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The Marnoso Project

Unique Field-Data on Turbidite Bed Shape: Implications for Flow Processes and Reservoir Geometry

This project is supported by the NERC Ocean Margins Link programme together with ConocoPhillips, Shell, BHP Billiton and Blackbourn GeoConsulting. The NERC grant provides substantial 'added value' through funding of experiments, fieldwork and poroperm analysis of outcrop samples. The project is run by UK-TAPS in collaboration with Dr Carl Amos (National Oceanography Centre, Southampton), Dr Paul Reynolds (Director of Bristol Colloid Centre at Bristol University) and Dr Andrew Hogg (Applied Maths Dept at Bristol University). Existing sponsors should click here.

Note that this project started in autumn 2003, but new sponsors can still join for a negotiated cost.

For further information please contact the UK-TAPS co-ordinator.

Bed shape in ancient outcrops

Individual turbidites have been correlated within a grid of 85 sections, across an area of 120 by 60 km, in the Miocene Marnoso Arenacea Formation of Northern Italy. These correlations now provide the most detailed and extensive constraints on turbidite bed shape yet achieved in any ancient deep-water sequence. This unique information on bed shape, and associated internal structures, offers an opportunity to test unequivocally controversial models for turbidite deposition (e.g. the 'high-density turbidity current' and 'sandy debris flow' models which predict bed geometries with tapered or abrupt margins respectively). Indeed, intriguingly, the shape of each and every correlated bed differs substantially from that predicted by existing models for bed shape.

Many beds comprise two fundamentally different types of sandstone; (i) clean graded sandstones with good reservoir quality, and (ii) ungraded mud-rich sandstone intervals ('slurries') with very poor reservoir quality. Inter-fingering of these low and high quality sandstone intervals records surprising flow transformations between multiple flow phases during a single flow event. Even beds that comprise only clean sandstone, repeatedly show abrupt thinning that is related to flow hydrodynamics, rather than to basin floor topography. These new data indicate a need to re-assess our understanding of turbidity current behaviour and consequent deposit geometry.

The field work has also clearly documents how bed-shape determines the frequency distribution of turbidite thickness. Understand this linkage is an essential pre-requite for attempts to use bed thickness distributions to diagnose deep-water depositional setting.

Experimental analysis of sand-mud settling behaviour

Distal deposits comprising thick muddy intervals demonstrate that most turbidity currents contain significant amounts of mud. Recent work has shown that even a few percent of mud can profoundly alter flow structure and sand deposition. The settling behaviour of sand through a non-Newtonian and colloidal muddy fluid is very poorly understood. Complex colloidal interactions between mud particles and surrounding water molecules leads to a rich variety of behaviour, such as flocculation and the formation of particle-networks (gels) with significant yield strengths.

An accurate understanding of settling behaviour is the crucial link that allows the character of a parent flow to be inferred from its deposit. Mud content also strongly influences sandstone permeability and reservoir quality. Sand-mud settling experiments will show how a deposit records the nature of its parent flow. Controlled laboratory experiments will be undertaken using consistent clay mineralogy (kaolin) and water geochemistry chosen to mimic seawater. Mud concentration, sand concentration and sand grain size(s) will be varied. Experiments will be combined with theoretical analyses to predict the settling behaviour of sand and mud particles, and to determine how mud and sand particles modify flow structure. 

Field data documenting individual bed geometries and experimental results will be combined to (i) test controversial depositional models for turbidity currents, and (ii) determine reasons for flow transformation that produce beds with inter-fingering intervals of high and low reservoir-quality sandstone, and (iii) predict turbidite extent and variations in reservoir quality within specific reservoir units chosen by sponsors.

Additional expertise

In addition to a wide range of expertise, this project brings together a consortium with access to well-equipped experimental laboratories suitable for this experimentally led study. Laboratories in Bristol's Earth Science and Applied Mathematics Departments, which form part of the University Centre of Environmental and Geophysical Flows, have recently been refurbished and provide suitable space for annular flume and settling tube experiments. Bristol Colloid Centre has an extensive range of equipment suitable for characterising the rheology and behaviour of colloidal suspensions. The Fluids Laboratory in the School of Earth Sciences at Leeds has a long track record in cutting edge experiments on particulate gravity currents, often using novel non-intrusive measurement techniques that measure both velocity flow fields and concentration fluctuations in opaque flows. Project results will be compared to geophysical surveys and samples from modern turbidite systems, provided by UK-TAPS partners.

To find out more about UK-TAPS, please use the links at the top of the page

© UK-TAPS Group
November 2005