To minimize the formation damage that drilling induced, CNOOC has launched oil-based mud (OBM) drilling campaign since 2019 in Xihu sag in East China Sea to create more user-friendly environment for acquisition of better data quality for formation evaluation. However, OBM system has its own challenges in their reservoirs: (1) Hydrocarbon from multiple source rocks was charged into the system, condensate and volatile oil distribution undergo the complex fluid system cross reservoir; (2) Both faults and lithology are interacted to control reservoir trap, which makes the fluid distribution complex; (3) Though oil-based mud system can protect formation from damage more effectively compared with water-based mud, its contamination to reservoir fluid presents a strong challenge to fluid types and fluid phase envelope changes. Downhole Fluid Analyzer (DFA) isn’t a new technology in the industry, and its OBM contamination monitor is well known method published by many authors. In this paper, we presented a comprehensive case study of 6 wells in Xihu sag and implemented quantifying OBM contamination real-time, to understand the fluid distribution across the reservoir [1] integrating with PVT analysis in this faulted geological structure. On the other hand, the formation mobility and OBM contamination relationship was studied to understand the impact of formation property on the sample quality, so it provided the operation suggestion for the future exploration to improve the data acquisition and reduce fluid measurement uncertainty further for reservoir characterization.

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