Characterization of free and bound bitumen fractions in a thermal maturation shale sequence. Part 1: Acidic and neutral compounds by negative-ion ESI FT-ICR MS

Abstract

To evaluate petroleum generation and expulsion processes, pyrolysis experiments that simulate increased overburden pressures were conducted on an immature organic-rich shale with type-II kerogen. Expelled oils and residual rocks with different maturities covering the oil window were recovered from the experiments. The residual rocks were then subjected to stepwise extraction in conjunction with stepwise dissolution of the carbonate and silicate minerals to obtain free and bound bitumen fractions. The compositions of acidic and neutral compounds in both expelled oils and bitumen fractions were investigated by negative-ion electrospray (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Neutral nitrogen compounds were found to only exist in expelled oil and free bitumen, while acidic compounds (typically contain carboxylic functional group(s)) were prevalent in bound bitumen. Oxygen-containing functional groups with high polarity in oxygen-containing compounds and oxygenated nitrogen/sulfur compounds are proposed to be the dominant bonding forces that facilitate the interaction between bitumen and mineral matrix or kerogen. Semiquantitative results of acidic compounds reveal that the distributions of acidic compounds in both free and bound bitumens vary throughout the oil window stage, especially for those in the bound phase after an intense oil expulsion from the shale at Easy%Ro > 0.9. During maturation, acidic compounds in free and bound bitumens gradually decreased in their abundances. However, oil expulsion from the shale brought about a significant redistribution of acidic compounds among expelled oil, free and bound bitumens. These results indicate that polar components in shales are closely related to thermal maturation of organic matter and oil expulsion from the shales in a petroleum system.