data analysis. The expansive range of data and parameters contained within the database improves the quality of predictive models, while the ability to produce statistical outputs enables deep-marine analogues to be more objectively selected. An initial investigation into a single case-study – the Golo system – helps to demonstrate the DMAKS research capabilities. Here, an original lobe-type architectural model is tested against a larger data-pool. The DMAKS quantitatively verifies the existence of distinct parent lobe-types (small, single-story versus larger, composite lobes associated with leveed-channels) through the analysis of geometrical, hierarchical and spatial relationships. It also provides new insights, as the hierarchical status of a lobe is seen to influence its resultant geometry.

Keynote Speech - Validation of Numerically Generated Stratigraphic Architecture

The preserved architectures of sedimentary successions serve as a valuable archive from which to determine factors that governed the form and evolutionary behaviour of the geomorphic landform systems that prevailed at the time of sediment accumulation, and their mechanisms of preservation. Deciphering the significance of this sedimentary archive represents an active research avenue that has potential for gaining a better understanding of the complex and interdependent relationship between extrinsic (allogenic) and intrinsic (autogenic) controls on sediment accumulation and long-term preservation. Forward numerical stratigraphic modelling represents a long-established and extensively used approach to discern and quantify the relative roles of allogenic controls, such as sea-level, baselevel, tectonic and climatic changes, from autogenic controls, driven by processes such as channel migration, avulsion and lobe switching. However, some fundamental problems remain to be resolved. Perhaps chief amongst these is how to effectively incorporate field-derived data – which are typically collected in a qualitative or semi-quantitative form – into numerical stratigraphic modelling workflows, such that model results can be conditioned and validated.