The Contribution of Logging Data to Sedimentology and Stratigraphy

Abstract

Abstract Until recently, the lithology and sedimentology of formations penetrated by a well could be studied only through the analysis of cores. Now a technique has been developed using multivariate analysis of logging data to give a high-resolution sedimentological description of any sequence of formations. The number of different logs and their range allow determination of many of the physical characteristics of the rock. The definition of a lithofacies has been extended by introducing the concept of an electrofacies, constructed on the basis of all the logging data at any depth interval. Thus, each logging datum is considered a descriptor for purposes of establishing electrofacies in a logged interval. Once established, electrofacies then can be correlated with actual geologic facies, if the logged interval has been cored. Introduction The evaluation of the potential and performance of hydrocarbon reservoirs includes the study of the sedimentary series in which they are found. Essential tasks to be performed during such a study includedescription of the rock facies and the relationships between facies,determining the geometry of the sedimentary bodies,reconstruction of the vertical column and analysis of sequences and cycles of facies types, andan accurate estimation of the common petrophysical parameters (porosity, permeability, etc.). The sedimentary environment in which deposition took place and diagenetic changes that may have occurred since are inferred from the data. There are several possible sources of information for the sedimentologist, including surface outcrops, surface geophysical measurements. cuttings obtained while drilling cores, wireline core samples and wireline log data. While the study of surface outcrops always has been useful, it has become less important as the depth of drilling has increased and more importance is placed on the analysis of deep-seated structures such as faults and traps. Cuttings obtained during drilling are a valuable source of information. However, there is always some depth uncertainty, and, of course, the samples obtained are not large enough for accurate measurement of porosity, permeability, and so on. Additionally, some samples are lost because of their size (silts) or by dissolving in the drilling mud (salts), and there is often a problem of cuttings falling in from above the bit (shales or sands). Cores, on the other hand, provide an excellent source of information since their depth can be located accurately and they are large enough for analysis. However, coring is an expensive process and is sometimes impossible because of safety reasons or formation conditions. For these reasons, extensive coring usually is not done, even in exploration wells. With these constraints on sample data, the trend in recent years has been toward the use of wireline log data, not only to predict general petrophysical parameters but also as a tool for sedimentologists and reservoir engineers. Toward this end, a number of wireline tools (e.g., LDT TM, NGS TM and GST TM) and computer programs (e.g., GEODIP TM) have been developed, and more recently, as outlined in this paper, detailed information on sediments has been extracted from log data. An advantage of logs is that they usually cover the entire interval of interest and at a sampling rate providing exceptional detail. Logs often can be obtained in conditions where coring is impossible, and they are cheaper, all costs considered. A combination of (short) cored sections and log data gives the sedimentologist an excellent information base. We consider the referencing of log data against cores later. SPEJ P. 117^