FIELD EXAMPLES OF THE COMBINED PETROPHYSICAL INVERSION OF GAMMA-RAY, DENSITY, AND RESISTIVITY LOGS ACQUIRED IN THINLY-BEDDED CLASTIC ROCK FORMATIONS

摘要

Limited vertical resolution of logging tools causesshoulder-bed effects on borehole measurements, thereforebiases in the assessment of porosity and hydrocarbonsaturation across thinly-bedded rock formations.Previously, we developed a combined inversion procedurefor induction resistivity and density logs to improvethe petrophysical assessment of multi-layer reservoirs.In this paper, we include the inversion ofgamma-ray (GR) logs in the interpretation method andevaluate three field cases that comprise hydrocarbonsaturatedTertiary turbidite sequences. Formations underconsideration are unconsolidated to poorly consolidated.All wells were drilled with oil-base mud (OBM),logged with triple-combo tools, and sampled withwhole and sidewall cores.We transform layer-by-layer inversion results intopetrophysical properties via a shaly-sand model. Onaverage, inversion results yield 19% better agreement tocore measurements and lead to 28% increase in hydrocarbonreserves when compared to standard well-loginterpretation procedures.The wide variety of sand-shale distributions andlayer thicknesses included in the example data sets enablesus to generalize recommendations for best practices of combined inversion, including criteria for bed-boundary detection, sensor selection, and modificationof our "UT Longhorn Tool" flux sensitivity functions(FSFs) to replicate those of commercial tools. The mostcritical step for reliable and accurate inversion results isthe detection/selection of bed boundaries. Inversion offield data also indicates that the minimum bed thicknessresolvable with combined inversion is about 0.7ft, andthat inflection points of density logs are the best optionfor bed-boundary detection.We show that combined inversion allows the detectionof noisy, inconsistent, and inadequate measurements,including cases of abnormal measurementcorrectionbiases otherwise difficult to diagnose onprocessed logs.