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Pr Kevin T. PICKERING Department of Geological Sciences University College London Gower Street LONDON WC1E 6BT e mail: ucfbktp@ucl.ac.uk http://www.ucl.ac.uk/GeolSci/people/pickering/books.htm
TURBIDITE SYSTEMS OF SOUTHEAST
FRANCE APPLICATION TO HYDROCARBON
PROSPECTIVITY
Frontispiece
• Inherited seafloor topography exerted the primary control on the location and overall shape of depocentres for the accumulation of sand, i.e. the establishment of sub-basins. • Syn-depositional tectonics do not appear to have exerted a major control on sand accumulation, but are recognised as having been important in increasing local seafloor gradients leading to over-steepening and mass flow (e.g. sediment slides). • The sedimentary structures associated with all grain sizes are consistent with turbidity currents being the principal sediment transport and depositional process, with relatively minor debris flow, sediment reworking (to isolated megaripple and dune bedforms) and slide processes. Assuming that individual beds are of basin-wide extent, and using the present-day (preserved) sub-basin dimensions, typical sediment volumes associated with the sandy turbidity currents were in the range 0.03-0.12 km3. • Water depths are very poorly constrained, apart from in the remnants of the feeder system. There are no shallow-marine sedimentary structures; the faunas merely indicate upper bathyal water depths, and basin-wide considerations (topographic reconstructions) favour water depths of at least ~500 m. • Sedimentation was controlled by abrupt, metronomic, changes in the sediment flux to result in the vertical alternation of sand-prone (between 8-80 m thick) and mudstone-prone (between 1-17 m thick) intervals, typically of sub-basin-wide extent. • In general, the packets of coarser grained clastics show an overall high continuity relative to the dimensions of the sub-basins, i.e. over distances <10 km. • Internally, sandstone packets typically show amalgamation of medium- to very coarse-grained, thick- to very thick-bedded Tab deposits. Large-scale erosional surfaces cutting down 15 m over ~200 m laterally occur within the sand-rich packets. • The mudstone packets are typically siltstone-rich, with isolated thin- to medium-bedded fine- to medium-grained sandstones. • Erosion into mudstones at the base of sandstone packets is up to 50 m over ~300 m laterally. A total of four examples of large-scale erosional features have been identified throughout the entire formation with only one showing both margins. Despite a channel-like geometry associated with such sandstone packets, no convincing levée and overbank environments could be recognised. These features are interpreted as having formed entirely by erosion and then passive infill. • Sediment dispersal patterns, together with petrographic data, suggest that only one side of the basin, in the up-current direction (to the south) acted as a major sediment source. There are no unequivocal preserved submarine canyons associated with the basin (or sub-basins) and, therefore, the nature of the feeder system(s) remains obscure. Locally, the remnants of contemporaneous fan-delta deposits suggest that much of the deep-marine deposits may have accumulated as fan-delta-to-slope basin/basin-slope systems. • The Grès d'Annot Formation cannot be interpreted as a classic radial submarine fan system (e.g. non-radial palaeocurrent pattern, lack of convincing channel-levée-overbank complexes, and the absence of any logical distribution of channel and non-channel environments), but rather as a basin-slope and slope-basin system in which sub-basins may have been connected by relatively narrow corridors. Two possible end-member options exist for explaining the pattern of sand accumulation: (1) a terraced and compartmentalised slope in which sub-basins (depocentres) were disconnected and separated by submarine ridges over which turbidity currents spilled (overspill and flow-stripping); (2) a terraced and compartmentalised slope in which sub-basins were connected by relatively narrow corridors. In the latter scenario (2), sediment gravity flows would still experience some overspill and confinement during travel between sub-basins. Although key marker beds were identified within individual sub-basins, this proved impossible between the various sub-basins. A reconstruction in which the depocentres must have been discrete sub-basins within a larger basin is suggested by the occurrence of well-defined onlap surfaces (up to at least 400 m high over lateral distance of ~5 km), showing onlap of the sandstones onto the marls in proximal-to-distal and axial-to-lateral directions, up- and down-current. The thickness of the sub-basin fills, however (up to ~770 m), suggest that depocentres remained stable for long time intervals and, by inference, that their sediment supply routes probably remained relatively stable. Circumstantial evidence leads to the latter option (2 above) as the favoured candidate. The Tertiary Grès d'Annot Formation of SE France is a sand-rich deep-marine system deposited in a basin with a complex basin-floor topography subdivided into local topographic highs and lows. The topographic lows acted as sites for the preferential accumulation of sediments from sediment gravity flows, mainly turbidity currents, debris flows and slide deposits. The deep-marine system appears to have lacked integrated typical deep-marine fan-like features such as a distinct radial distribution of fan environments (from canyon, through inner- and middle-fan to outer fan sheet-like deposits). This lack of typical submarine fan features may be due to the type of feeder system (e.g. multiple-sourced fan-deltas) supplying sediment to the deep-marine basin. For the Grès d'Annot Formation no single point source for sediment supply is observed, and a multiple sourced feeder system is inferred, akin to that described from