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Estimation Of Siltstone Oil Saturation In Kern River Using Spectroscopy Logs


A key measurement in the surveillance of the steam flood at Kern River is the current oil saturation derived from carbon/oxygen logs. Accurate quantification of oil saturation from carbon/oxygen logs depends on calibrating the logs in zones of known saturation. Typically this would be done using a known wet zone. However, in a brown field, it is rare to encounter the oil/water contact, and many wet zones may contain residual oil making them unsuitable for calibration. An alternative is to calibrate in silt/shale zones in which the saturation is expected to stay constant throughout the life of the well. To calibrate the oil saturation in the silt/shale intervals we ran an advanced spectroscopy log capable of estimating total organic carbon (TOC). This estimate of TOC can then be converted into an estimate of oil saturation. By measuring TOC in the silt/shale intervals, we could potentially use spectroscopy logs to baseline the oil saturation for future carbon/oxygen logs. In addition to spectroscopy and cased-hole saturation logs, we also cut 800ft of core. From this core we took more than 600 plugs for saturation analysis and more than 40 siltstone plugs for TOC analysis.The agreement between the spectroscopy derived TOC and laboratory measurements was quite close, although extremely low in each sample (< 5% TOC). The average TOC from core was 0.21% TOC and from the logs was 0.25% TOC. After converting the TOC measurements from the spectroscopy log to oil saturation, the average oil saturation in siltstone from spectroscopy was ~4 times smaller than from the carbon/oxygen, suggesting that there is potentially room for improvement by using spectroscopy. However, the agreement between the core and carbon/oxygen oil saturation in the sand was extremely close (< 2 su), despite the discrepancy in the siltstone oil saturation. This leads us to conclude that while carbon/oxygen logs are capable of extremely accurate oil saturation measurements in sandstones, the oil saturation estimated in the inter-bedded siltstones will be overestimated.