Geochemical Logging of a Middle East Carbonate Reservoir

Abstract

Summary Geochemical logs run through a complex carbonate formation yield accurate elemental concentration logs. These are instrumental to the computation of detailed formation mineralogy and derivative properties, such as matrix density and sigma. The quantitative mineralogy logs also are used to estimate permeability empirically with a correlation observed from core data. Introduction The Utility of geochemical logging was evaluated in a Middle East reservoir described as extremely complex and heterogeneous in terms of its degree of dolomization, argillaceous content, secondary porosity, and porosity distribution. Geologists, reservoir engineers, and petrophysicists working in this reservoir identified well-to-well correlation as a problem, and for log interpretation, they specifically cited problems with the accuracy of (1) porosity determination, (2) degree of dolomitization, (3) characterization and quantification of clay minerals and clastics, and (4) permeability estimations. It appeared that these problems were interrelated, and it was anticipated that, by providing a detailed interpretation of the rock composition, geochemical logging might aid in geologic characterization, petrophysical interpretation, and ultimately, interwell correlation. The objectives of the study were to determine the quality of the primary geochemical interpretation products (elements and minerals) and to investigate the effects these answers had on geological and petrophysical interpretation. The study was composed of a combined logging and core analysis program. The pertinent core analysis was divided into two parts, with standard petrophysical measurements of grain density, porosity, and permeability made on plugs spaced approximately at 1-ft intervals throughout the reservoir, and a subset of samples analyzed for chemistry and mineralogy. The logging program included the Geochemical Logging Tool (GLTSM), which measures elemental concentrations of aluminum, silicon, calcium, iron, sulfur, gadolinium, titanium, thorium, uranium, potassium, and magnesium.1 In addition to a full suite of resistivity and nuclear logs. Results show that the geochemical elemental concentration and mineral concentration logs describe the formation with an accuracy that could not be obtained previously without extensive core analysis. The new logs clearly delineate the zones of dolomitization and quantitatively interpret the amount of noncarbonated minerals. The mineral logs were used to compute a continuous matrix density, which was in turn used to determine porosity. The results of the mineralogy and porosity interpretations are compared directly with core measurements and also with the results of a volumetric log interpretation that does not include geochemical data. The elemental concentration logs and mineral concentration logs were used together to derive matrix sigma and apparent fluid signs, which were in turn used to estimate near-wellbore saturation. Finally, the core and log data also were used to illustrate a relationship between mineralogy and permeability. The result is an accurate formation description in terms of composition and petrophysics, which should enhance correlation and reservoir evaluation. The interpretation is a scientifically based analysis that does not require numerous log-analyst picks or a large degree of regional modification. Instead, the elemental concentrations are transformed through a fixed composition matrix into mineral concentrations. Many of the petrophysical components, such as grain density and matrix sigma, are natural consequences of the mineralogy, and therefore, can be directly computed from the mineralogy. Other properties, such as permeability, can be more affected by diagenetic events and fracturing. Some diagenetic events, like dolomitization, are implicit in the mineralogy, and in this case, the degree of dolomitization can be used to estimate the permeability.