Evaluation of the characteristics of the oil rim of Cenomanian gas reservoir based on pyrolysis data

Introduction. The presence of unidentified oil rims during the development of gas deposits can lead to problems in gas production. If it is impossible to collect conditioned oil samples, the only way to obtain the characteristics of the oil rim is to conduct laboratory studies of the oil-saturated core. However, the presence of petroleum hydrocarbons in the composition of hydrocarbon-based drilling fluids leads to an overestimation of the Kn values during petrophysical studies and erroneous detection of “light” oil by pyrolysis data.

The aim of the work was to clarify the characteristics of the oil rim (thickness, oil saturation coefficient) of a large Cenomanian deposit in one of the fields in Western Siberia based on the results of the analysis of a historical core with traces of penetration of hydrocarbon-based drilling fluid filtrate.

Materials and methods. To assess the properties of the oil rim, the authors developed a special laboratory research scheme, including petrophysical, pyrolytic and chromatographic studies of the core and rock extracts, as well as an auxiliary stage consisting in determining the pyrolytic profiles of Cenomanian oils of different densities to plot the dependence of oil density on pyrolysis parameters.

Results. Based on the data of pyrolysis studies of core samples, the density values, “group” composition of the oil phase were estimated and the oil saturation coefficient values in the intervals of the oil rim of the gas-saturated formation were calculated. In the course of the studies, an innovative methodological approach to assessing the saturation nature, quantity and properties of the oil phase in the oil rim of the PK1 formation of one of the fields in Western Siberia was developed and implemented. The methodological approach is based on the results of pyrolysis studies of core samples using core photography data in UV, chromatography and petrophysical studies.

Conclusion. The developed methodological approach has demonstrated high efficiency and promptness of data acquisition and is recommended for express assessment of hydrocarbon phase properties in the core sampling interval when implementing complex core study programs from gas-saturated formations. 

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