ABSTRACT
This paper gives the geological aspect of a 3-D study on fluvio-deltaic sedimentary bodies and corresponding 3-D conditional simulations obtained by geostatistical methods. It is the continuation of paper SPE 16752 and 16753 (Dallas, 1987).
It describes the new sedimentological interpretation of the Scalby formation (U.K.) from the core-drill data, the different correlations performed between the core-drills, the quantification in terms of proportion curves and of variograms of lithofacies sets – and at last some conditional simulations.
The image of the cliff is described with the topographical corrections. This new image allows establishing the true correlation with the eleven core-drills located just behind the cliff, and to carry out the accurate structural analysis – in the geostatistical sense of the term – required for the simulations.
Henceforth, the core-drill study shows that the evolution of the system and the deposit environment are somewhat different from those deduced from the field data. Therefore, there exists a marine influence in the basal terms, and the unconformity separating the major units is significant and probably regional. However, the field data remain essential for the geostatistical study. The upper unit is composed of multistoried and highly sinuous channels deposited in a typical fluviodeltaic environment. The lower unit is principally fluvial-dominated but it has been influenced by the tides. The well-to-well correlations show that the overall geometry is of a sand-sheet type. The intervals are more or less of the same thickness; this fact, together with the low rate of sedimentation and the low thickness/lateral extension ratio of the terms suggest low subsidence and high lateral divagations of the distributaries.
Correlations in terms of 1ithostratigraphy, lithology, porosity and permeability have been made between the core-drills in the grid where the distance was progressively reduced. The illustrations from the correlation; given in this paper emphasize the lateral variations in several parameters (facies, petrophysical…) between wells in the same lithostratigraphic unit over short distances.
Interpreted geological data have been digitised and then quantified. This quantification is represented by vertical proportion curves and by variograms which are the input parameters for the calculation of the simulations.
2 and 3-D conditional simulations of the studied reservoirs are carried out and then analysed. These random set conditional simulations are obtained by an original method elaborated by our IFP-CG group.
In this paper, we show the influence of the input parameters on the results. These parameters have been relatively easily fitted due to a large amount of information. The obtained simulations then display an aspect very similar to that of the digitised cliff. The horizontal ranges have an influence over the continuity restoration. Thus a large range gives a strong continuity of the simulated image, while a short range reduces the size of the simulated sedimentary bodies.
The ranges are calculated and then fitted using available data. When these data are insufficient, other "a priori" information is introduced.
The vertical proportion curves of the different facies plays an important part in the lithofacies distribution. These curves are easily calculated from the core-drills. The fastness of the simulation calculations allows testing several hypotheses, and various sedimentological concepts with regard to the geometry of bodies to be simulated.
