Synergistic diagenesis of sandstone and mudstone in Chang 73 sub-member of Triassic Yanchang Formation in Ordos Basin

Key words:  synergistic diagenesis  reservoir quality  Chang 7 member  Ordos Basin

Abstract:

This study of the Chang 7₃ sub-member of Triassic Yanchang Formation in Ordos Basin used an integrated approach of core observation, thin sections analysis, XRD analysis, AMICS mineral analysis, SEM, and vitrinite reflectance analysis to clarify the diagenesis characteristics of thin sandstone interlayers and mudstones. Three types of sandstone-mudstone combinations were observed in the Chang 7₃ sub-member. Type I is the massive mudstones and low-density turbidity current deposited sandstones combination, whose mudstones had little influence on sandstones, and sandstone interlayers are characterized by strong compaction with poor reservoir properties. Another type (type Ⅱ) involves "organic-rich + silt-grained feldspar-quartz" laminae combination shales and hyperpycnal flow deposited sandstones combination. In this type, fluid evolution and diagenetic response mainly occurred inside the shales and had limited influence on sandstones, which has medium compaction, weak feldspar dissolution and carbonate cementation, and medium reservoir properties. The third type (type Ⅲ) include “organic-rich lamina + tuff-rich lamina” binary laminae combination shale and sandy debris flow deposits sandstone or hyperpycnal flow deposited sandstones combination. When “organic-rich lamina + tuff-rich lamina” binary laminae combination shales are combined with sandy debris flow deposits sandstones, the shales not only provided organic acid to promote feldspar dissolution in sandstone, but also provided calcium-rich fluid to make carbonate cementation develop at the sandstone-mudstone interface, which made desirable reservoir properties of middle sandstones beneficial. When “organic-rich lamina + tuff-rich lamina” binary laminae combination shales are interbedded with hyperpycnal flow deposited sandstones, the abundant tuff and pyrites in shale can catalyze organic matter to generate large amount of oil and gas at the early stage of burial time, which is accumulated in intergranular pores of the thin interbedded sandstones. Crude oil can inhibit the latter compaction and cementation in sandstones, and resulting in best reservoir properties.