Mud gas data represent the "first breath" of a well being drilled. However, despite significant progress in technology, techniques and theoretical understanding of mud gas detection and analysis, such data are still largely under-utilized by formation and fluid evaluation specialists.

This paper addresses such issue; it presents how new mud gas data realistically expand the limits of surface logging and redefine interpretation uncertainties.

The modern analysis process of hydrocarbons detection while drilling rests on three legs: First, advances in mud gas detection enable to analyze a broader spectrum of hydrocarbons directly on site, detecting more than 30 hydrocarbon species from methane to n-octane. This is coupled with the analysis of the carbon isotopic fingerprint of light components at the wellsite.

The second leg is constituted by a robust procedure covering all aspects of data acquisition, quality control and interpretation, applied systematically to the analysis process.

Finally, the third leg is the integrated utilization of all formation data available, from surface and downhole sources.

The principles described above have been applied in several projects worldwide. One of the main results is a significant reduction in the uncertainty in fluid-type indication. In fact, the utilization of chromatographic and isotopic data, simultaneously collected by entirely independent instruments, leaves limited doubt in fluid typing.

Furthermore, early isotopic data have permitted understanding some of the reservoir characteristics without waiting for laboratory sample tests.

Another important result is that the utilization of early formation evaluation data has enabled to optimize downhole data acquisition programs, with significant cost savings.

In all the successful cases, data interpretation has been carried out taking into account the geological context and utilizing models validated with laboratory data. The application of specific analysis procedures has also resulted in identifying reducible and non-reducible limits of mud gas detection and analysis.

The overall result of this approach is a significant reduction in data uncertainty in mud gas data. In the same time, it provides a realistic review of how far the analysis of light hydrocarbons can be extended, given the recent technology advances. Ultimately, a critical approach to surface logging-based formation evaluation techniques leads to a better understanding of the significance of such under-utilized although cost-effective data, and indicates the way forward for further development in this discipline, based, it should not be forgotten, on the direct and early analysis of the formation fluids.

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