Role of bitumen and NSOs during the decomposition process of a lacustrine Type-II kerogen in semi-open pyrolysis system

The purpose of this work is to investigate the generation characteristics of bitumen, NSO compounds, oil, and HC compounds during the artificial maturation of a lacustrine Type-II kerogen (which has not been given enough attention before) in order to determine its decomposition process. The analysis is based on the data itself, on the premise of jumping out of the generally accepted sequential reaction model. By taking the kerogen of the Chang 7 shale as an example, seven parallel experiments, in the temperature range from 300 degrees C to 420 degrees C were conducted on newly designed temperature-based semi-open pyrolysis system. The overall products are classified into oil and bitumen according to their phase, the C15+ fractions are classified into C(15+)sat, C(15+)aro and NSOs based on chemical compositions, and the NSOs are further classified into n-pentane NSOs and DCM NSOs according to Behar et al., (2008) and (2010). Results show that large proportion of oil is not merely a result of thermal cracking of bitumen, but also directly from the decomposition of kerogen itself. Both C(15+)sat and C(15+)aro are generated as soon as kerogen starts to decompose. It is not until the initial productivity of kerogen is basically exhausted that NSOs become the main precursor of hydrocarbons. The comparison with Behar et al. [2008, organic geochemistry 39, 1-22] further reveals that, for Chang 7 kerogen, initial decomposition of kerogen generates much more HCs than DCM NSOs. These results contradict the sequential reaction model described as: kerogen -> bitumen -> oil or kerogen -> NSOs -> hydrocarbons. Instead, they confirm the alternate pathway mechanism proposed by Burnham a al. [ACS symposium, 1989] 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 that are independent of each other. This study adds geochemical insights into the decomposition mechanism of lacustrine Type-II kerogen.