A new geochemical logging tool has been designed and developed for the precise determination of formation chemistry, mineralogy, and lithology, as well as the identification of total organic carbon (TOC). The primary elements identified by the system include aluminum, calcium, carbon, chlorine, hydrogen, iron, magnesium, oxygen, potassium, silicon, sulfur, thorium, titanium, and uranium. These elements are utilized to identify the minerals present in both conventional and unconventional formations.
Tool operation begins by emitting high energy 14 MeV neutrons into the formation from a pulsed neutron generator, and the resulting gamma rays are intercepted by a high resolution, state of the art, LaBr3(Ce) detector. In order to exclude background gamma rays and provide a clean capture spectrum, a boron coating has been placed on the housing. The 3.25-inch tool diameter makes the system easier to operate in small boreholes as well as in horizontal wells.
The extensive set of detected elements is made possible by the PNG, where high speed electronics are incorporated to accrue both capture and inelastic energy spectra. A Levenberg-Marquardt matrix inversion algorithm is employed to separate the spectra into their fundamental elemental components. Characterization of the system has been achieved through numerous measurements in more than 30 formations from a newly constructed Rock Formation Laboratory in Fort Worth, Texas as well as at the Callisto Facility in the United Kingdom. A significant number of core samples were obtained from these formations and analyzed for elemental and mineralogical composition. Extensive use of MCNP modeling was exploited for the design and characterization of the system.
The final lithological and mineralogical interpretation is guided by the elemental concentrations of the various elements, as well as the computation of intrinsic sigma. Magnesium is used to differentiate between calcite and dolomite in carbonate formations. Aluminum, iron, and potassium, in addition to silicon, provide the information required to distinguish the various clays in sand/shale formations. Sulfur is vital for the identification of both pyrite and anhydrite. Ternary plots are generated to aid in the final interpretation. To demonstrate the effectiveness of this work, log examples from the field are provided.