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Abstract Since currently producing hydrocarbon reservoirs can no longer supply the world's energy needs, the search for new sources has led to another look at formations that have posed difficult evaluation problems. Several formations encountered in the Rocky Mountain region are in this category. In the past, drilling or evaluation techniques could not accurately indicate productive and nonproductive zones. Three of the productive and nonproductive zones. Three of the stratigraphic units which have proven difficult to evaluate with conventional surveys are the Niobrara, Mowry and Phosphoria. Problems in evaluating these formations can result Problems in evaluating these formations can result from several factors. Most can be related to determining the true clay content for accurate lithology and resultant porosity determinations, the location of fractured or fracturable zones, and evaluating formations that contain a low or unknown salinity formation water. Recent developments in wireline logging instruments provide measurements which enable these and other provide measurements which enable these and other sequences with similar problems to be correctly evaluated. These methods rely on the spectral analysis of natural and induced gamma rays. Two commercially available instruments that provide these measurements are the Spectralog and Continuous Carbon/Oxygen Log. This paper will present the theory of operation and parameters measured with these instruments. parameters measured with these instruments. Applications of these measurements will be discussed, with field examples showing how these measurements provide an improved evaluation of these formations. Introduction Recent changes in the energy situation have necessitated a more refined approach to the evaluation and often re-evaluation of potentially productive hydrocarbon zones. Much of the exploration today consists of re-examining some of the formations that have proven hard to evaluate in the past, and checking proven hard to evaluate in the past, and checking old wells for bypassed hydrocarbons. "Hard to evaluate" infers that these formations did not have the expected response on available wireline surveys or did not conform to conventional methods of interpretation. Most logging instruments measure certain parameters and then the Analysts use empirical relationships to arrive at the desired evaluation. Many available surveys are a compromise between attempting to obtain the desired parameters while using instruments designed only for normal drilling conditions. However, recently introduced methods of measurements and evaluations have proven successful in evaluating some of the more complex formations, such as those found in the Rocky Mountain region. Two such developments are the Spectralog and the Continuous Carbon/Oxygen Log instruments. The Spectralog provides a spectral analysis of naturally occurring provides a spectral analysis of naturally occurring gamma rays, while the Continuous Carbon/oxygen Log survey provides a spectral analysis of induced gamma rays. Gamma rays are electromagnetic waves produced by unstable radioactive elements as the produced by unstable radioactive elements as the atoms are undergoing spontaneous or induced transformations. One of the benefits of measuring gamma rays is that they can be measured equally well in cased holes or in open holes. The measurement of natural radioactivity has long been used and proven valuable in formation evaluation. While gamma ray measurement is used extensively as a method of correlation, it is also one of the most universally accepted clay indicators. Earlier gamma ray measurements indicate the concentration of all the radioactive elements without distinction to the type of source element. The most beneficial use of natural gamma rays has been identification of lithologies which could be potentially productive. However, there are several formations in which the lithology cannot be interpreted using the total natural gamma ray measurement. These formations are usually referred to as radioactive dolomites or limestones, radioactive sands and often fractured shales. The Spectralog provides the measurements necessary for the evaluation of such lithologies. |
