oa Role of Bitumen and Nsos During Thermal Maturation of a Lacustrine Type-Ii Shale in Semi-Open Pyrolysis Experiment

By taking the Chang7 shale in the Ordos Basin of China as an example, the decomposition pathway of lacustrine Type-II kerogen was studied. Results show obvious characteristics of concurrently generation of bitumen and oil and simultaneously generation of NSOs and HCs during the initial kerogen decomposition. The results not only contradicts with product-precursor relationship between the bitumen and oil of lacustrine Type-I and marine Type-II kerogen, but also negates the sequential reaction model between NSOs and HC of lacustrine Type-I kerogen, marine Type-II kerogen, and Type-III lignite.

The reaction process of the Chang7 shale under semi-open pyrolysis conditions follows the “alternate pathway”, in which hydrocarbons can be formed immediately from kerogen in parallel with NSOs and the formation of the two species are controlled by bond-breaking reactions independent of each other. From this point of view, the Chang7 kerogen is likely to contain sufficient branched structures that bond-breaking has a high possibility of generating HC at the onset of pyrolysis. The mechanism not only applies to the Chang7 shale, but also can be extended to source rocks of the same kerogen type worldwide. This work has broadened the understanding of the decomposition pathway of a less studied kerogen type.