Influence of hydrocarbon components in drilling fluids on the results of geochemical core analyses

Introduction. Geochemical studies of core samples are widely used to characterize the composition and origin of reservoir fluids. However, the reliability of data may reduce due to contamination by hydrocarbon components of drilling fluids, which alter chromatographic and biomarker characteristics.

Aim. To evaluate the effect of hydrocarbon additives in drilling fluids on core geochemical data and to demonstrate contamination patterns observed in wells from different regions of Russia.

Materials and methods. Core samples from three wells located in the Orenburg region, Khanty-Mansi Autonomous Okrug, and Yamal-Nenets Autonomous Okrug were analyzed. Bitumoids were extracted via solvent chloroform extraction. The extracts were fractionated by group-type analysis. Saturated and aromatic  hydrocarbon fractions were examined using gas chromatography and gas chromatography-mass spectrometry. Principal component analysis was applied to distinguish clean and contaminated samples.

Results. Samples were considered contaminated when they showed clear signs of hydrocarbons that did not belong to the native organic matter. Chromatograms typically displayed a strong naphtenic hump and irregular or series of n-alkanes in such extracts. The fraction composition also changed: saturated hydrocarbons dominated, while aromatic compounds, resins, and asphaltenes were noticeably reduced. Because of the mixing between native extract hydrocarbons and drilling-fluid additives, key biomarker ratios became unreliable and could not be used for genetic interpretation. Statistical analysis further confirmed this pattern: clean and contaminated samples formed separate clusters, with contaminated ones grouping closer to the composition of drilling-fluid hydrocarbons.

Conclusion. Hydrocarbon additives from drilling fluids can significantly distort the geochemical characteristics of core extracts, affecting chromatographic patterns, group composition, and biomarker ratios. To ensure more reliable interpretation, it is important to record the composition of drilling fluids, analyze them together with core samples when possible, and use statistical tools to identify and exclude contaminated material. 

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