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Sedimentary rocks can be described an distinguished from others not only by their lithology, geometry, sedimentary structures, and fossil content but also by their overall response on the electric logs--their "electrofacies." FACIOLOG processing zones together those sections of a lithological sequence which have a comparable suite of electric log responses to give an electrofacies zonation.
Geologists have traditionally used electric logs as a basis for zonation when drawing up composite (lithological) columns. FACIOLOG processing now offers the potential of treating all the logs, including the dipmeter, on an objective and quantitative basis. The electrofacies zonation is made by cross-plotting all the log responses on a multi-dimensional set of axes and then using cluster analysis to identify locally dense areas. Each cluster represents a series of depth intervals with a similar suite of log responses (an electrofacies). The degree of similarity of the various clusters is then expressed in the form of a dendrogram, and the complete well section is displayed with each level assigned to its own particular electrofacies.
How closely does the "electrofacies zonation" correspond to the more conventional lithofacies zonation? Generally there is good agreement because electric logs, especially with new services such as the litho-density and natural gamma ray spectrometry tools, respond to the basic mineralogy of the rock matrix as well as the fluid content. FACIOLOG processing also incorporates the high resolution information from the dipmeter, which corresponds to the basic sedimentology.
Because of the usually good match between the electrofacies zonation and the lithological zonations in cored sequences (especially in siliclastic sequences), FACIOLOG processing can be used to extrapolate the results of core analysis into those sections in the well where there is no core.
A new 22 in. (56 cm) wide presentation format allows all the logs, the dipmeter curves (with GEODIP or CLUSTER tadpoles), and the electrofacies zonation itself to be used as a basis for integrating all the information acquired when a well was drilled. Lithological descriptions from cuttings and cores or other stratigraphic information can easily be integrated onto the flexible format. Presenting all the logs together and having the additional advantage of the quantitative electrofacies zonation is clearly an aid to well-to-well correlation. With data banks, specific electrofacies zones may now be automatically traced across
Identifying zones with a consistent and continuous log response greatly aids manual log interpretation. It also provides a valid means of data reduction for the first passes of a computer-processed interpretation because an interpretation model may be tested by treating a limited, yet representative, number of points. Cross-plot interpretation can also be simplified by using the electrofacies type on the z-axis, and displaying averaged log values for each electrofacies.
The FACIOLOG approach is designed not to compete with conventional facies analysis but to put electric logs into a framework which the geologist can easily integrate with his own studies and thereby squeeze the maximum amount of geological information out of them.
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