Negative Inversion Process and Hydrocarbon Accumulation of Yaha Fault Belt in Northern Uplift,Tarim Basin

Key words: 
• negative inversion fault /
• main inversion phase /
• displ acement-distance plot /
• hydrocarbon accumulation /
• Tarim basin

Abstract: The Yaha negative inversion fault belt,which include main Yaha fault,Ya N o.1 fault and Ya No.2 fault,is located at the north uplift of Tarim basin. The fault belt is the boundary between the north Tarim uplift and Kuche depression.The main Yaha fault,elongating more than 200 km.is a listric fault that trends NE E and dipps southward,and the dip angle of the fault plane is steep in the shall ow strata and become flat in the deep strata.Ya No.1 fault,dipping northward and elongating 35 km,is on the south of main Yaha fault.Ya No.2 fault, dipping nort h ward and elongating about 38 km,is on the south of main Yaba fault and Ya No.1 f ault.Ya No.1 and No.2 faults are also negative inversion faults,controlling"grab en-type"fault-blocks with main Yaha fault.Yaha fault belt is developed along pre -existing basement faults or weak zones.The evolution of the fault belts has the characteristics of thrustion in the early phase(pre-Cretaceous)and negative in version in the late stage(Cretaceous-Tertiary). The characteristics of geomety,kinematics,main inversion phase and hydreocarbonc o ntrolling of Yaha negative inversion fault belt,north Tarim uplift,are discussed in this paper according to the analysis of sequence thickness,displacement-dist ance plot,inflection point,null point and fault growth index.The point of change from thrusting to negative inverstion faulting in the displacement-distance plo t is the inflection point which represents the main inversion phase.And the poi nt of transformation from net compression to net extension is the null point.The inflection point of displacement-distance plot of main Yaha fault is at the bot tom of Lower Cretaceous,indicating that the negative inversion of main Yaha faul t began at the initial Early Cretaceous.The main negative inversion phase is at the Early Cretaceous,henceforth the negative inversion weakens gradually.The sec ondary inversion phases exist in the late Cretaceous-Paleogene,Suwiyi and Kanchu n epoch of Miocene.Fault growth index of main Yaha fault in the main inversion p hase of Lower Cretaceous is about 1.44～1.54.And the growth index in the seconda ry inversion phase is about 1.25～1.28.The negative inversion terminates at the Kanchun epoch of Miocene and Kuche epoch of Pliocene. The inflection point of displacement-distance plot of Ya No.1 fault is at the bottom of Suwiyi Formation of Miocene,indicating that the main inversion phase of the fault is at the Suwiyi epoch of Miocene,and the growth index of the fault at this time is about 1.1～1.69. The inflection point of displacement-distance plot of Ya No.2 fault is at the bottom of Upper Cretaceous-Lower Paleogene,indicating that the main inversion phas e of the fault is at the Late Cretaceous-Paleogene,and the growth index of the f ault at this time is about 1.1～1.18. A series of local structures.including fault-blocks,fault-anticlines and buried- h ill-drapes,are developed along Yaha fault belts.The formation of these structure s has a rerationship to negative inversion of the fault belt at the late Yanshan and Himalayan.The Yaha fault belt has the excellent hydrocarbon accumulation con ditions because of matched time of main inversion,trap formation,petroleum migra tion and accumulation.