Generation of HC and Non-HC Phases With Thermal Maturation and Their Dependency on the Chemistry of the Immature Kerogen
One of the earliest and most unambiguous chemical methods of kerogen characterization is based on a Van Krevelen diagram, onto which the elemental ratios of H/C and O/C are projected (van Krevelen, 1961; Tissot and Welte, 1978). The H/C ratio of kerogen is the controlling factor for the production of oil vs. gas yields from the primary hydrocarbon-generating reactions. Therefore, the diagram classifies the various kerogens based on these ratios where the highest H/C ratio is classified as type I (oil prone), the lowest as type III (gas prone) and the intermediate as type II. As defined by Tissot and Welte (1978), as the kerogen thermally matures, the various kerogen types follow distinct pathways where the H/C and O/C values in the kerogen decrease. These pathways cross the bitumen, oil, wet and dry gas generative windows which were defined using these same ratios. Apart from these three elements, the kerogen consists of other heteroatoms such as N and S, which in addition to O, can account to more than 20 wt. % of the kerogen. These heteroatoms (i.e., NSO’s) are released upon thermal maturation mainly as non-HC compounds (e.g., CO2, CO, H2S, H2O) and therefore affect both the products yield, composition and the maturation path (defined as the decrease in HC generation potential with maturation). Whereas the projection of a maturation pathway of a certain kerogen on a van-Krevelen diagram looks at the evolving CHO composition of the kerogen, it does not take into account: (1) N and S (2) generated products yield and composition, (3) determination of HC vs. non-HC products. In this research, we apply a mass balance approach in order to track the CHNOS elemental budget and redistribution of elements as HC and non-HC products before, after and during the maturation process. To this end, 3 different source rock types (I, II-S and III) were artificially matured in the laboratory in a semi-open pyrolysis reactor. This system allows quantifying the yield and distribution of HC and non-HC components as a function of maturation. Moreover, the generated data set allows quantifying the onset of the generation of certain components as well as their decay and the relationship to other components. It is shown that for Type I kerogen, the onset and decay of all bulk pyrolysis products (C1-C4, H2S, H2, CO, CO2 and oil) except water are coupled together in a late and narrow maturation window. This is not the case for type II-S and type III kerogens where the components are not coupled together and are also spread on a wider maturation window which starts at lower maturation levels than type I kerogen. It is also shown that for all kerogen types, most of the water generation commenced at early maturation stages, prior to thermogenic HC generation, despite the release of C in the form of CO2 and CO. We therefore suggest that source rock classification should include also the heteroatoms in the kerogen alongside with reference to their generated HC and non-HC products and maturation scales.
AAPG Datapages/Search and Discovery Article #90341 ©2019 AAPG Geoscience Technology Workshop, Exploration and Development of Siliciclastic and Carbonate Reservoirs in the Eastern Mediterranean, Tel Aviv, Israel, February 26-27, 2019