Abstract:

The Jiufotang Formation has huge hydrocarbon resources in the Ludong depression of Kailu basin, but there is less research on the relationship between Milankovitch cycle and sedimentation patterns, especially fine-grained sedimentation, which restricts the prediction of source rock distribution. In this study, wavelet and spectrum analyses of natural gamma log data were used to divide the Milankovitch cycle of the Upper Member of the Jiufotang Formation, and the sedimentary environment at that time was compared with the divided cycle to clarify the stratigraphic sedimentary response characteristics. The results show that 13 Class Ⅳ cycles, 25 Class Ⅴ cycles and 66 Class Ⅵ cycles can be identified in the Upper Member of  Jiufotang Formation. According to the lithology and long eccentricity characteristics of the sedimentary rocks in the Upper Member of  Jiufotang Formation, it can be divided into two stages of sedimentary environment evolution: Warm and humid climate half-cycle and cold and dry climate half-cycle. In the warm and humid climate half-cycle, the increasing depth of waterbody benefits the deposition of fine-grained sedimentary rocks. The warm and humid climate period is the period of hydrocarbon source rock development, and the cold and dry climate period is the period of reservoir development. Combining with relevant geochemical data, the development sites of reservoirs and hydrocarbon source rocks can be predicted.

Key words: Kailu basin; , Ludong depression, Milankovitch cycle, Upper Member of Jiufotang Formation, sedimentary response, fine-grained sedimentary rock, source rock