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Effect of Thermal Maturation on the Composition of Polar Organic Compounds in the Eocene Lacustrine Shales of the Bohai Bay Basin in Eastern China as Revealed by FT-ICR-MS Analysis of Hydrous Pyrolysates


The lacustrine shales in the Es3 and Es4 sections of the Eocene-Oligocene Shahejie Formation represent one of the most economically important source rocks for conventional oil resources in eastern China. This organic-rich shale/mudstone sequence occurs with thicknesses of a few tens meters and has been the principal target for lacustrine shale oil in recently years. The present study investigates the effect of thermal maturation on the distribution of polar organic compounds in rock extracts from a laboratory simulated maturity sequence of the Es4 shale from the Dongying Depression of Bohai Bay Basin. The measured apparent thermal maturity of the studied shale sample increases from 0.37 to 1.71 % Ro. The influence of this increase in maturity on gross hydrocarbon yields and SARA compositions was studied using conventional analytic techniques, while the expelled oils and total rock extracts have been investigated by direct infusion electrospray ionization experiments with Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS). In negative ESI mode, clear differences in the major acid species were observed from the residual oils representing different thermal maturation stages. At low maturity level (early oil window), for example, fatty acids (DBE=1) were the most abundant species. Within the main oil window, naphthenic acids and aromatic acids dominated. At higher maturity levels, most of the acidic species underwent thermal cracking and aromatization reactions, thus only the O2 class with short chain and high DBE value (aromatic acids) were detected. With increasing maturity, a significant increase and compositional change of heterocompounds containing one nitrogen atom (and one oxygen atom) were also observed in the residual oils, including the increase in the relative amounts of dibenzo-/naphthocarbazoles and benzonaphthocarbazoles compared to those of the carbazoles and benzocarbazoles. Similar trends hold also true for the pseudohomologous polyaromatic phenolic compounds. Systematic changes observed in the expelled versus residual oils include the relative enrichment of more alkylated and less aromatized species in the expelled oils. The results, once validated from natural sample sequence, have clear implications in geochemical processes at and close to interfaces, and may exert significant controls on how hydrocarbon fluids are stored in and produced from tight oil/shale oil reservoirs.