Three-dimensional stratigraphic complexity within mixed Eolian-Fluvial successions: implications for reservoir connectivity

On-going exploration of conventional hydrocarbon plays is increasingly focused towards the development of geologically complex reservoirs for which stratigraphic heterogeneity is difficult to predict. Many such current reservoirs, and an increasing proportion of likely future ones, are characterized by sedimentary bodies that accumulated as mixed eolian-fluvial systems that competed and interacted synchronously. Well-known reservoir examples include the Permian Unayzah Formation of Saudi Arabia, the Permian Rotliegend Group of the North Sea, the Triassic Ormskirk Sandstone of the East Irish Sea, the Jurassic Norphlet Sandstone of the Gulf of Mexico, and the Cretaceous Agrio Formation, Argentina. These mixed depositional systems typically exhibit highly variable lateral and vertical facies configurations that preserve complex juxtapositions of architectural elements composed of stratal units with markedly variable reservoir properties. Such stratigraphic partitioning is intrinsically difficult to predict from limited subsurface data. As such, there exists a requirement for more sophisticated geological models to better account for reservoir architecture and connectivity. This work uses outcropping case-study examples of eolian-fluvial interactions from the Triassic Sherwood Sandstone Group of the UK and the Permo-Pennsylvanian Cutler Group of southeast Utah, USA, to develop a suite of predictive models that depict common styles of stratigraphic complexity within eolian-fluvial systems. Studied successions accumulated in response to a variety of system interactions, deposits of which are preserved at a range of spatial scales from 100–104 m: (i) short-lived and localized fluvial reworking of eolian dune deposits in response to flash flood events; (ii) eolian reworking of fluvial deposits via winnowing; (iii) the fluvial exploitation and possible damming of open interdune corridors; (iv) the flooding of isolated (spatially enclosed) interdune hollows in response to an elevated water table. Identified types of interactions are characterized within a spatial scheme whereby occurrences can be used as a predictor of relative position within the larger-scale zone of transition between coeval eolian dune-field and fluvial systems. Application of this spatial scheme allows for prediction of the type of eolian-fluvial interactions expected for a range of paleogeographic settings, thereby serving as a tool for ranking exploration targets within larger prospect areas

Anatomy and facies distribution of terminal lobes in ephemeral fluvial successions: Jurassic Tordillo Formation, Neuquén Basin, Argentina

In terminal fluvial‐fan systems, characteristic proximal to distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation. This work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal‐fan systems, which record transitions from deposits of channel elements to lobe‐like and sheet‐like elements. This is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well‐exposed Kimmeridgian Tordillo Formation (Neuquen Basin, Argentina). The fine sand‐prone and silt‐prone succession associated with the medial to distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures. Facies and architectural‐element analyses, and quantification of resulting sedimentological data at multiple scales, have been undertaken to characterize sedimentary facies, facies transitions, bed types, architectural elements and larger‐scale architectural styles. Eight bed types with distinct internal facies transitions are defined and interpreted in terms of different types of flood events. Channelized and non‐channelized architectural elements are defined based on their constituent bed types and their external geometry. The most common elements are terminal lobes, which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner; a hierarchical arrangement of internal components is recognized. Proximal feeder‐channel avulsion events likely controlled the evolution of terminal‐lobe elements and their spatiotemporal shifts. Stratigraphic relations between architectural elements record system‐wide trends, whereby a proximal sector dominated by channel elements passes downstream via a gradational transition to a medial sector dominated by sandy terminal‐lobe elements, which in turn passes further downstream to a distal sector dominated by silty terminal lobe‐margin and fringing deposits. This work enhances current understanding of the stratigraphic record of terminal fluvial systems at multiple scales, and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions.Centro de Investigaciones Geológica

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

Meandering rivers in modern desert basins: Implications for channel planform controls and prevegetation rivers

