A methodology to rescale the vertical coordinate into geological time using a geostatistical analysis of log data

Abstract : The aim of this paper is to outline a methodology to transform depth intervals into geological time intervals. Working in a geochronological system aids in the understanding of the physical properties of rocks and to do very precise correlations between wells. To transform the present coordinates a high resolution tool is necessary : the FMI® (Fullbore Formation MicroImager, Schlumberger). First, well log correlations are established between seven wells at different scales. Relative variations of sedimentation rate are observed locally. Second, FMI® images analysis are used to reveal clear carbonate - clay interbedding which display cycles. Third, to study these cycles, geostatistical tools are used. The variographic analysis of conventional log data shows cycles of one meter. With FMI® data, smaller cyles can be detected. Variogram modelling and factorial kriging analysis suggest that three spatial periods exist. They vary vertically and laterally in wells but cycle ratios are stable and similar to orbital cycle (Milankovitch cycle) ratios. The three periods correspond to eccentricity, obliquity and precession. If orbital cycles are known, depth intervals can be converted into geological time intervals and thus give real sedimentation rates and quantification of hiatus duration.