Enhanced Reservoir Characterization In A Deep Water Turbidite System Using Borehole Images And Spectroscopy Logs

It is commonly accepted that deep marine reservoirs are dominated by submarine channels and channelized lobes. Submarine channels can be broadly classified as gravity-flow channels and leveed channel complexes. While gravity-flow channels are characterized by strong textural heterogeneity, thin-bedded channels and associated levees show rapid lithological changes at cm scale, well beyond the vertical resolution of standard logging techniques. This paper highlights a technique for enhanced lithofacies prediction in a deep-water clastic turbidite reservoir, offshore India, principally using resistivity images and spectroscopy logs. We also show how an integration of the micro-resistivity images together with other well log data enabled a more accurate estimation of gas in place compared with using only conventional logs in this ‘low resistivity pay’, thin-bedded reservoir. The micro-resistivity images, together with the spectroscopy log derived dry weight lithologies were used to generate an enhanced lithofacies using a novel lithofacies processing and classification scheme. The system uses the dry weight mineralogical output from the spectroscopy data, and with a corresponding set of rules to create dry-weight mineralogy-based lithofacies. This output is further sub-divided to a finer scale using calibrated micro-resistivity image data. Advanced image processing and interpretation enabled an estimation of reservoir heterogeneity. A heterogeneity index is calculated from the percentile resistivity distribution of the image spectrum. Since the heterogeneity index is related to a distribution of resistivities around the borehole, it is independent of the absolute resistivity values. The integration of these results also reinforce the fact that lithology and texture are equally important as the petrophysical characteristics of the reservoir units and provide important input for lithological and textural heterogeneity during advanced formation evaluation. As continuous coring is not always feasible due to its innumerous cost, borehole image logs and spectrometry data, when calibrated with core, provide sufficient resolution and quantification for a continuous and detailed facies description.