Diagenetic facies distribution in high resolution sequence stratigraphic framework of Chang 8 Oil Layers in the Jiyuan area

Abstract: Lithologic characteristics, reservoir property and pore structure, diagenesis, diagenetic minerals, diagenetic environment, diagenetic stage and diagenetic evolution sequence of Member 8 reservoir of Yanchang Formation were studied by making full use of normal thin sections, casting thin sections, cathodoluminescence, X-ray diffraction, scanning electron microscopy and core data. The results show that the reservoir has experienced such diagenetic processes as compaction, cementation, corrosion and fracture, and it is now at stage A of middle diagenetic process. 5 types of diagenetic facies were recognized based on diagenesis, diagenetic minerals and their effect on the reservoir quality, namely, weak corrosion with chlorite mat; corrosion of unstable components; compaction density, kaolinite filling, and carbonate cementation. The well logging response characteristics of different types of diagenetic facies were summarized on GR, SP, DEN, AC and CNL, and thus the logging recognition model and standard of diagenetic facies could be set up. Then the diagenetic facies was matched with the high resolution sequence stratigraphic framework. The results show that the sequence boundary between the Chang 81 lowstand systems tract and Chang 82 highstand systems tract controls the dissolution of the silicate minerals, the filling of the kaolinite and the cementation of carbonate cement, which suggests that diagenetic facies as corrosion of unstable components and kaolinite filling are very common in the vicinity of the sequence boundary, and they are strongly comparable between various wells; nevertheless, the carbonate cementation is also very common under the sequence boundary. The maximum flooding surface of the medium-term base level cycle corresponds to the compaction density diagenetic facies, the reservoir quality of Chang 82 highstand systems tract (the middle-term base-level's ascending semi-cycle) is better than that of Chang 81 lowstand systems tract (the middle-term base-level's falling semi-cycle) due to the following factors: (1) the Chang 82 sand bodies tend to be dissolved since they are located under the sequence boundary; (2) the Chang 82 sand bodies have high content of feldspars and are of coarser grain size and, what is more, the degree of compaction is lower than that of Chang 81 sand bodies due to the depositional hiatus after the formation of sequence boundary.