Log Interpretation Problem in Low Resistivity Sands

Abstract

Abstract All recent investigations have attributed low electric log resistivities in producing formations to a conductive ion layer associated with clay material present in the sands. In this study another mechanism is proposed. It is believed that connate water saturation variations (which may or may not be associated with shale contact) also contribute to low resistivities. In addition to the mechanistic studies, evaluation of various recently proposed interpretation methods have been made. Of these, the Poupon, Loy, and Tixier and de Witte methods seem best. Statement of Problem Low resistivity sands are often encountered which turn out to be commercial hydrocarbon zones but appear from electric log data to be water bearing. Adequate methods of recognizing these cases are needed. Industry-wide interest in this problem is evidenced by the number of papers published in recent years. It seemed that several zones in the Mustang Island field (Nueces County, Tex.) were affected by the shaly sand problem. This was evidenced by:low hydrocarbon saturation calculations from logs and measured formation factor data in producing zones,changes in apparent measured formation factors with changes in water resistivity, andcore descriptions which indicated the presence of shaly "streaks" or "spots." A section of a typical log is shown in Fig. 1. All sands on this log are hydrocarbon zones. Scope of Investigation The Mustang Island field, Nueces County, Tex., has been chosen for the study of the shaly sand problem because the oil-water contacts are well established for most of the sands and some core data were available for study. This problem has been approached by both empirical and experimental methods. The empirical portion of this study is a comparison of calculated hydrocarbon saturations from logs by various methods with expected or known production (oil, gas, or water).