Observatoire des sciences de l'Univers Grenoble

Stages de DEA proposés pour 2000-2001 au LGIT

1.      Diagenèse d'une série gréseuse en relation avec la mise en place de nappes ; implications sur les mécanismes de fracturation.

Responsable : Pierre Labaume
Téléphone : 04 76 82 80 59
Autres encadrants : Elisabeth CARRIO et Jean-François GAMOND (LGIT), Simon SHEPPARD (ENS Lyon)
Lieu du stage : LGIT

Résumé :
Lors de son enfouissement, une série gréseuse subit une diagenèse qui associe la compaction mécanique, la compaction chimique (transfert de silice par pression solution) et la cimentation par les circulations de fluides, le développement de ces processus étant variable en fonction des conditions physiques et chimiques de l'enfouissement (pression, température, durée, chimie des fluides). Cette diagenèse transforme les propriétés mécaniques (acquisition de rigidité) et pétrophysiques (réduction de porosité) de la roche, ce qui a des conséquences majeures sur les mécanismes de la fracturation ainsi que sur les propriétés hydrodynamiques (perméabilité). Dans les Alpes externes françaises, les Grès d'Annot déposés à l'Eocène supérieur ont été recouverts à l'Oligo-Miocène par les nappes alpines dans les parties est et nord du bassin, alors qu'ils sont restés sans couverture dans la partie sud-ouest. En conséquence, ils sont devenus très rigides et très peu poreux sous les nappes, alors qu'ils sont friables et poreux là où elles sont absentes. L'étude consiste à étudier dans ces différents domaines, en les comparant, les caractéristiques de la diagenèse (mécanismes de la compaction, cimentation), ses conséquences pétrophysiques (porosité) et ses implications sur les mécanismes de la fracturation. Le travail sera basé sur l'analyse pétrographique et texturale en microscopie (optique, électronique, cathodoluminescence). Des analyses de géochimie isotopique des carbonates effectuées par un collaborateur extérieur permettront de préciser les interactions fluides-sédiments.

Characterisation of lateral heterogeneities in an exceptionally exposed turbidite sand-body, Grès d'Annot (Eocene-Oligocene), SE France

Department of Geology & Petroleum Geology, University of Aberdeen, King's College, Aberdeen AB24 3UE, UK. s.lomas@abdn.ac.uk & SEAN KELLY
Shell UK Exploration & Production, Altens Farm Road, Aberdeen AB12 3FY, UK.

The Trois Evêchés outcrop of the Grès d'Annot (Annot Sandstones: Eocene-Oligocene, SE France) represents one of the world's best-exposed examples of a confined sandy turbidite system and has exceptional exposures of 'channelised lobe' or 'amalgamated sheet' sand-bodies. The sand-bodies have simple tabular external geometries (lobe/sheet-like) but show complex internal organisation characterised by a combination of scouring, bypass and aggradational features (channel-like). We have targeted the best-exposed, most laterally continuous sand-body (termed the FB unit) for very detailed studies at a scale appropriate to reservoir modelling. The result is a WNW-ESE panel 35 m thick, 1700 m long parallel to palaeocurrent direction ('downdip') within which all zones are characterised in terms of key properties (grain-size, sorting, cementation, primary and secondary structures) and all key surfaces are absolutely correlated (i.e. directly physically traced). This unique database allows quantification of lateral facies relationships and deterministic definition of both architectural components and stratal hierarchy. The FB body is thick, tabular, high-net:gross unit with a simple tabular external geometry but a complex internal structure. Key observations are that: 1) the system shows considerable lateral variability; 2) the well-defined base of the FB sand-body is a different genetic surface in different places; 3) a few beds and surfaces are laterally persistent but, because of erosion, many are not traceable for more than 100-200 m; 4) packages within the sand-body are erosive-based and vary laterally in thickness, with an element of compensation between successive packages; 5) the erosive-based packages show a back-stepping arrangement of successive points of deepest erosion (i.e. migrating towards the ESE, up-palaeocurrent). The FB sand-body appears relatively uniform at a distance but in detail contains numerous heterogeneities at variable scales which would have a considerable impact on fluid flow through an apparently homogeneous sandstone.

