Smectite-illitization difference of source rocks developed in saline and fresh water environments and its influence on hydrocarbon generation: A study from the Shahejie Formation, Dongying Depression, China

Abstract

Samples of argillaceous source rocks from three sub-members of the Shahejie Formation (Es) in the Dongying Depression, China, were collected to investigate the differences in hydrocarbon generation among the sub-members, which developed in fresh (Es32, Es33) and saline (Es41) water environments. Pyrolysis, XRD and thermo-XRD analyses were used to compare the characteristics of organic matter (OM), clay minerals and OM occurrences. Total organic carbon and hydrocarbon potential proxies suggest that the samples from Es33 were much better than the other two intervals, which agrees with previous studies. The characteristics of clay minerals suggest that the samples from Es41 have the most abundant illite, with a maximum illite percentage in mixed-layer illite-smectite (ISm), and the best crystallinity of ISm with a main stacking mode of R1.5. However, the stacking modes of ISm in Es32 and Es33 were primarily R0 and R1, respectively, and the crystallinity was relatively poor. Thus, the smectite illitization process was faster in Es41 than in the other two intervals, and a saline environment was a primary cause for the acceleration of the process. Moreover, OM occurrence indicates that the samples from Es41 had the lowest amount of interlayer OM, whereas Es33 had the largest amount. Therefore, the rapid illitization in Es41 caused abundant interlayer OM to be desorbed and discharged, which in turn caused the amount of residual interlayer OM in Es41 to be less than that in the other two intervals. Thus, the source rocks of Es41 made a more significant contribution to hydrocarbon generation than those of the other two units. In conclusion, the inconsistent illitization among these intervals was a major cause of the differences in hydrocarbon generation.