Fault Genesis Analysis And Numerical Modeling For The Southern Fault Step Zone In The Gaoyou Sag

The southern fault step zone, as a fourth-order tectonic unit, is a substructure of theGaoyou Sag in the southern Subei basin and very rich in oil and gas resources. Thesouthern fault step zone include the Zhenwu fault step zone between the Zhen-①fault atthe southern margin and three left-step, en echelon Zhen-②faults at the northern marginand the Wubao fault step zone between the Wu-①fault at the southern margin and theWu-②fault at the northern margin. There were two types of basement faults beforeformation of the Gaoyou sag exists. One was ENE-striking, N-dipping thrusts withdeveloped during collision of the North China and Yangtze plates in the Indosinian period.The other type of the basement faults are NNE-striking, sinistral faults formed due tohigh-speed oblique subduction of the Izanagi Plate underneath the eastern China cintient inLate Jurassic. The whole Gaoyou Sag initiated during the Yizheng movement of latest LateCretaceous and experienced the Wubao movement, Sanduo movement and Yanchengmovement that resulted in its present tectonic framework. Previous studies suggest thatPaleogene stress field of the Gaoyou sag was under a state of N-S extension.Several classes of faults appear in the southern fault step zone of the Gaoyou Sag. TheZhen-①and Wu-①faults belong to the first-order faults. the Zhen-②, Wu-②and Ji-3faults belong to the secondary-order faults. Other third-order and fourth-order faults have arole in dividing fault-blocks or oil-bearing fault-blocks. After analyses for time-spacechanges of activity rates of main faults and cross-section balances, active features of themain faults and southern fault step zone are summarize in this work. During the rifitingperiod of the latest Late Cretaceous to Paleogene, the Zhen-①and Wu-①faults show thesimilarity in activities with synchronous intensifying and weakening. Both of them initiatedduring the Taizhou period, reached into peak activity during the Fu-2to Fu-4periods andweakened then gradually. The three Zhen-②faults experienced a evolution sequence fromthe Zhen-②-3fault, Zhen-②-2to Zhen-②-1faults. The Zhen-②-3fault was developedduring the Fu-2period while the Zhen-②-2and Zhen-②-1faults formed during the Dainanperiod. They all acted until the Sanduo period. The Wu-②fault initiated in the Dainanperiod and reached its peak activityt during the Sanduo period.After analysis for fault features and numerical modeling, genesis of different classesof faults is summarized in this study. During the N-S extension, the Zhen-①faultreactivated ENE-striking basement faults and local NNE-striking basement faults, and propagated into EW-striking segment locally, leading to its present curved fault plane as alarge-scale boundary faul. The Wu-①fault reactivated NNE-striking basement faults earlyand followed NEE-striking faults. The development of Zhen-②faults controlled by theZhen-①fault is produced by the Zhen-①fault and is newly-formed, substitutableseparation-fault on the large and shovel-shaped Zhen-①fault. The majorWu-②faultoriginated from reactivation of the NNE-to ENE-striking basement faults. ENE-strikingand NNE-striking fault systems in the southern fault step zone originated from reactivationof the basement faults or result from main fault activity as synthetic, parallel, normal faults.EW-striking faults in the zone result from newly-formed faults during the S-N extensionand propagation of the reactivated basement faults.Two small-scale accommodation zones occur between the Zhen-②-2and Zhen-②-1faults and the Zhen-②-2and Zhen-②-3faults along the northern margin of the step zone.The former is called as the Xuzhuang whereas the latter as the Fangxiang accommodationzones. As special deformed zones in the basin, the accommodation zones often had specialdeformation under local stress field and affected by boundary faults in order to keep strainconservation in the stretching region, leading to development of a complicated fault system.By numerical modeling, types and formation mechanism of faults with differentorientations in the two accommodation zones are summarized in this study. These faults aredivided into two types, i.e. connection faults and non-connection faults.