Differences in hydrocarbon composition of shale oils in different phase states from the Qingshankou Formation, Songliao Basin, as determined from fluorescence experiments

Abstract

The phase state of shale oil has a significant impact on its mobility. The mineral and organic matter in shale reservoirs play an important role in oil phase. This study attempts to evaluate the properties of shale oils in different phase states and to investigate how these differences are related to initial shale composition. Samples from the first member of the Qingshankou (Q1) Formation were analyzed using X-ray diffraction, total organic carbon content, rock pyrolysis solvent extraction and group component separation. Subsequently, fluorescence techniques were used to quantitatively determine the content and properties of the free oil (FO), the adsorbed oil associated with carbonate (ACO), and the adsorbed oil associated with silicate and clay-organic complexes (AKO). The results showed that non-hydrocarbons and asphaltenes are the primary fluorescing compounds on shale grain. FO is the dominant phase in the Q1 Formation. The quantitative grain fluorescence on extraction (QGF-E) and total scanning fluorescence (TSF) spectra of ACO and AKO show a significant redshift compared to the FO. The TSF spectra of FO have a characteristic skew to the left and a single peak distribution, suggesting a relatively light hydrocarbon component. The TSF spectra of ACO show a skew to the right and an even, double-peaked distribution. The TSF spectra of AKO show a single peak with a skew to the right, indicating that ACO and AKO hydrocarbons are heavier than FO hydrocarbons. In summary, enrichment of carbonate minerals in shale may result in mis-identification of “sweet spots” when using QGF. The normalized fluorescence intensity of QGF-E and TSF are effective indexes allowing oil content evaluation. As an additional complicating factor, hydrocarbon fractionation occurs during generation and expulsion, leading to a differentiation of oil composition. And FO has high relatively light hydrocarbon content and the strongest fluidity.

Keywords

stepwise extraction / fluorescence spectroscopic techniques / shale grain / shale oil phase

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Acknowledgments

This study is funded by the National Natural Science Foundation of China (Grant No.41972156) and the Science and Technology Project of Heilongjiang Province (No.2020ZX05A01).

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