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
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 (Es 3 2 , Es 3 3 ) and saline (Es 4 1 ) 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 Es 3 3 were much better than the other two intervals, which agrees with previous studies. The characteristics of clay minerals suggest that the samples from Es 4 1 have the most abundant illite, with a maximum illite percentage in mixed-layer illite-smectite (I?Sm), and the best crystallinity of I?Sm with a main stacking mode of R1.5. However, the stacking modes of I?Sm in Es 3 2 and Es 3 3 were primarily R0 and R1, respectively, and the crystallinity was relatively poor. Thus, the smectite illitization process was faster in Es 4 1 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 Es 4 1 had the lowest amount of interlayer OM, whereas Es 3 3 had the largest amount. Therefore, the rapid illitization in Es 4 1 caused abundant interlayer OM to be desorbed and discharged, which in turn caused the amount of residual interlayer OM in Es 4 1 to be less than that in the other two intervals. Thus, the source rocks of Es 4 1 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.
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