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 inter-well sandbody 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 impact and significance of analog choice in subsurface workflows for characterizing fluvial reservoirs. A 3.2 km long architectural panel based on a Virtual Outcrop from the Cretaceous Blackhawk Formation (Wasatch Plateau, Utah, USA) 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: (i) analogs that partially match with the Blackhawk Formation in terms of depositional setting, and (ii) 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 on the impact of analog choice, and draws attention to associated pitfalls

Geological Modeling of Outcrop Successions to Assess Analog - Based Predictions of the Sedimentary Heterogeneity in Fluvial Reservoirs

In the generation of static reservoir models, it is common to apply quantitative information derived from outcrop analogs, especially in techniques for well correlation and stochastic modeling. Sand-body well correlations can be guided by reference to ‘correlability’ models, which quantify the likelihood of correlation of sand-bodies across well arrays, based on analog sand-body width distributions. Pixel-based geostatistical simulations of reservoir architecture can be conditioned by indicator variograms that are parameterized using empirical relationships based on geologic properties, whereas objectbased reservoir models are commonly constrained using analog data on the geometry of sedimentary units. Through application of such geostatistical techniques, analog information also enables the construction of training images for conditioning reservoir models based on multi-point statistics. This study applies these techniques to model large-scale fluvial architecture of extensively exposed outcrops of various successions, as constrained by data from outcrops of analog successions. A typical subsurface workflow has been replicated, the aim being to test the value and limitations of the methods mentioned above, and to assess the impact of analog choice in workflows involving their use. Vertical ‘dummy’ wells (minimum spacing = 50 m) were placed across outcrop architectural panels representing km-wide exposed sections; the intervening architecture was predicted by correlability models and geostatistical simulations, constrained on outcrop-analog data drawn from an architectural database (FAKTS). The relevant FAKTS output was filtered to obtain composite analogs that match with the outcrop successions being modeled in terms of key system parameters (e.g. river discharge regime) and architectural properties (e.g. net-to-gross ratio). The value of the predictive methods was assessed by quantifying the degree of match between forecasted and known outcrop architecture, in terms of channel-complex correlability and static connectivity. Comparisons of correlability and geostatistical models vs. outcrop observations highlight the effectiveness of the different methods and demonstrate the influence of well density on the confidence assigned to predictions. Results support modeling approaches based on the use of a varied range of analogs as a way to consider uncertainty in analog choice, and on alternative modeling methods to account for potential algorithm-related pitfalls

Experimental distributive fluvial systems: Bridging the gap between river and rock record

A debate has called into question as to which fluvial channel patterns are most widely represented in the stratigraphic record, with some advocating that distributive fluvial systems (DFS) predominate and others that a broad diversity of fluvial styles may become preserved. Critical to both sides is the adequate recognition of original channel planform from geological outcrops separated from their formative processes by millions or even billions of years. In this study the river and rock record are linked through experimentally created DFSs with both aggrading channel beds and floodplains. This approach allows depositing processes and deposited strata to be studied in tandem. Proximal areas comprise coarse, amalgamated channel-fills with scarce fine-grained floodplain material. The overall spread of sandbody dimensions become far more varied in medial stretches, with an overall reduction in mean width and depth. In these areas channel-fills may be sand-rich or mud-rich and, following avulsion, all channels are covered by floodplain sediment. Channels, levees and splays form discrete depositional bodies each with varying aspect ratios; a novel breadth of deposits and morphologies in aggrading experiments largely concurrent with proposed trends indicative of DFSs. The proportion of floodplain material increases distally, resulting in decreased interconnectedness of distal channel-fills. Muddy floodplain sediments significantly change DFSs behaviour and subsequent stratigraphic architecture by enhancing bank stability and reducing avulsion through the filling of floodbasins. The laboratory methods utilised here open up the possibility of controlled experimentation on the effects and mechanisms of DFSs sedimentation, which is important since the modelled stratigraphic trends are rarely so tractable in ancient geological outcrop belts