The South Fuwaris Field is located in the Partitioned Zone (PZ) between Kuwait and Saudi Arabia. The primary productive intervals are the Lower Cretaceous Ratawi Limestone and Ratawi Oolite reservoirs. The degree of vertical continuity between the Ratawi Limestone and Ratawi Oolite, and within the Ratawi Oolite reservoir is uncertain, and multiple methodologies have been employed to resolve this uncertainty.
The objective of the current study was to utilize gas data derived from Geolog's Advanced Mud Gas Detection System for formation evaluation. This system provided real-time continuous measurements of the concentration of formation gases, from very light components, such as methane, to heavy components, such as toluene.
The primary components of the system utilized are: a constant volume gas extractor, a gas sample flow control system, and a high resolution chromatographic system. These components are used to extract and measure the light and heavy fraction of hydrocarbon gases, extending the detection range of the standard mud chromatograph beyondmethane (C1) to pentane (C5), to include the dominant C6 and C7 hydrocarbon species, comprising n-hexane, n-heptane, benzene and toluene.
A series of gas ratios, based on the analysis of these gases, was utilized to validate reservoir zonation and potential vertical flow barriers. Specific proxies from the gas data were also used to confirm fluid characterization and identify oil/water contacts (OWC). Gas ratio analysis was also found to have value in geosteering in the lateral sections of the wells, where gas proxies were used to confirm that the well had steered in and out of the target intervals. A probabilistic approach was developed in order to extract maximum value from the measurements of heavier volatile components (C6+).
Following analysis of data from a number of wells, the Advanced Gas Analysis technique was confirmed as a powerful tool for reservoir evaluation and the identification of formation zonation while drilling. In particular, the analysis of heavy components (hydrocarbon gases heavier than n-pentane) has enabled the validation of vertical flow barriers between the Ratawi Limestone and Ratawi Oolite and within the Ratawi Oolite. Oil/water contact identification was successfully correlated with contacts picked from nuclear magnetic resonance data.
It was also found that data quality enabled direct comparison between mud gas data and the PVT samples laboratory data. To the authors' knowledge, this represents the first time that heavy gas components such as C6 and C7 have successfully been compared with PVT data.
At this time, some interpretative doubts remain regarding the relationship between some of the gas ratios, and the relationship of the gas data to productivity. It is anticipated that answers to these questions will be found through the acquisition and interpretation of additional data.
