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The effect of diagenetic environment on hydrocarbon generation based on diagenetic mineral assemblage in mudstone

Abstract The diagenetic mineral assemblages in mudstones during diagenesis can indicate the changing diagenetic environmental characteristics, providing possibility to study the influence of the diagenetic environment on the evolution of organic matter (OM). In the present study, mudstones from the Dongying Depression were selected and evaluated by X-ray diffraction (XRD), thin section, and scanning electron microscopy (SEM). The SEM and XRD results indicated that detrital minerals are depositional minerals, whereas clay minerals are diagenetic minerals; carbonate are a combination of depositional and diagenetic minerals. According to the diagenetic mineral content and content change rates, three diagenetic mineral assemblages were distinguished: I. smectite/I-S (1,000–2,000 m); II. Smectite/I-S + illite + kaolinite (2,000–3,000 m); III. Smectite/I-S + illite + chlorite + ankerite (>3,000 m). Kaolinite is formed in acidic environments, while the other diagenetic minerals are formed and stable under alkaline conditions. Therefore, the evolution of the diagenetic environment was investigated using the K/(I + Ch + A) ratio (K:Kaolinite; I:Illite; Ch:Chlorite; A:Ankerite), and the formations were found to have undergone alkaline-acidic-alkaline changes. For OM with different chemical properties, huge differences exist in hydrocarbon generation between acidic and alkaline diagenetic environments. Therefore, for hydrocarbon generation, attention should be paid to the changes in diagenetic environment, which are important for understanding the hydrocarbon generation, hydrocarbon expulsion and hydrocarbon accumulation in mudstones.