Hybrid Sedimentation in Late Permian-Early Triassic in Western Sichuan Basin, China

Mixed carbonate-siliciclastic sedimentation is common in the Upper Permian-Lower Triassic in the western Sichuan (四川) basin. The extensional movement was strong in the Late Permian, resulting in differential uplifting of fault blocks. During this period, there was an extensive retrogression. The Kangdian (康滇) ancient land progressive uplifted and enlarged, forming the major provenance of terrigenous clastics. In Changxingian age, mixed terrigenous-carbonate sedimentation was distributed in the area between Dafeishui (大飞水)-well Dashen (大深)-1-well Shoubao (寿保)-1 and Beichuan (北川)-well Guanji (关基). The terrigenous clastics sourced from Kangdian ancient land input into the relatively shallow water, resulting in an extensive limited platform-mixing tidal flat where moderate-thick laminated grey limestone, thin muddy limestone and purplish red sandy shale were deposited. The tectonic framework in Feixianguanian (飞仙关) stage was inherited from the Changxingian stage. The hybrid sedimentation was limited to the south of Dujiangyan (都江堰)-Xindu (新都). In Jialingjiangian (嘉陵江) stage, the palaeogeographic features were also unchanged and two complete transgression-retrogression cycles also occurred. The mixing tidal flat was distributed to the west of Baoxing (保兴)-Ya'an (雅安)-Hongya (洪雅)-Leshan (乐山), where purplish red sandstone with thin limestone and shale interbeds and grey-dark grey silty mudstone with limestone were deposited. The carbonates in the mixed facies tract have few primary pores preserved and poorly-developed secondary pores due to the relatively high content of terrigenous clastics, strong compaction and weak dissolution. Therefore, they are unfavorable for the development of effective reservoirs.