Simulation de la sédimentation dans un bassin évaporitique à niveau d'eau sous influence eustatique. Application au bassin paléogène de Mulhouse (Alsace, France)
Simulation of Sedimentation in an Evaporitic Basin At Water Level under Eustatic Influence. Application to the Paleogene Mulhouse Basin (Alsace, France)
Institut Français du Pétrole
2 Institut Français du Pétrole-MNHN
Le programme SIMSALT permet, à partir de fonctions sinusoïdales simulant des variations climatiques et eustatiques, de modéliser la sédimentation dans une maille élémentaire d'un bassin évaporitique en communication restreinte permanente avec l'océan, les arrivées d'eau marine se faisant par l'intermédiaire d'un seuil topographique. En fonction des salinités calculées, le programme détermine s'il y a précipitation de sels ainsi que les épaisseurs sédimentées. Les principaux résultats fournis sont représentés par des colonnes lithologiques en fonction des épaisseurs cumulées et en fonction du temps. Ce programme renferme également un module permettant de tester des fonctions simulant la production-dégradation de la matière organique. Ce modèle a été appliqué à une partie de la série évaporitique d'âge Eocène supérieur - Oligocène inférieur du bassin de Mulhouse. L'application du modèle aux cyclothèmes carottés du sondage Max montre que la sédimentation évaporitique et organique peut être contrôlée principalement par des variations climatiques résultant de la somme de trois sinusoïdes, respectivement de période de l'ordre de 13000, 500 et 100 ans, associées à des variations eustatiques de période 13000 ans déphasées par rapport aux variations climatiques de même période. Avec le modèle SIMSALT, il est donc possible de reconnaitre les influences respectives de l'eustatisme et du climat sur la sédimenation évaporitique et organique dans un bassin en communication restreinte avec le domaine marin.
The SIMSALT program takes sinusoidal functions simulating climatic and eustatic variations and uses them to model sedimentation in an elementary mesh of an evaporitic basin in permanent restricted communication with the ocean, with arrivals of seawater being determined via a topographic threshold (Fig. 1). For this, the mesh is assumed to be made up of several superposed elements (Fig. 2, i. e. a lower body of salt water (salt saturated), an upper body of salt water (with varying salinity) and sometimes at the surface a body of low-salt water linked to the supply of continental water. The lower body of salt water, called captive brine , is defined as the water layer underneath the threshold. Its height during time depends on the difference between the depth of the sediments deposited and the subsidence. We will assume that the depth of the basin (and hence the subsidence) is always sufficient for the captive brineto be able to exist. The upper body of salt water, called active body of water , is defined as the salt water layer located topographically above the threshold. It is in communication with the transfer zone or the open marine domain/basin . Its height during time thus depends on eustatic variations. The body of low-salt water corresponds to a body of pellicular water whose existence is defined by the presence or absence of influxes of fresh continental water in the mesh. On the scale of the basin, this body of low-salt water has a geographic extension that is governed by the regional hydrologic balance (influxes of continental water and evaporation). It may not exist at the level of the mesh considered, and its presence at the top is linked to the very strong contrast of density between the low-salt water and the underlying brines. In the model, sedimentation as a function of time is governed by the alternating passage of the regional hydrologic balance between the two states:(a) State 1 ( dryperiod) in which the volume of continental water supplied to the top of the mesh is lower than the volume of water evaporated. There is no body of low-salt water, and sedimentation is evaporitic. (b) State 2 ( wetperiod) in which the volume of continental water supplied to the top of the mesh is greater than the volume of water evaporated. A body of low-salt water exists at the top of the mesh, and sedimentation is shaly. The duration of each stateand the regional hydrologic balance during the two types of sedimentation are governed by the sums of sinusoidal functions. The program computes the contents of calcium sulfate, sodium chloride and potassium in the water in the mesh as a function of the hydrologic deficit or excess. Depending on the salinities computed, the program determines whether salts were precipitated as well as the sedimented thicknesses. The main results obtained are represented by lithologic columns as a function of the cumulative thicknesses and of time. This program also includes a module for testing the functions simulating the production-degradation of organic matter. The distribution and origin of organic matter depend on variations in organic production, on the rate of degradation of the organic matter and on its dilution by sedimentary influxes and chemical precipitates. By checking the lithology, the model can be used to determine the climatic and eustatic factors compatible with the sedimentary recording. It thus gives access to accumulation rates. When these rates are known, it thus becomes possible to test the validity of the assumed relations between productivity, degradation, climate and eustatism by comparing the results provided by the model with geochemical data. This model has been applied to part of the Upper Eocene-Lower Oligocene evaporitic series in the Mulhouse basin (France, Fig. 3). This part of the series (Sel IV-Sel V) corresponds to a succession of cyclothems formed by an alternation of sometimes potassic halite beds and argillo-anhydritic beds (Fig. 4). These beds were themselves formed by an alternation of evaporitic and shaly horizons. The organic matter, which is present solely in the shaly horizons, reveals a rhythmic vertical evolution (Fig. 5). The applications of the model to cored cyclothems from the Max borehole shows that evaporitic sedimentation can be controlled (Figs. 7-8) by :(a) Climatic variations resulting from the sum of three sine curves with respective periods of 13,000, 500 and 100 years. (b) Eustatic variations with a period of 13,000 years, out-of-phase in relation to climatic variations having the same period. (c) Variations having a longer period (about 26,000 years), which can be used to account for the vertical evolution of the cyclothems. To the scale of Sel IV and Sel V, a drift for the eustatic function must be added, corresponding to the regressive evolution of a third-order sequence, as well as a climatic drift to account for the evolution of the climate during the Oligocene. Tests performed on the organic productions corresponding to the cored interval show (Figs. 10-11) that :(a) High organic-matter production could have existed only during State 2. (b) The volume of continental organic matter is proportional to the influxes of fresh water. (c) The production of phytoplanktonic organic matter is limited to the fresh-water bodies and to the active-water bodies (which are situated in the euphotic zone). (d) Phytoplanktonic production in the active-water body depends on eustatic variations and on influxes of fresh water. (e) Production of bacterial origin is mainly situated in the two bodies of salt water and remains slight. With the SIMSALT model, it is thus possible to recognize the respective influences of eustatism and climate on evaporitic and organic sedimentation in a basin in limited communication with the marine domain.
© IFP, 1992