PROCESS RESPONSE MODELLING OF FLUVIO-DELTAIC ARCHITECTURE

Ir. I. Overeem (Ph.D. student)

Department of Applied Earth Sciences

Section Earth Sciences

The research aim is the development of a simulation model of the fluvial-deltaic processes in order to quantify the influence of sea level change, tectonics and climate on the architecture of these deposits. The model links process-based modelling and geometrical interpretation, which is applied to 3-D seismic data. An example of modelling fan deposits is elaborated.

Introduction

The project falls in the framework of NEESDI (Netherlands Environmental Earth System Dynamics Initiative). The objective of NEESDI is to advance the development of numerical modelling tools as an instrument to address geological processes in coastal lowland settings. This specific study focuses on the Neogene-Quaternary and a time scale of 103 - 105 years. Over the Neogene-Quaternary an enormous delta system extended over entire N-W Europe. Huge volumes of sediments were deposited and completely filled the North Sea basin. Recently interest arose in the architecture of the specific system because it contains possibly economic gas reservoirs. Both 2D and 3D seismic data has been interpreted with innovative image processing techniques in co-operation with the Department of Applied Geophysics and NITG-TNO. The challenge is to extend insights on local architecture, resulting from 3D seismic data, to generalised information about delta system dynamics. The project has to be finalised in 2001.

Modelling approach

To study the system dynamics of fluvio-deltaic sedimentation, under the influence of tectonics, sea level change and sediment supply, a numerical process-response model, AQUATELLUS, has been constructed. The model integrates process descriptions of fluvial and marine processes for large spatial and temporal scales. Over large (geological) time scales (> 103 years) major floods and storms are the relevant transport events. A large spatial scale is chosen and a typical longitudinal profile extends over 10 – 100’s of kilometres. The chosen macroscopic scale requires simplification of the process descriptions, while on the other hand the dynamic nature of the processes requires a certain degree of freedom. Separate algorithms describe erosion and sedimentation fluxes. This allows us to easily incorporate multiple grain size classes. Erosion depends on discharge, slope and water depth. In the fluvial domain we are experimenting with incorporation of avulsions and bifurcations. Offshore, the used description of erosion fluxes can be seen as a proxy for both gravity flows in submarine channels and downslope diffusion. Sedimentation is described as dependent on the sediment flux and a travel-distance factor, which is grainsize dependent.

A 2D cross section of the coastal zone is simulated, parallel to the main flow direction. Unsteady external controlling parameters like sea level and tectonics can be posed unto this profile per time step (generally 1 year). Climatic changes over time are incorporated in the discharge function by increasing or decreasing initial discharges and sediment loads for each time step.

Modelling fan deposition

Interpretation of 3D seismic data of the L08 and F09 block in the Southern North Sea revealed extensive mass movements in Cenozoic delta deposits. The dimensions of these fans can be upto 6 km wide and 75 m thick, making them interesting potential reservoirs. Several mechanisms are proposed for their origin; they coincide with a large Late Pliocene sea level fall and a significant climatic cooling, which may have increased sediment supply. The 2D model has been used to investigate different scenarios of origin of the mass movements. The mass movements are described in a separate routine. Firstly, the threshold for slumping depends on exceeding of a critical slope. The dimensions of the eroded volume depend on the intersection of the critical slope angle and an angle of repose. Subsequent deposition of the slumped mass depends on the local slope along which the sediment moves. High local slopes induce larger transport rates, resulting in elongated slumps. Low local slopes, in contrast, result in compact lens-shaped fans. In Fig 1 the testing of a scenario of lowstand fan deposition is illustrated. Lowering of the sea level yields extensive deposition of coarser sediments in largely fine-grained distal sediments. The mass movements are caused by oversteepening of the delta front with coarse sediments, due to the falling sea level. In the distal permeability log a coarsening trend due to this progradation can be recognised. The slump-induced lens clearly stands out in the permeability log as a distinctive higher permeable body.

Fig. 1 Model output of a scenario with falling sea level, showing coarse slump-fans in a finer matrix.

Research plan for 2001

Some small improvements and experiments have to be incorporated in the 2D model. In addition, papers and thesis are finalised. Presentations at the EAGE, June 2001, Fluvial Sedimentology Conference, August 2001 and IAMG, September 2001 are planned and abstracts submitted.

Publications and presentations in 2000

Kroonenberg, S.B., I. Overeem, N.I. Alekseevski, E.A. Baldina, G.V. Rusakov; Volga delta dynamics during the last 3-m Caspian Sea level (1929-1995), Fluvial Archives Group, March 2000, Mainz, Germany.

Overeem, I., W. van Amstel, G.J. Weltje; AquaTellus; a process-response model of fluvio-deltaic sedimentation. British Sedimentological Research Group Meeting, Abstract Volume, 11-12 September 2000, 36.

Steeghs, P., I. Overeem, S. Tigrek;Seismic Volume Attribute Analysis of the Cenozoic Succession in the L08 Block (Southern North Sea). In press 2001. Global and Planetary Change.

Overeem, I., C. Bishop, G.J. Weltje, S.B. Kroonenberg; The Late Cenozoic Eridanos delta system in the Southern North Sea Basin: a climate signal in sediment supply? In press 2001. Basin Research.

Fluvio-deltaic sedimentation in the North Sea Basin over the Neogene, presentation NEESDI, Den Haag, 29 May 2000.

Numerical modelling of fluvio deltaic sedimentation, invited presentation at INSTAAR, Colorado 17 February 2000

The Late Cenozoic Eridanos delta in the Southern North Sea Basin, presentation at TransCanada, Zoetermeer, 6 October 2000.

Supervised courses and MSc theses

Petroleum geological fieldwork in Huesca, 2000

Amstel, W. van; Development of a 2-D process-response model of multiple grain size sedimentation in a fluvio deltaic environment. 2000.

Bilt, B. v.d.; The architectural evolution of the Late Cenozoic delta lobes in the F09 block, North Sea. In cooperation with G.G. Drijkoningen, section Applied Geophysics and J.Woodside, Section Sedimentology VU Amsterdam. 2000.