The influence of biotic processes in controlling the development of meandering channels in fluvial systems is controversial. The majority of the depositional history of the Earth's continents was devoid of significant biogeomorphic interactions, particularly those between vegetation and sedimentation processes. The prevailing perspective has been that prevegetation meandering channels rarely developed and that rivers with braided planforms dominated. However, recently acquired data demonstrate that meandering channel planforms are more widely preserved in prevegetation fluvial successions than previously thought. Understanding the role of prevailing fluvial dynamics in non- and poorly vegetated environments must rely on actualistic models derived from presently active rivers developed in sedimentary basins subject to desert-climate settings, the sparsest vegetated regions experiencing active sedimentation on Earth. These systems have fluvial depositional settings that most closely resemble those present in prevegetation (and extra-terrestrial) environments. Here, we present an analysis based on satellite imagery which reveals that rivers with meandering channel planforms are common in modern sedimentary basins in desert settings. Morphometric analysis of meandering fluvial channel behaviour, where vegetation is absent or highly restricted, shows that modern sparsely and non-vegetated meandering rivers occur across a range of slope gradients and basin settings, and possess a broad range of channel and meander-belt dimensions. The importance of meandering rivers in modern desert settings suggests that their abundance is likely underestimated in the prevegetation rock record, and models for recognition of their deposits need to be improved

Timing of uplift and evolution of the Lüliang Mountains, North China Craton

This study analyses evidence for reformed basin development and basin-mountain coupling associated with development of the Ordos Basin and the Lüliang Mountains, China. Gaining an improved understanding of the timing and nature of uplift and evolution of the Lüliang Mountains is important for the reconstruction of the eastern sedimentary boundary of the Ordos Basin (a major petroliferous basin) as well as for providing insight into the evolution and breakup of the North China Craton (NCC). Based on systematic sampling for fission track analysis, it is suggested that the main phase of uplift of the Lüliang Mountains occurred since later part of the Early Cretaceous. Three evolutionary stages of uplift and development are identified: slow initial uplift (120–65 Ma), accelerated uplift (65–23 Ma), and intensive uplift (23 Ma to present), with the majority of the uplift activity having occurred during the Cenozoic. The history of uplift is non-equilibrium and exhibits complexity in temporal and spatial aspects. The middle and northern parts of the Lüliang Mountains were uplifted earlier than the southern part. The most intensive episode of uplift activity commenced in the Miocene and was associated with a genetic coupling relationship with the eastern neighboring Cenozoic Shanxi Grabens. The uplifting and evolutionary processes of the Lüliang Mountains area since later part of the Early Cretaceous share a unified regional geodynamic setting, which was accompanied by uplift of the Mesozoic Ordos Basin and development of the neighboring Cenozoic Shanxi Grabens. Collectively, this regional orogenic activity is related principally to the far-field effects of both the compression sourced from the southwestern Tibet Plateau and westward subduction of the Pacific Plate in Cenozoic

Data summary of 34 meander-belt cases simulated by PB-SAND model

Data summary of 34 meander-belt cases simulated by PB-SAND model, including preservation ratio for different architectural hierarchies (accretion package pairs, accretion stages and meander belts), their accumulation time, accretion rate, and platform characteristics (apex rotation, channel sinuosity, and migration angle)

A classification scheme for sedimentary architectures arising from aeolian-fluvial system interactions: Permian examples from southeast Utah, USA

International audienceThe preservation of the sedimentary deposits of arid environments is determined by both geomorphic and geologic processes. Sedimentary evidence of aeolian-fluvial system interactions in arid-climate settings are preserved in both recent and ancient sedimentary successions. However, despite considerable prior sedimentological research, there is no unifying scheme to provide generalized definitions of commonly occurring types of preserved aeolian-fluvial interactions. This study addresses this shortcoming by introducing a novel classification scheme for sedimentary architectures arising from such system interactions. The scheme is demonstrated through reference to examples from the Permian Cutler Group, Paradox Basin, Southeast Utah, USA - a sedimentary record of competing aeolian dune-field and fluvial-fan systems along a palaeo-coastline. Well-preserved, laterally continuous outcrops arranged in different orientations enable three-dimensional architectural characterization. The sedimentary record of eight distinct types of aeolian-fluvial interaction are identified: (i) water-table-controlled interdune sedimentation; (ii) deposits of low-energy fluvial floods; (iii) isolated fluvial channel-fills originating from episodic and confined flooding of interdunes in orientations parallel to the trend of dune crestlines; (iv) channel fills oriented perpendicular to the trend of dune crestlines; (v) amalgamated fluvial channel elements resulting from persistent, long-lived but confined dune-field flooding; (vi) deposits of unconfined sheet-like flood deposits; (vii) fluvial breaching of dunes and their reworking by catastrophic flooding; (viii) aeolian reworking of fluvial deposits. Each interaction type is characterized in terms of preserved sedimentary facies, architectural element geometries and associated proprieties, to demonstrate sedimentary variability in three dimensions. Results provide a guide with which to make sedimentological comparisons and interpretations between active systems and their preserved depositional record