Low Silicate Concentrations Accurately Quantified in Carbonates Using Combined Outputs from Geochemical Well Logs
Authors: Bradford Charles M. , Clerke Edward A. , Pemper Richard R. , Mendez Freddy E. , Longo John M. , Bruner Michael W. 
Published: September 2011

Formation mineralogy is fundamentally important to the proper characterization of carbonate reservoirs. Knowledge of the dominant calcite and dolomite components, as well as the anhydrite evaporitic precipitate component, is important to the assessment of carbonate petrophysical properties. Core samples have always provided an excellent but time consuming source for carbonate formation mineralogical data. Now such information can be obtained — continuously and rapidly in carbonate formations — using modern nuclear logging tools that incorporate the principles of natural and induced gamma ray spectroscopy. The calcium, carbon, magnesium, and sulfur elemental concentrations — from the inelastic and capture spectra of the pulsed-neutron spectroscopy tool — provide the data required to compute formation quantities of calcite, dolomite and anhydrite.

Accurate quantification of low concentrations of the quartz, illite, feldspar, and mica (QIFM) silicate mineral phases — present in carbonates as a result of windblown clastic influx — is important for carbonate net pay delineation and time stratigraphic layer models. The dilute QIFM is indicative of aerial exposure-related carbonate diagenesis and low sedimentation rate carbonates, which may be porous but impermeable. Core studies of important carbonate sections obtained and previously published (Clerke, 2009; Clerke et al. 2008; Clerke, 2004; Kamal et al., 2004, Clerke, 2003) show that the amounts of aluminosilicate material within these carbonate formations are typically less than 10%, but that important carbonate textural changes are associated with small variations in the abundance of QIFM.

This work investigates the combination of the FLeXSM (Formation Lithology eXplorerSM) geochemical outputs of silicon, aluminum and iron — coupled with the potassium output from the natural gamma ray spectroscopy tool. The combination of these elemental outputs provides a high accuracy and high precision QIFM abundance at low concentrations in carbonate formations. Within typical carbonate formations, this combination is shown to provide data quality and accuracy over the range of 1-5% QIFM with a statistical precision of less than one percent. This is demonstrated using computer simulations of the FLeX tool performance, in addition to measurements in numerous laboratory carbonate-block formations. The mineralogies of the test-block formations are obtained by using XRD and XRF analysis. The computer simulations of tool performance use Monte-Carlo generated elemental concentrations that are then converted to mineralogy, using the software employed in well log processing of subsurface FLeX data.

This paper demonstrates the ability of the tools’ combined elemental signals to provide accurate and precise volumes of low concentration (1-5%) QIFM silicate material in a predominantly carbonate formation. This ability is an important and economically-leveraging technical development, with application to the carbonate reservoir quality modeling of the Middle Eastern Khuff gas-bearing formation, in addition to other Permian age formations in the West Texas basins. Accurate identification and quantification of the QIFM mineralogical component of the rock matrix provides an important input for refined models of carbonate reservoir quality, diagenesis and aerial exposure, as well as serving as a utility for time marker beds in stratigraphic reservoir models.