the Eocene Tyee Formation, Western Oregon (Heller and Dickinson 1985). In sand-rich systems which have a point source and no complex basin topography, typical submarine fan features develop, such as channel-levée overbank complexes, and radial fan-like development, such as the modern Redondo fan of southern California (Haner 1971). The type of feeder system and sediment calibre supplied to a confined basin play a major role in the type and pattern of sediments deposited. The three main types of feeder system include: (i) submarine fan point source; (ii) multiple sourced ramps, and (iii) slope apron linear source (Pickering 1982, Mutti and Normark 1987). classified deep-sea depositional systems by sediment calibre and feeder system type; sediment calibre was divided up into mudstone, sandstone and gravel dominated systems. In a confined basin where the volume of individual flow events tends to exceed the aerial extent of the basin floor, the basin slopes inhibit the radial expansion of decelerating turbidity currents and hence produce elongate sedimentary bodies. Also, the scale of the confinement in the receiving basin will play a major role in controlling the types of sediment distribution patterns observed. Normark (1985) uses examples from modern submarine fans to explain how large-scale morphological features in the basin control fan development and growth, citing examples from the Monterey and Navy Fans. In the Monterey Fan there were five stages during fan growth, in part controlled by topography (Normark 1985). In confined basins, which are to some extent comparable to at least parts of the Grès d'Annot Formation system, the radial expansion of decelerating turbidity currents is suppressed but not inhibited and, hence, produce relatively elongate sedimentary bodies, a feature of the sub-basins within this system.
An understanding of the nature and range of onlaps is important for understanding the fluid dynamics and resulting deposition / erosion caused by the interaction between sediment gravity flows (e.g. turbidity currents) and basin slopes. Also, it is important for hydrocarbon prospectivity because sand-bed and sand-packet terminations may be associated with poor reservoir sealing and consequent hydrocarbon leakage from stratigraphic traps. The Grès d'Annot Formation contains a variety of onlap types, ranging from abrupt bed termination against a marl slope to a more gradual thinning of beds as a drape onto a marl slope. Two types of onlaps (with examples) have been defined: (1) TYPE 1 ONLAP: Abrupt edge to the sands which all onlap the same depositional surface, approximating to a near-isochronous surface, and (2) TYPE 2 ONLAP: Significant draping of the onlap surface, resulting in an apparent onlap surface which upon closer inspection is in fact diachronous, i.e. with multiple stacked onlap surfaces. The resultant sedimentary characteristics in the vicinity of any particular onlap relationship are a function of: • Angle of slope; In some cases described from SE France, individual sandstone beds and packets of sandstone beds show an increase in thickness in the vicinity of the onlap surface, commonly associated with liquefaction and fluidization features such as convolute bedding. Such thickness changes are attributed to abrupt momentum changes and the rapid dumping of sand at the base of local slopes as turbidity currents interact with the slope. Quantifying deep-water high-continuity sandy systems: Application to Grès
d'Annot, other deep-water systems, Quantifying the geometry of high-continuity sand-prone systems is important both for a better understanding of the intrinsic nature of these systems (volumetrics, stacking patterns, etc.) and in the application of this data to modelling the depositional and reservoir characteristics of such systems, e.g. in basin analysis and reservoir modelling, including reservoir characterisation. There have been attempts to quantify shale lengths, for example Schuppers (1993) produced a shale length distribution curve for the Eocene Lower Ainsa (Ainsa I) channel in the south-central Pyrenees. This book presents a new and simple method for quantifying the geometrical characteristics of architectural elements, in any depositional system, with particular reference to high-continuity sand-prone systems. The method should prove particularly useful both for industrial and academic research because it: • Provides an objective means of defining
architectural elements (for all sedimentary environments - in this case applied
to deep-water high-continuity or less-confined sandy systems); Such datasets can be incorporated into CD-ROM-based information system to include quantitative data linked with more qualitative data. In industry, this approach should prove particularly valuable in computer modelling packages which attempt to populate accommodation space with geologically reasonable sedimentary geometries at all scales from individual beds to stratally-bound packages of sediment such as submarine channels and lobes. Furthermore, the scheme can equally be adapted to characterise shale lengths and reservoir sands. The scheme is not scale invariant. It is important that elements of comparable scale are compared, and ideally in sections with similar orientations relative to the palaeoflow directions. A further potential limitation of the scheme outlined here concerns the "representativeness" of the measured section/panel. If the true extent and shape of architectural elements is at variance with that exhibited by the outcrop/panel, then the resulting statistics will be severely skewed by the available data; e.g., in situations where only the marginal parts of a system are visible and/or measured such as in the vicinity of a basin onlap.
Note: References cited in this section can be found in the book site web: http://www.ucl.ac.uk/GeolSci/people/pickering/books.htm
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