The source of highly overmature solid bitumens in the Permian coral reef paleo-reservoirs of the Nanpanjiang Depression

Abstract

Solid bitumens occur extensively in Permian coral reefs of the Nanpanjiang Depression. Both potential source rocks and solid bitumens in the study area are highly overmature and have similar bulk carbon isotope values. It is difficult to perform an oil–source rock correlation study in this area based on only regular molecular geochemical methods and bulk carbon isotope values. Thus the covalently bound biomarkers released from solid bitumens and source rock kerogens by catalytic hydropyrolysis (HyPy), together with the geological settings, were taken into account in this oil–source rock correlation study. The distribution characteristics of covalently bound biomarkers suggest that the major source rock of the Longlin paleo-reservoir (in the midwest of the depression) solid bitumen should be the Middle Devonian mudstone, whereas the source rock of the Ziyun paleo-reservoir (in the north of the depression) solid bitumen should be the Lower Permian source rock. However, solid bitumens in the Ceheng and Wangmo paleo-reservoirs (in the middle of the depression) may be mainly sourced from the Middle Devonian source rock, but partly from the Permian source rock. Our bitumen–source rock correlation results are also supported by the petroleum geological settings of the study area, which indicate that the filling of those paleo-reservoirs was controlled by the matching of hydrocarbon generation and trap formation. Basically, the timing of hydrocarbon generation of the Middle Devonian source rocks matches well with the formation of Permian coral reef traps in the middle and midwest portions of the depression, but it is earlier than the formation of the Permian coral reef trap in the north of the depression. We show that our oil–source rock correlation study based on covalently bound biomarkers can provide reliable information for petroleum system analysis when highly overmature strata in South China are involved.