Geostatistical Modelling of Heterogeneities in a Mixed Carbonate Platform: The Characterization of Complex Progradational and Retrogradational Mechanisms

Abstract

Abstract In mixed carbonate siliciclastic platforms, deposition is strongly constrained by sea-level fluctuations which induces heterogeneities with complex lateral and vertical sequences. In such sequences, a logical scheme of deposition can be identified over several orders of cyclicity. Although many geostatistical methods are available, very few are suitable to represent simultaneously the three main characteristics of such environments: the order of deposition, the evolution of facies proportions (lateral and vertical), and the correlation length of the facies. In this paper, we present a methodology well suited to reproduce those features; it is based on probability field simulation, facies are obtained by the truncation of a continuous random variable. The truncation thresholds are determined by indicator kriging. The technique has been applied in a practical case: a recursive application of such a treatment allows the characterization of lithofacies and petrophysical classes, in consistency with the geological model and with the distributions of petrophysical parameters. Subsequently, porosity values arc assigned using a multiple sequential gaussian simulation which provides the appropriate porosity (f) distribution for each petrophysical class. Effective permeabilities (K) are then deduced from f-Log(K) correlations which are different from one petrophysical class to another. Finally, a sub seismic fracturation is roughly taken into account by a deterministic anisotropy factor on permeability. In addition, the main fracture axes are represented, after restoration of the actual geometry, using structural maps containing this information. The obtained reservoirs' structure is therefore constrained by all the expected geological and petrophysical properties. Introduction A Geological Context Sedimentation in mixed carbonate siliciclastic environments is mainly controlled by the relative position of sea-level towards the surface of deposition. The vertical distance between those surfaces is called the accommodation space. Its magnitude results from an equilibrium between detritic sediments' supply, subsidence mechanisms, and sea level fluctuations. As these components are gradual in space, they lead to heterogeneous reservoirs characterized by a fairly good lateral correlation, and a typical cyclical order of deposition.