Short length-scale variability of hybrid event beds and its applied significance

Hybrid event beds (HEBs) are a type of deep-water sediment gravity flow deposit that generally comprise a basal clean sandstone overlain by a variety of muddier and less-permeable sandy facies. They are thought to be emplaced by combinations of turbidity currents, transitional flows and debris flows, all as part of the same transport event. To date, a number of studies have highlighted the common presence of HEBs mainly in the outer and marginal parts of deep-water systems where they replace beds composed dominantly of clean sand up-dip and/or axially over scales of km to 10s km. In addition to these broad patterns, important yet poorly understood short-length facies changes (over metres to 100s m) occur, modifying the overall texture and reservoir characteristics at or beneath typical spacing of production wells. The nature and origin of the short length-scale transitions is here addressed in four well-exposed HEB-prone outcrops: the Cretaceous-Paleocene Gottero Sandstone and the Miocene Cilento Flysch, both in Italy, the Carboniferous Mam Tor Sandstone in northern England, and the Carboniferous basal Ross Sandstone Formation, Western Ireland. A series of detailed correlation panels show marked lateral variations in internal bed make-up for most of the hybrid event beds studied. This variability typically involves lateral changes in the proportions of the cleaner basal sandstone and the overlying muddy sandstone division that occur without substantial change in the overall event bed thickness. The variability is inferred to reflect the complex fingering between the up-dip sandstone-dominated part of the event bed and the down-dip linked debrite due to internal erosion (ploughing) of the debrite into the basal clean sand. Where the upper part of the bed is dominated by large mudstone rafts, these may have foundered into liquefied sand and been injected and partly fragmented by the sand intrusions. The variable thickness and continuity of the basal clean sandstones have important implications for reservoir characterisation; significant variability in bed character at interwell scale can be anticipated. Rugose contacts between the intra-bed facies divisions may impact on drainage and sweep efficiency during hydrocarbon production

Ichnofossils, cracks or crystals? A test for biogenicity of stick-like structures from vera rubin ridge, mars

New images from Mars rover Curiosity display millimetric, elongate stick- like structures in the fluvio-lacustrine deposits of Vera Rubin Ridge, the depositional environment of which has been previously acknowledged as habitable. Morphology, size and topology of the structures are yet incompletely known and their biogenicity remains untested. Here we provide the first quantitative description of the Vera Rubin Ridge structures, showing that ichnofossils, i.e., the product of life-substrate interactions, are among their closest morphological analogues. Crystal growth and sedimentary cracking are plausible non-biological genetic processes for the structures, although crystals, desiccation and syneresis cracks do not typically present all the morphological and topological features of the Vera Rubin Ridge structures. Morphological analogy does not necessarily imply biogenicity but, given that none of the available observations falsifies the ichnofossil hypothesis, Vera Rubin Ridge and its sedimentary features are here recognized as a privileged target for astrobiological research

Hybrid Event Beds Generated By Local Substrate Delamination On A Confined-Basin Floor

The outer parts of deep-water fans, and the basin plains into which they pass, are often described as areas where erosion is negligible and turbidite systems have net aggradation. Nevertheless sedimentological and stratigraphic analysis of outer fan lobe and confined basin plain deposits in Cretaceous-Paleocene Gottero Sandstone (NW of Italy) has revealed extensive but cryptic bedding-parallel substrate-delamination features at the base of many sheet-like event beds. These comprise a variety of shallow but wide scour structures showing evidence of lateral expansion by sand-injection. The scours commonly occur at the base of beds made up of a basal clean sandstone overlain by argillaceous sandstone containing abundant mudstone clasts and locally large substrate rafts (up to 20 meters long). These strata are interpreted as a type of hybrid event bed. Field observations suggest that mud-clast entrainment occurred by delamination at the base of dense sandy flows. The large rafts, in some cases only partly detached, were incorporated in the flows locally and then carried for short distances (100s m to a few km) before partly disaggregating and undergoing deformation due to internal shearing. The development of such features may be common in flat and/or confined basin settings where high-volume flows interact with a cohesive and well layered substrate (e.g. muddy outer fans or confined or ponded basins with thick mudstone caps). Delamination is therefore suggested as an alternative mechanism leading to the formation of hybrid event beds following local substrate entrainment on the basin floor as opposed to on more remote slopes and at channel-lobe transition zones

Facies and architecture of a sand-rich turbidite system in an evolving collisional-trench basin: a case history from the upper Cretaceous-Palaeocene Gottero system (Nw apennines)

This study documents the main depositional elements of a dynamically-controlled sand-rich deep-water turbidite system (Upper Cretaceous\u2013Palaeocene Gottero system, north-west Italy). The large exposures and the wide range of facies and deep-water sub-environments recognised, ranging from proximal channels, unconfined proximal and distal lobes and confined basin plain deposits make this an instructive case study to investigate the spatial-temporal relationships between fan features (channels and lobes) and confined to ponded basin-plain deposits developed in a trench-fill basin. The study focus on stratigraphic and palaeo-environmental reconstruction of the Gottero system in the western sector of the basin. Bed types, facies associations and depositional sub-environments are described in main outcrop locations and used to feed a comprehensive bed-scale database. A coherent stratigraphic framework of the system is proposed for the first time, linking its stratigraphic evolution with the collisional-trench context of the Ligurian units during the Upper Cretaceous-Palaeocene. It includes a first stage in which the Gottero was a prograding extensive basin-floor fan developed in a relatively unconfined setting (Gottero 1 and 2), and a second stage in which the system deepens and got progressively confined and segmented in multiple distal depocentres, dominated by sheet-like high magnitude events, meanwhile the proximal area forms a series of fan elements which display an overall retrograding trend (Gottero 3). The basin fill terminates with the deposition of the Giaiette mass-transport complex interepreted to represent the final collapse of the growing Ligurian accrectionary wedge

