Formation Evaluation of Some Limestone Reservoirs With Particular Reference to Well Logging Techniques

Abstract

Introduction Evaluation of carbonaceous rocks as to their productive possibilities was for many years virtually impossible from electrical logs. True, many carbonaceous formations had sections of sufficient homogeneity and thickness that methods of interpretation normally associated with sands could be used with a fair degree of success. Unfortunately, the formations having the proper characteristics for such methods were the exception rather than the rule; hence, the log found little use except as a correlation tool. In recent years many new logging tools have been presented to the industry with the result that under proper conditions it is often possible to arrive at a satisfactory interpretation of the carbonaceous beds traversed by the drill bit. The so-called "proper conditions" are the key to a successful approach to any logging program or interpretation. However, it is not to be concluded that in every instance the conditions can be controlled or anticipated to the extent that the logs obtained will lend themselves to a simple interpretations Too often the combination of Mother Nature and the limitations of even the latest well logging equipment will result in a log from which the logging experts can do little more than make an educated guess as to its interpretation. The theory of the various logging devices and quantitative interpretations has been amply covered in the literature. This discussion will concern itself with the more common type of carbonaceous reservoir rocks and approaches that may be made toward their evaluation by means of various logging techniques. Viewed strictly from a logging standpoint, the depth, lithology, porosity, fluid content, borehole diameter variations, and formation dips are the parameters that may be determined. Those that cannot be determined are permeability and, to a degree, the depth of invasion of the mud filtrate and the physical characteristics of the rock. All of these have been discussed in numerous papers; however, their importance to carbonate formation evaluation is such that it is felt time will not be wasted in reviewing some of the more salient points as related to the examples to be discussed.