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

Mixed carbonate-siliciclastic sedimentation is common in the Upper Permian-Lower Tri-assic 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 retrogres-sion.The Kangdian(康滇) ancient land progressive uplifted and enlarged,forming the major prove-nance of terrigenous clastics.In Changxingian age,mixed terrigenous-carbonate sedimentation was dis-tributed 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 de-posited.The tectonic framework in Feixianguanian(飞仙关) stage was inherited from the Chang-xingian 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.