Estimation of Productivity of Lobo 6 Sand, Lower Wilcox, Texas, by Identifying Diagenetic Clays With Well Log Data

Abstract

Summary The laminated, structural, and grain-coating primary clays in the Lower Wilcox Sand, Lobo 6, tend to be potassium-bearing illite and some smectite, whereas diagenetic clays, the cause of pore-throat plugging, are largely potassium-deficient kaolinite and chlorite. These diagenetic clays largely control reservoir permeability, depending on the degree of pore-throat plugging. This paper presents different methods for qualitative and quantitative evaluation of these diagenetic clays, which are related to productivity of the Lobo 6 sand. Qualitative evaluation consists of crossplot techniques using natural spectral gamma ray and other log data plus selected computed reservoir parameters. For quantitative evaluation, a sophisticated clay-analysis program that uses log-derived cation exchange capacity (CEC) and hydrogen index (HI) values is used. The average volume of different clay types in the zones of interest is calculated, presented, and correlated with production and core data. This approach was used on eight wells on which production data are available to evaluate the presence of diagenetic clays and to estimate productivity of the Lobo 6 sand. The reliability of the log-derived method applied is verified by the excellent correlation with actual production data.