The origin of solid bitumen in the Honghuayuan Formation (O1h) of the Majiang paleo-reservoir—Evidence from catalytic hydropyrolysates

Abstract

Large amounts of solid bitumen occur in the Honghuayuan Formation of the Lower Ordovician (O1h) within the Majiang paleo-reservoir. Most of the potential source rocks and solid bitumens in the Southern Guizhou Depression are of high maturity (%Ro>2). Consequently, the yields of extractable organic matter (EOM) from the potential source rocks and solid bitumens are too low to satisfy the requirement of instrumental analysis. Moreover, the reliability of oil–source correlations based on biomarkers in EOM may be questionable because the routine biomarkers obtained by Soxhlet extraction may have been severely altered by secondary alterations. In this research, catalytic hydropyrolysis (HyPy) is used to release covalently bound biomarkers from highly overmature kerogens and solid bitumens. We analyzed the covalently bound biomarkers and n-alkanes released from the O1h solid bitumens and potential source rock kerogens via catalytic hydropyrolysis by using gas chromatography–mass spectrometry and gas chromatography–isotope ratio mass spectrometry to clarify the oil–source correlation of the Majiang paleo-reservoir. Compared with free biomarkers in EOM, the covalently bound biomarkers released by HyPy exhibit low-maturity characteristics and they retain inherited geochemical information. The oil–source correlation indicates that the Lower Cambrian marine mudstones are the main source for the O1h solid bitumens of the Majiang paleo-reservoir. Furthermore, this research also suggests that some of the O1h solid bitumens had suffered severe biodegradation ahead of thermal alteration.