IAMG 2001--Cancún Technical Program--Session J

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Applications of SpectroLith Quantitative Lithology in Petroleum Exploration and Development by Jack P. Horkowitz, Schlumberger, USA Accurate lithology is one of the most basic types of information required for meaningful evaluation of hydrocarbon bearing basins, formations and reservoirs. In the oil and gas industry, lithologic information is a fundamental input throughout all geoscience and engineering disciplines; from the analysis of geophysical attributes, to the selection of drilling fluids and bits, to the calculation of reservoir pore volume and hydrocarbon saturation for booking reserves, to the selection of completion fluids, and in the quantification of mechanical properties to design fracture stimulation programs to name a few. Our ability to consistently provide an accurate and unbiased estimate of lithology and mineralogy from conventional logging measurements such as gamma-ray (GR), spontaneous potential (SP), density (RHOB), neutron (NPHI) and photo-electric effect (PEF) is severely challenged in complex lithologic sequences and even in many "simple" shaley-sand reservoirs. A new quantitative lithology product derived from downhole geochemical measurements was recently introduced to the industry for both open- and cased hole applications. SpectroLith lithology is derived from elemental concentration logs obtained from neutron-induced, gamma ray spectroscopy. The new interpretation methodology is based on a high quality core database. Fourier transform infrared mineralogy and detailed chemical analyses were measured on over 400 samples to examine the relationships between lithology and a wide variety of geochemical signatures. The results provide the necessary relationships between elemental concentrations available from downhole measurements and measured mineralogic fractions. Specifically, total clay concentration is estimated from the elements silicon, calcium and iron; sulfate concentration (anhydrite and gypsum) is determined from the elements sulfur and calcium; carbonate concentration is determined from calcium with exceptional accuracy; and the remainder is composed of quartz, feldspar, and mica minerals. Recent enhancements include logic for pyrite, siderite, coal and salt. In many instances, the number of unknowns being solved for in a petrophysical evaluation can be two or three times greater than the number of independent measurements available from the logging suite run, resulting in a highly undetermined system. In this typical scenario, local knowledge, rules of thumb, and "close-ology" come to the rescue to provide a standard solution which may be highly inaccurate but is deemed acceptable or the "best guess". From a business perspective, the magnitude of these errors can have a significant impact on economic decisions due to inaccuracies surrounding the nature and value of the asset. When the SpectroLith products are used in combination with other measurements, definite improvements in the accuracy of porosity, water saturation and productivity predictions are observed based on the increased accuracy in lithologic volumes and matrix properties. Also, the additional measurements in many cases allow for the solution of a determined or over-determined system. A wide variety of applications have been developed and proven with this new technology. A number of the more common applications include: 1) quantitative clay volumes independent of gamma-ray, spontaneous potential or density-neutron, 2) identification of marls, clay-rich shales and silty shales for seismic attribute analysis, seismic stratigraphy and rock properties modeling, 3) smectite-illite discrimination for pore pressure prediction from seismic in the deep-water Gulf of Mexico, 4) accurate anhydrite/gypsum volumes in carbonate reservoirs, 5) matrix properties (density and sigma) for more accurate porosity and fluid saturation answers, 6) coalbed methane evaluation, 7) lithologic inputs for enhanced fracture stimulation design, 8) accurate lithology for selection of appropriate completion fluid (acid type), 9) accurate lithology for enhanced image log interpretation, 10) open- and cased hole geochemical stratigraphy for enhanced geologic correlation and reservoir modeling, 11) permeability, bound fluid and cementation exponent estimates based on mineralogy and specific surface area, and 12) robust cased hole formation evaluation answers.