A Computerized Method for the Zonation of Digitized Well Logs
Authors: Souder W.W. , Pickett G.R. 
Published: December 2007

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Approximating well logs by zones corresponding to stratigraphic units of the sedimentary section is the first step in development of computer programs to extract more information from log data. Advantages of computer zonation are:

production of subsurface stratigraphic sections more rapidly and economically by permitting computer correlation of logs from well to well in an area, automatic comparison of logs in the same well to adjust depths and reduction of computation time as compared to point-wise manipulation of unzoned data.

A technique has been developed to pick zone boundaries on digitized well logs by pick zone boundaries on digitized well logs by computer. This method consists essentially of searching a smoothed version of the well log for large changes in slope (i.e., searching for extrema in the derivative of amplitude with respect to depth). Examples are presented for the induction, FDC, gamma ray, and SP logs.

Introduction

It has been common practice in well log interpretation by manual techniques to employ a "zonation" process. This process consists essentially of selection of boundaries that divide the log into intervals, within each of which the measurement being considered is relatively constant compared to the change in the measurement from one interval to the depth-adjacent intervals. The intervals so defined are called zones.

For example, consider the schematic representation of a well log in Fig. 1. The log consists of some measurement, y, displayed as a continuous function of z. In practice, z might be depth and y one of several dozen measurements (resistivity, neutron response, density, etc.). A log interpreter might (by visual inspection) select zone boundaries as shown (A, B, C, etc.).

In well log interpretation, zonation is a natural and useful process. Sedimentary sections tend, of course, to be layered and a well log is a reflection of some measurable property of these layers. Proper zonation property of these layers. Proper zonation defines the boundaries between the layers and makes possible the attendant geological applications (well-to-well correlation, subsurface structural studies, isopach maps, etc.). In addition, the well log interpreter uses the measurements for such applications as porosity, water saturation and hydrocarbon volume estimates. These applications require the selection of a value of y to represent the interval, say, DE. The average value of y within the interval (y in Fig. 1) is often used. However, if the y in Fig. 1 were resistivity, for example, the maximum value of y, yp, might still be smaller than the actual value of resistivity because of thin-bed effects on the logging system.