Comparison Of Core And Log Carbon/Oxygen Ratios In South Belridge Diatomite

Core derived carbon/oxygen ratios (COR) are compared to log measurements made both open and cased hole in the Diatomite Formation of South Belridge Field, California. The log measurements were made with Schlumberger's Gamma Spectroscopy Tool (GST) operated in inelastic mode and logged continuously at 60 feet per hour. At a 2.5 foot vertical resolution this provides a COR precision of 0.016 which translates roughly into an oil saturation precision of about 5 percent, all at the 68 percent confidence level. Core porosities and fluid saturations, corrected for in situ conditions, were used in the comparisons, along with formation matrix and fluid compositions. The key result of this report is that both open and cased hole log measurements track the core-derived carbon/oxygen ratios very well, even though hole conditions were, in many cases, far from ideal. However, within the precision of the data, there is a statistically significant offset between the log and the core results with the log COR being consistently higher. This offset is believed to arise largely from the unique high porosity character of the Diatomite vis-a-vis standard laboratory calibration formations, though effects due to nitrogen foamed cement may be important in the comparisons with the cased hole logs. Not withstanding this offset, the excellent tracking between core and log results has obvious implications for evaluation and monitoring of fluid saturations in the Diatomite and elsewhere. Furthermore, these results, which represent a unique comparison in terms of both the type of formation and the use of Continuous COR logging, should provide additional incentives to improve COR logging precision and cost.