Applications Using Geochemical Logs

Colson, J.L., Schlumberger Well Services; Grau, J.A., Herron, M.M., Herron, S.L., and Schweitzer, J.S., Schlumberger-Doll Research; Hertzog, R.C.* and Preeg, W.E.,* Schlumberger Well Services; Nurmi, R.D.,* Schlumberger Middle East ; and Johnston, J.,* Schlumberger Overseas Abstract Elemental logs can be combined with mineral models to provide significant information for geological description provide significant information for geological description and evaluation of downhole formations. Mineral compositions from geochemical logs can be applied to a variety of lithologies, stratigraphic settings, and geological ages including those of the Middle East. The most abundant element concentrations in the rock forming minerals are obtained from the Geochemical Logging Tool (GLT). This tool measures natural, activation, and prompt neutron capture gamma-ray spectra. In addition to the traditional potassium, thorium, and uranium from natural gamma-ray spectrometry, a direct measurement of aluminum concentration is provided. A geochemically-based closure model is used to derive silicon, calcium, iron, sulfur, gadolinium, and titanium concentrations. The concentration of these elements, expressed in weight percent, significantly improves identification of mineral percent, significantly improves identification of mineral composition, and allows the estimation of important formation properties — an excellent basis for a broad range of applications useful to petroleum engineers, geologists, and petrophysicists. Introduction Three gamma-ray spectrometers have recently been combined into a single logging tool string to obtain elemental concentrations as a continuous function of depth for ten elements: Si, Ca, Fe, S, Ti, Gd, Al, Th, U, and K. This suite of elements has been shown to provide valuable information on the nature of the rock matrix, such as a detailed mineralogical description, sandstone classification, source rock evaluation, better porosity determination from logs, cation exchange capacity, grain-size distribution, and permeability. Much of this information could only be obtained previously through discrete core analyses. The Geochemical Logging Tool (GLT) includes measurements of natural activity for determining Th, U, and K concentrations, measurements of delayed activity induced by thermal neutron capture for determining Al concentration, and measurements of prompt gamma-rays following thermal neutron capture for determining Si, Ca, Fe, S, Gd, and Ti concentrations. While a fairly complete overall description of this tool has been presented previously, this paper provides a focus on the potential applications of geochemical logging in typical Middle East oil fields. Three examples are included to illustrate the broad range of geochemical applications:our first example of geochemical elemental concentration logging in the Middle East, taken from a clastic environment in south Oman;a second clastic example from sediments in California, similar to the problem shales that run from Egypt through the Arabian Platform into Oman and Yemen, that demonstrates detailed applications which can be derived from elemental concentration logs; anda third example from a major oil producing carbonate formation in North America that is typical of many carbonate reservoirs in the world containing a complex mixture of minerals such as those of the Arab, Khuff, Eocene of the Arabian Platform and the Kirkuk field of Northern Iraq. P. 365