Detection and potential geochemical significance of methyltrimethyltridecylchromans in mature crude oils

Methyltrimethyltridecylchromans (MTTCs) are biomarkers that are commonly used to identify immature-low mature source rocks or crude oil. MTTCs are abundant in the mature crude oils found in the Machang area of the southern Dongpu Depression in the Bohai Bay Basin. To explore the mechanism behind the enrichment of MTTCs in mature crude oils, a detailed study was conducted to investigate their distribution characteristics in crude oil samples. The study integrated tectonic evolution history, distribution characteristics, and thermal history of source rocks, molecular fingerprints characteristics of crude oils, and catalytic characteristics and combination features of clay minerals in the wall rock of crude oils. Based on this analysis, two enrichment models were proposed: (1) The first model suggests that crude oils were mixed with a small amount of immature-low mature soluble bitumen containing MTTCs; (2) The second model proposes that the evolutionary mechanism of clay minerals in the reservoir could reduce the decomposition rate and degree of MTTCs, resulting in their relative enrichment. Therefore, this study provides insights into the enrichment mechanism of MTTCs in mature crude oils and highlights the importance of considering various factors, such as tectonic evolution history, source rock characteristics, and catalytic properties of clay minerals, to understand the distribution and enrichment of biomarkers in crude oils.