Outcrop Analogues for Deep Water Channel and Levee Genetic Units from the Grès d'Annot Turbidite System, Southeast France

Clark, J.D. and A.R. Gardiner, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, UK


Abstract

Large-scale exposures of the Grès d'Annot in south-east France provide excellent opportunities to study detailed architecture of sand-rich turbidite systems deposited in a relatively confined basin. The deposits are dominated by laterally extensive sheet-like sandstone packets which, in general, have quite distinctive bed thickness characteristics and sandstone-to-shale proportions. Field studies in the Grand Coyer, Trois Evêchés, and Col de la Cayolle outcrop areas have identified, in addition to the sheet-like packets, a common type of turbidite channel succession. These channels are characterised by a relatively high aspect ratio; channel dimensions range from 900-4000 m wide and 14 to 110 m deep. The channel fill is sand-rich, moderately- to highly-amalgamated, but the sandstones show largely planar bedding architecture. The relatively low relief of the channels and their sheet-like fill makes them difficult to distinguish in areas without good lateral continuity of exposure, and it is possible that their importance within the fill of the Grés d'Annot basin has previously been underestimated.

Several channel exposures allow detailed examination of channel margin architecture, and clearly demonstrate several periods of reactivated channel activity. Typically, the margins show the most complex sedimentary architecture. Laterally away from the channel margin, and/or stratigraphically above or below the channel-fill, thin-bedded sandstones and shales form distinctive packets. Within these packets, the thin-bedded sandstones are relatively coarse-grained, cross-laminated, a nd planar-laminated, and beds are commonly discontinuous over relatively short distances (showing pinch-out in both directions). Cross bedding is commonly found in the thicker beds, and trace fossils such as Ophiomorpha and Thalassinoides a re abundant. Sandstones containing lignite and charred wood fragments are commonly found in these intervals. Megascours, and small-scale channel-fill sandstone bodies can also be found within these intervals. Together, these facies, and their association with the channels, suggest that the thin-bedded packets represent the levees to sandier channelised deposits.

These levees formed from the aggradation of sand and shale, deposited from flows, or parts of flows, that spilled from the channels. This facies association has been used to interpret other levee deposits elsewhere in the Grés d'Annot, where channels are not exposed. It is not, however, possible to interpret all thin-bedded packets in the Grés d'Annot succession as levee facies; for this interpretation to be valid, the packets must include some or all of the distinctive features described above.

The Grés d'Annot channel and levee deposits are characteristic of this turbidite system. These channels appear more common in the proximal part of the system (in the Grand Coyer outcrop area), and may have developed basinwards fr om highly confined flows that were channelled though the Annot Sub-basin. The channels are found interbedded with laterally continuous sheet sandstones, have a sheet-like fill, and relatively high aspect ratios, suggesting that they broadened out downstr eam to form connected depositional lobes. This type of channel-levee system may provide a new model for analogous channels in other turbidite systems.

 

Detailed Characterisation of Lateral Heterogeneities in Exceptionally Exposed Sand-Rich Turbidite Outcrops from the Grès d'Annot, SE France: Stratal Continuity and Reservoir Simulation

Lomas, S.A., B.T. Cronin, A.J. Hartley, Davide Duranti, Andrew Hurst, Emma Mackay, S.J. Clark, Department of Geology and Petroleum Geology, University of Aberdeen, King's College, Aberdeen AB24 3UE, UK, and Sean Kelly, Shell U K E&P, Altens Farm Road, Aberdeen AB12 3FY, UK

Abstract
The Trois Evêchés outcrop of the Grés d'Annot (Annot Sandstone) represents one the world's best-exposed examples of a confined sandy turbidite system. The Grés d'Annot is a large Eocene-Oligocene sand-prone turbidite basin-fill now exposed as a series of basin remnants fringing the southwest Alps in southern France. The Trois Evêchés remnant has exceptional exposures of sub-seismic to seismic-scale architectural geometries which, despite the scale and quality of the outcrop, have seen little previous work. Our work here shows the Grés d'Annot to be up to 980 m thick with an overall 'net:gross' >75%, a mean sandstone bed thickness of 0.70 m, and a mean sand-body thickness of 26.1 m. The sand-bodies have simple tabular external geometries (lobe/sheet-like) but show complex internal organisation characterised by a combination of scouring, bypass, and aggradational features (channel-like).
We have targeted the best-exposed, most laterally continuous sand-bodies for very detailed studies at a scale appropriate to the reservoir modelling scale. A key result is a panel 35 m thick, 1700 m long ('downdip') within which all zones are fully characterised in terms of key properties (grain-size, sorting, cementation, primary and secondary structures) and all key surfaces are absolutely correlated (i.e., directly physically traced). This unique database allows quantificatio n of lateral facies relationships and deterministic definition of both architectural components and stratal hierarchy. Numerical simulations of this outcrop panel are compared with an analogous turbidite field from the central North Sea to evaluate reserv oir modelling constraints and best practice in high-N:G turbidite reservoirs, where the scale and distribution of intra-reservoir heterogeneities are believed to have critical effects on production. Particular emphasis is placed on the modelling and produ ction impact of reservoir heterogeneity on long-reach horizontal wells.
Detailed Characterisation of Lateral Heterogeneities in Exceptionally Exposed Sand-Rich Turbidite Outcrops from the Grès d'Annot, SE France: Stratal Continuity and Reservoir Simulation
Lomas, S.A., B.T. Cronin, A.J. Hartley, Davide Duranti, Andrew Hurst, Emma Mackay, S.J. Clark, Department of Geology and Petroleum Geology, University of Aberdeen, King's College, Aberdeen AB24 3UE, UK, and Sean Kelly, Shell U K E&P, Altens Farm Road, Aberdeen AB12 3FY, UK
 

