Hydrocarbon potential and polycyclic aromatic compounds differences of Carboniferous-Permian coaly source rocks, Bohai Bay Basin: An implication for different sources of gas condensate and oils

Abstract

This study investigates the characteristics of Carboniferous-Permian coaly source rocks in the Bohai Bay Basin, Eastern China, which provides new insights for understanding of hydrocarbon potential of Carboniferous-Permian coaly source rocks worldwide. A total of 37 samples comprising different types of the coaly source rocks were selected for the analysis of lithofacies, total organic carbon (TOC) and pyrolysis, organic petrology, fourier transform infrared spectroscopy (FTIR) and two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GC × GC-TOFMS). The results show that there are six types of coaly source rocks, including clarain, durain, inferior coal, carbonaceous mudstone, deltaic shale, and lagoonal shale. The clarain and the durain are with Type III-II and II-III kerogen and have high hydrocarbon potential (51–237 mg HC/g rock), while the others are with Type II-III and III kerogen and have low hydrocarbon potential. The high hydrocarbon generation potential of clarain and durain are mainly attributed to hydrogen-rich desmocollinite, cutinite, and sporinite in them. Variance in the abundance of methylfluorene (MF), fluorene (F), dimethylfluorene (DMF), methylphenanthrene (MP) and phenanthrene (P) among different types of coaly source rocks can reveal the genetic relationship between condensates and their source rocks. The condensate associated with shales presents relative low MF/F ratios but high MF/DMF ratios. The condensate associated with coals presents a series of low MF/DMF, MF/F and MP/P ratios. Natural gas and condensate that are associated with coals have been discovered in the Dagang Oilfield of the Bohai Bay Basin, indicating notable hydrocarbon potential of Carboniferous-Permian coaly source rocks in this area.