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

Palaeoclimatic and palaeoenvironmental evolution of the Lower Pleistocene Arda River succession

The Arda River marine succession, cropping out in western Emilia (northern Italy) represents an excellent site to study past ecosystems dynamics in the frame of Early Pleistocene climate change and tectonic activity. This one-day excursion leads the participants to discover the palaeoclimatic and palaeoenvironmental evolution of the Lower Pleistocene Arda River marine section, unraveled through an integrated use of sedimentological, palaeoecological (molluscs and trace fossils) and geochemical tools. Upsection, the succession was deposited in progressively shallower water and colder climate during phases of advance of fan deltas affected by hyperpycnal flows. It culminates at the top with clast supported alluvial conglomerates and freshwater/terrestrial biota indicating a sea level drop and the establishment of a continental environment. It is very rich in fossils: in the marine part molluscs, brachiopods, corals and echinoderms, besides well preserved trace fossils, are abundant; whereas in the continental part a mammal fauna and freshwater/terrestrial molluscs are occasionally found. Sclerochemical analyses undertaken on bivalve shells indicate that seawater temperature seasonality was the main variable of climate change within the study area during the Early Pleistocene. In particular, strong seasonality and low winter palaeotemperatures were assumed to be the main drivers for the widespread establishment of Arctica islandica populations in the palaeo-Adriatic Sea around 1.80 Ma. During the excursion not only fossils are shown, but also interesting biocalcarenitic bodies with a complex geometry cropping out in the town of Castell\u2019Arquato. The excursion is complemented by the visit to the Giuseppe Cortesi geological and palaeontological museum, housing vertebrate and invertebrate fossil collections

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

Magnetostratigrafia della serie Pleistocenica di Castell’Arquato (PC)

Abbiamo presentato i risultati preliminari relativi all'indagine magnetostratigrafica della sezione affiorante nel torrente Arda (PC) da cui \ue8 poi stato ricavato l'articolo Monesi et al., (21016

A hierarchical multiple-point statistics simulation procedure for the 3D reconstruction of alluvial sediments

A correct representation of the heterogeneity of porous formations and of their preferential flow paths is crucial for a reliable modelization of the contaminant transport processes. Several geostatistical tools have been developed to tackle this challenge. Many of these tools are often applied in a multi-scale framework, where the geostatistical simulation is applied fist trying to reproduce the big scale features of the sedimentary formations, and finally to reproduce their small scale features. However, many of the developed multi-scale and hierarchical techniques have a quite complex work-flow and rely on diverse simulation methods. Here a simplified hierarchical simulation procedure is proposed, where only multiple-point statistics (MPS) is used to simulate the target heterogeneities at different scales. The simulation procedure is organized in a tree-like frame, where MPS is applied at each simulation branch using a simplified binary training image and the corresponding available conditioning data. At each simulation branch, the MPS simulation is performed in a sub- domain defined by one of the two facies codes simulated at the parent branch. The proposed procedure is tested in the three-dimensional (3D) reconstruction of two model blocks of alluvial sediments, using the available two-dimensional (2D) outcrop information as training images. It is compared against a non hierarchical MPS simulation procedure in terms of connectivity indicators and breakthrough curves obtained from 3D particle tracking numerical experiments. All the aforementioned tests are performed considering 100 equiprobable realizations for each simulation technique. This allows to make statistically reliable comparisons, and to extract statistical distributions of the transport parameters by fitting analytical curves to the results of the particle tracking experiments. These statistical distributions are used to perform one-dimensional transport experiments on spatial scales ten times bigger than the block scale using the Kolmogorov-Dmitriev approach in a Monte Carlo framework

Three Geostatistical Methods for Hydrofacies Simulation Ranked Using a Large Borehole Lithology Dataset from the Venice Hinterland (NE Italy)

A large borehole lithology dataset from the shallowly buried alluvial aquifer of the Brenta River Megafan (NE Italy) is used in this paper to model hydrofacies with three classical geostatistical methods, namely the Object-Based Simulation (OBS), the Sequential Indicator Simulation (SIS), and the Truncated Gaussian Simulation (TGS), and rank alternative output models. Results show that, though compromising with geological realism and rendering a noisy picture of subsurface geology, the pixel-based TGS and SIS are better suited than OBS for their ease of conditioning to closely spaced boreholes, especially in fine-scale simulation grids. In turn, SIS appears to provide better prediction and less noisy hydrofacies models than TGS without requiring assumptions about relationship among operative facies, which makes it particularly suited for use with large borehole lithology datasets lacking detail and quality consistency. Flow simulation on a test volume constrained with numerous boreholes indicates the SIS hydrofacies models feature well-connected sands forming relatively fast flow paths as opposed to TGS models, which instead appear to carry a more dispersed flow. It is shown how such a difference primarily relates to ‘noise’, which in TGS models is so widespread to translate into a disordered spatial distribution of K and, consequently, a nearly isotropic simulated flow