Log-Core Correlations in the Athabasca Oil Sands

Abstract

Accurate bitumen saturations in Alberta's Athabasca tar sands are obtained with fairly routine coring and core-analysis procedures. The paper investigates the applicability, of various log-analysis approaches by comparing logs and cores for several wells. Despite the importance of clay in controlling the bitumen saturation, a simplified Archie model works well in the area studied. Introduction The bitumen content of the Athabasca tar sands is determined primarily from cores, which are easy to obtain because of the limited amount or absence of overburden. As more detailed evaluation programs are undertaken, there are definite economic advantages to substituting logs for cores. While logs have been used satisfactorily for many years in tar-sands evaluation, few quantitative estimates of their accuracy have been published. published.A common practice in log evaluation for engineering studies is to "calibrate" the logs to the core, usually with respect to porosity. In the case of the Athabasca tar sands, the primary core measurement is the bitumen content expressed as weight percent. The validity of that measurement is accepted because of the limited flushing of the extremely viscous bitumen. Because of the unconsolidated nature of the sand, porosity is usually measured incidentally as the sum of fluids. This creates a need to relate the usual terms of log analysis, expressed as volume percent, to the bitumen measured as a weight percentage. percent, to the bitumen measured as a weight percentage.Two additional conditions complicate the correlation problem. The influences of shale on both core and log problem. The influences of shale on both core and log measurements can be profound. Clay is present in the formation and controls, to a large degree, the bitumen saturation. Evidence of variations in water resistivities in the area of interest became apparent during this study. The latter phenomenon can only be inferred because water-bearing zones are not common and data from measured samples are rare. The main purpose of this study was to estimate the accuracy of the common log methods. This required the development of correlative procedures to handle the factors mentioned. Saturations as measured in cores were compared with those calculated from logs to fulfill the primary objective and to validate the developed primary objective and to validate the developed correlations. The indicated accuracy suggests that further improvement is required. Whether the improvement takes the form of a new tool or a different analysis model, its performance can be measured against other methods. performance can be measured against other methods.Some additional information about the tar sands and the correlation technique that was used is provided before the quantitative comparison is presented. Background Information With the exception of a paper by Fetzner et al., there is little in the literature that deals with logs and core analysis in the Athabasca tar sands. The subject is addressed in documents submitted to the provincial government in support of recovery schemes. Papers dealing with geological aspects are more plentiful in the literature. Athabasca Oil Sands Reserve The Athabasca deposit shown in Fig. 1 is the largest of the heavy-od deposits that occur in northern Alberta and that, as defined in the Oil and Gas Conservation Act of Alberta, contain oil "not recoverable in its natural state through a well by ordinary production methods." JPT P. 1157