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Abstract

Biostratigraphical resolution has increased markedly over the past few decades. The use of origination and extinction data has been supplemented, particularly at the reservoir scale, by more subjective local ‘bioevents and biozones’ in the form of successions of unique abundance fluctuations of individual taxa and distinct faunal/floral assemblages. Herein, a methodology is described that uses a sample-by-sample multivariate similarity matrix of biostratigraphical census data to define a high-resolution correlation path between two wells. Three different similarity/dissimilarity metrics are used to calculate the matrix (cosine-theta, and Euclidean and Manhattan distance) for each of three transformations of the data: raw data (counts), presence-absence, and percent-range transformation. Recommendations are presented for use of various metrics and transformations, given different types of data sets and the objectives of the analyst. The correlation path through the matrix is monotonic and must increase in depth in one or both wells. The correlation path moves one sample-pair or matrix element at a time to an adjacent element that has the highest average, local path. After an adjacent element is selected it becomes the current state and the algorithm repeats the process until the matrix has been transited. The correlation path can be constrained using a priori information, if available (e.g. maximum flooding surfaces). Such events are established as tie-points, and the correlation path is required to go through these points. Additional steerage of the correlation path may be accomplished interactively, by graphical analysis of individual taxon abundance plots, followed by establishment of additional tie-points. The method also includes calculation and graphic representation of a 95% confidence interval about the calculated faunal/floral percentages. This enables the biostratigrapher to take into account the statistical confidence one has in a peak faunal abundance. The method is demonstrated on palynological data from two wells from the Blake Field, Outer Moray Firth, UKCS.

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