Abstract: The diagenetic minerals in mudstones generally change with diagenetic systems during diagenesis. Therefore, in order to
deeply understand the difference of hydrocarbon generation in mudstones, effectively identifying the diagenetic mineral associations
and dividing diagenetic systems are essential. The Paleogene mudstone samples from the Dongying Sag, China, buried at a depth of
1200-4500 m were selected and evaluated by rock thin section, scanning electron microscopy and X-ray diffraction, to distinguish the
differences in microscopic features between exogenously input minerals (e.g., quartz) and diagenetic minerals (e.g., sparry calcite and
dolomite). Based on the results, a method on quantitatively estimating the diagenetic transformation content of clay minerals is
established: (CTerrestrial=(C/Q)Es1 ×Qsample and Cdiagenetic=CXRD-CTerrestrial (C: clay mineral content, Q: quartz content)). This method can better
reflect the diagenetic evolution of clay minerals. During the burial process, the diagenetic minerals and associations in mudstones
display a two-stage feature. Above 3000 m, the diagenetic transformation of clay minerals and dolomite is slow, forming a smectite+
kaolinite association, which is inferred to have experienced an acidic diagenetic environment and open diagenetic system. The
diagenetic transformation is accelerated below 3000 m, forming an illite+chlorite+dolomite association, which is inferred to have
experienced an alkaline diagenetic environment and closed diagenetic system. This fully demonstrates the differences between deep
and shallow diagenetic environments and systems. This understanding is consistent with the previous results that the boundary of open
and closed systems is caused by the abnormal pressure which is generated by hydrocarbon generation in the mudstones. This implies
that the diagenetic systems revealed by diagenetic minerals have a good response to the pressure system generated by hydrocarbon
generation. Thus, dividing diagenetic systems is significant for understanding the heterogeneities in hydrocarbon generation processes
and mechanisms under different diagenetic systems.

Key words: diagenetic system, diagenetic mineral, evolution characteristics, mudstone, Dongying sag