Abstract
The Trois Evêchés outcrop of the Grés d'Annot (Annot Sandstone) represents one the world's best-exposed examples of a confined sandy turbidite system. The Grés d'Annot is a large Eocene-Oligocene sand-prone turbidite basin-fill now exposed as a series of basin remnants fringing the southwest Alps in southern France. The Trois Evêchés remnant has exceptional exposures of sub-seismic to seismic-scale architectural geometries which, despite the scale and quality of the outcrop, have seen little previous work. Our work here shows the Grés d'Annot to be up to 980 m thick with an overall 'net:gross' >75%, a mean sandstone bed thickness of 0.70 m, and a mean sand-body thickness of 26.1 m. The sand-bodies have simple tabular external geometries (lobe/sheet-like) but show complex internal organisation characterised by a combination of scouring, bypass, and aggradational features (channel-like).
We have targeted the best-exposed, most laterally continuous sand-bodies for very detailed studies at a scale appropriate to the reservoir modelling scale. A key result is a panel 35 m thick, 1700 m long ('downdip') within which all zones are fully characterised in terms of key properties (grain-size, sorting, cementation, primary and secondary structures) and all key surfaces are absolutely correlated (i.e., directly physically traced). This unique database allows quantificatio n of lateral facies relationships and deterministic definition of both architectural components and stratal hierarchy. Numerical simulations of this outcrop panel are compared with an analogous turbidite field from the central North Sea to evaluate reserv oir modelling constraints and best practice in high-N:G turbidite reservoirs, where the scale and distribution of intra-reservoir heterogeneities are believed to have critical effects on production. Particular emphasis is placed on the modelling and produ ction impact of reservoir heterogeneity on long-reach horizontal wells.

 

Changes in architectural style as a confined turbidite system aggrades: the Peïra Cava system, south-east France

Lawrence A. Amy, School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, United Kingdom, lawrence@earth.leeds.ac.uk, William D. McCaffrey, School of Earth Sciences, Leeds University, Leeds, LS2 9JT, United Kingdom, and Ben Kneller, Institute for Crustal Studies, University of California, Santa Barbara, CA 93101.

Confined turbidite systems are an important component of many deep-water hydrocarbon provinces. Seismic studies have identified that the large-scale architecture may change as these systems aggrade. However, there are few documented field examples of such architectural development - particularly in systems with good 3D control.

The Eocene/Oligocene Grès d'Annot Formation comprises deep marine clastic sediments deposited within the Tertiary foreland basin of SE France. Deposition initially occurred in a series of linked, topographically-confined sub-basins. This study is focused upon one such sub-basin, and in particular on the architectural changes within one well correlated 700 m stratigraphic interval over a down-stream length of 10 km that preserves back/lateral slope, base-of-slope and basinal settings.

Moving up section in certain parts of the basin, three distinct architectural styles are observed: Type 1, a high net-to-gross proximal slope-related facies dominated by complex amalgamated beds and sand filled mega-scours, interpreted as the result of sedimentation close to the break-of-slope; Type 2, a low net-to-gross basinal facies composed of sand-mud couplets with few thick sand beds or thick-bedded packages, interpreted as reflecting deposition in a basinal setting away from the break-of-slope; Type 3, a moderate net-to-gross basinal facies consisting of repeated thick-bedded sand packets separated by thinner-bedded packets, interpreted as indicating either the up-stream growth of a submarine fan or the initiation of spill-over into a downstream sub-basin. This sequence may represent a general basin-filling pattern common to confined turbidite systems and identifiable at a seismic-scale.

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