Log Analysis of Sand-Shale SequencesA Systematic Approach

Abstract

Poupon, A., Poupon, A., Schlumberger Technical Services Clavier, C., SPE-AIME, Schlumberger-Doll Research Center Dumanoir, J., Schlumberger-Doll Research Center Gaymard, R., Schlumberger Technical Services Misk, A., SPE-AIME, Schlumberger Well Services A primary feature of this interpretation method is that it uses all the logging data in a coherent manner. The formations are analyzed for clay, shale, quartz, water, hydrocarbon content, and changes in sand-grain mineralogy. Even where hole conditions are adverse, reliable results can be achieved through extensive crosschecking for likeness. Introduction During the past 7 years methods have been developed for interpretation of clean and shaly sands using combinations of sonic, density, and neutron logs, along with resistivity and auxiliary logs. Corrections for the presence of light hydrocarbons in the formation, which affect the log readings of the sonic, density, and neutron logs, were also developed. For these methods the shale parameter values are either assumed or deduced from the log readings in adjacent shale beds. This is satisfactory as long as the shales have uniform properties. Frequently this is not so, and the log properties. Frequently this is not so, and the log analyst is faced with determining the properties of the shales occurring in the shaly sands. Intuitively it would seem that sands and shales deposited in sequence during a continuous sedimentation cycle should possess logging properties related to their common geological background. The study of neutron and density logs made in sand and shale sequences has substantiated this. From this study a conceptual model of shales and shaly sands has evolved that is consistent with geological considerations as well as logging tool responses, and in turn has led to the interpretation method described here. The method makes maximum use of all the following logs: neutron, density, resistivity,* gamma ray, SP, microresistivity, sonic, and caliper. Some of these logs may be omitted (microresistivity, sonic, caliper, and either SP or gamma ray); however, doing so decreases the reliability of the results. Formation clay content is determined from several clay indicators and the formation is analyzed for shale, quartz, water, hydrocarbon content, and changes in sand-grain mineralogy. In favorable cases an estimate of hydrocarbon density is made. The associated computer program, SARABAND, solves the interpretation, crossverifies the input data and results and determines automatically many of the required parameters. The interpretation results of main interest are listed in special tabulations and displayed on a film especially coded for easy identification. The Sand and Shale Models Fig. 1 is a neutron-density frequency crossplot generated by a computer. The plotted values of andD, are the apparent porosities from neutron and density logs. Each of the one- or two-digit numbers on the plot represents the total number of readings, over a 390-ft interval in a sand-shale sequence, having the values of N and D, corresponding to the location of the number. The distribution shown on Fig. 1 is typical of sandshale sequences. JPT P. 867