Comparison of in-source solid bitumen with migrated solid bitumen from Ediacaran-Cambrian rocks in the Upper Yangtze region, China

Abstract

In-source solid bitumen was found in some cracks within the calcareous concretion in an overmature (Rbitumen > 3.5%) black shale of the Lower Cambrian Niutitang Formation from Songtao, Guizhou, China. The solid bitumen (grain size >2 cm) concentrated in source rock, which was formed by the transformation of petroleum-like substances that have not been expelled from source rock. This provides a substitute for studying the physical and chemical properties of in-situ solid bitumen, which usually occurs as finely dispersed particles in black shale. On the other hand, Ediacaran-Cambrian carbonate rocks (which serve as reservoir in the area) contain pyrobitumen derived from petroleum that migrated from the black shale of the Niutitang Formation.A comparative petrological and geochemical analysis was conducted on the in-source solid bitumen from the Lower Cambrian Niutitang Formation and the migrated solid bitumen in the Ediacaran-Cambrian carbonate rocks. The in-source solid bitumen displays a relatively homogeneous optical structure with very weak anisotropy and is visibly different from the migrated bitumen in the Ediacaran-Cambrian carbonate reservoir. This migrated bitumen in the reservoir displays strong anisotropy and optical structure similar to that of coke. Considering a strata like the lower Palaeozoic in South China, which has no vitrinite and is highly over mature, the in-source solid bitumen is likely to be the most suitable organic component for measuring the reflectance and subsequently determining the thermal maturity of organic matter.Interestingly, the in-source solid bitumen displays some inheritance of its parent material. The stable carbon isotope ratios are very similar to those of kerogen in the black shales (approximately −34‰ to −33‰), and the distribution pattern of the trace elements and rare earth elements (REEs) is consistent with that of the Niutitang black shales. The distribution patterns of the trace elements and REEs in the migrated bitumen are basically consistent with those of in-source solid bitumen, however, their stable carbon isotope composition is slightly heavier (−33.6‰ to −31.2‰) than that in the source rocks. In other words, as a key link in the chain, the in-source solid bitumen could connect the migrated bitumen in the reservoir with its source rock. This provides direct support for tracing oil and gas sources in the area by using the element geochemical characteristics of migrated solid bitumen in reservoir.