BOUROULLEC, R., Imperial College, London, UK, C. LANSIGU, J.-M. QUEMENER, and D. SAVANIER, Geosciences, Université de Rennes, France

Abstract: High Resolution Sequence Stratigraphy of a Turbiditic System: Synsedimentary Deformation and Reservoir Implications - Grès d'Annot, French Alps

Estimation of reservoir geometry, hydrocarbon migration, trapping history, sedimentary facies prediction and fault sealing characterisation can only be improved in accuracy if the fault kinematics and the 3D stratigraphic model are well constrained.The quantification of periods of syn-depositional fault activity and the characterisation of the growth history is crucial in petroleum geology. Good outcrops showing large synsedimentary extensional faults such as those seen in Gulf Coast or the West-african margin subsurface are rare. A tectono-stratigraphic study as been carried out to quantify in time and space the processes involved in a deep offshore depositional system located in south-east France. Annot basin is one of the peri-alpine basins stretching from the French Italian Alps to the western Swiss. Its substratum, Mesozoic in age, has been deformed by the "pyrénéo-provencale" compressive tectonics. The sedimentation (Lutetian-Oligocene), following a flooding event, is marine.The deposits, named "Trilogie Priabonienne," consist of three main units, which are: 1) the Calcaires Nummulitiques, marine limestones, 2) the Marnes Bleues, a homogeneous formation of slope marls and 3) the Grès d'Annot, gravitational deposits (c.f. Fig. 1). Many controversies remain about the tectonic context contemporaneous with the "Trilogie Priabonienne" deposition (compressive or transtensive).

The relationship between this turbiditic depositional system and an extensional synsedimentary deformation has been analysed on cliff faces outcropping on the western part of the French southern Alps. The late Eocene - early Oligocene siliciclasitic succession of the Grès d'Annot Formation located on the 4 km long Barre de la Mauvaise côte and Moutière Cliffs show a series of 4 major SW/NE extensional faults with downthrow sides to the NW, the same direction that the average regional palaeo-current trend (c.f. Fig. 1). The décollement zone used by these syn-Grés d'Annot deposition faults is believed to be the Marnes Bleues formation, a 50-200 m thick slope marls underlying the turbiditic succession. A series of 26 logs constructed at a scale of 1:50 with a total thickness of 3300 m and a photographic survey from land and air have been used to build a sedimentological model, a 3D high resolution stratigraphical model and to analysed the fault systems in term of geometry and kinematics.

The Grès d'Annot consist essentially of 5-10 m thick sandstone beds separated by metric heterolithic successions (cf. Fig. 2).These beds are described as elementary geometric pattern and are present within the entire succession.The sedimentary study of these beds, considered as homogeneous and as representing an single event, indicate a very important rate of amalgams in both homolithic and heterolithic facies.These amalgams are represented as thin shale layers, centimetric bioturbated sandstones beds and rapid granulometric variations. They are the result of facies variation in time and space linked with different hydrodynamisms: 1) ripples, megaripples and large obliques indicate a traction regime, 2) planar/parallel laminations and growing-up structures are typical of a suspension regime.These facies are spatially organised within the elementary sandstone beds, following a recurrent pattern and showing some differential preservation motifs.They are integrated in a elementary succession depending on their hydrodynamism, from a progradational trend for the tractive facies to a.aggradational trend for the suspensive facies.These geometries are genetically linked with erosions shelfward.

Three types of faults have been identified (cf. Fig 1): 1) some major NE/SW extensional faults with throws between 30 and 200 m, 2) some secondary antithetic and synthetic faults accommodating the major faults, and 3) some secondary faults strike normally to the main tectonic system. Many of the secondary faults have a listric shape and have used shaly successions (heterolithic intervals and intra-debris flows shale-filled channels) as décollement zones. The intensity and style of deformation depend directly on the distance to the major faults. The morphology of the fault system indicate a complex syn-depositional history with cyclical periods of activity and non-activity. Figure 3 illustrates an example of a middle-scale fault system, some kinematic results on the fault NF' and a detailed view of syn-deposition deformation. The chronology of the faults activity also indicate a migration of the deformation toward the basin. Gentle roll-overs and thickenings of 10-15% are observed on the hanging walls. Particular geometries linked with turbiditic deposition on a deformed sea floor have been analysed at different scale, such as complex erosions around the fault tips and subsequent depositional geometries (palaeo-current variation, instability features such as small-scale diapirs, slumps, sedimentary dykes, eroded tilted blocks).The deformation of the sediment involved in the synsedimentary events indicate a clear lithological control on small-scale deformation and also a very shallow and early diagenesis.This study shows the interest of using a multi-disciplinary approach for a good understanding of the relationship between sedimentary and tectonic processes in an deep offshore environment.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England