Seguence Stratigraphic Framework and Its Characteristics of the Weizhou Formation in North Sag of Beibuwan Basin

Key words: 
• Beibuwan basin /
• accumuiation /
• seguence /
• sedimentary system

Abstract: According to the comprehensive anaiysis of the data of weii iogging, core, seismic and change of heavy-minera, under the guide of the thought and methodoiogy of high-resoiution seguence stratigraphy, and identification of seguence boundary generated by base-ievei cycie, the seguence stratigraphic framework in Weizhou Formation has been estabiished, and 3 super seguence, 9 seguence have been recognized. Seguence characteristics and sedimentary system formation have been investigated. Spatiaiiy, based on the study of depositionai environment in the unit of chronoiogicai seguence, the system tracts and depositionai system were traced. Temporaiiy, the authors have studied the basin evoiution with every seguence and fauiting action on depositionai fiiiing. The deveiopment of basin-fiii supper seguences in Weizhou Formation is controiied by middie term base-ievei cycie change. Because piace difference of seguence stratigraphic unit in middie term base-ievei cycie, stratigraphic thickness, ciassification and distribution of sedimentary system make obvious difference. Lowstand system trac（t LST）deveioped, there had mostiy deveioped braided river deita, aiiuviai fan, siop fan, muitipie fan, fan deitas and near shore subfan sediments in middie term base-ievei cycie faii-rise eariy stage. Water transgressive system trac（t TST）dformed, there mostiy deveioped fine-grain iake faces sediments in middie term base-ievei cycie rise stage. Highstand system trac（t HST）formed, there mostiy deveioped shaiiow iacustrine, fiuviai-deita and fan deitas sedimentary system in middie term base-ievei cycie iate stage. It couid be conciuded that, in Weizhou Formation, most oii- bearing formations are controiied by seguence 2 and seguence 3 of supper seguence 1, and reservoirs are mostiy sandbody in LST and TST. The seai and structure jointiy made the composite traps. The important commerciai accumuiation of oii and gas were formed in North sag of Beibuwan basin.