FAULT FORMATION AND EVOLUTION IN RIFT BASIN--SANDBOX MODELING AND ITS INFERENCE

Fault distribution and evolution is the core on structure research, and also the key problem on oil and gas exploration in rift basin. Revealed by the high-precision 3-D seismic data, the fault distribution and assemblage are very complicated in rift basins and it can not be provided a reasonable explanation with classical theory of structural geology– Anderson’s model. The essence of the fault complexity and the intrinsic law of fault formation and evolution in rift basins have not been revealed clearly with the existing models (multi-phase superposition model and oblique extension model etc). The objective sandbox modeling are completed in the paper, with two typical rift basin regions - Zhangjuhe structural belt in Bohai Bay Basin and Weixinan Sag of Beibuwan Basin in China - as the prototype, and finally the basic structural characteristics of the target area are well reproduced. The basic characteristics of fault system in rift basin revealed by sandbox modeling are: 1) The faults controlled by basement pre-existing fabric constitute the basic framework of basin fault system; 2) The fault formation and evolution are controlled by basement pre-existing fabric trend, regional extension direction and extension amount. 3) At early stage of extension, fault strike is mainly controlled by the trend of basement pre-existing fabric. With the increasing of extension amount, the affection of extensional direction become stronger, and the faults formed in late extension stage can usually reflect regional extensional direction. 4)Uncoordinated progressive extension process is the root cause to lead to complex fault system in rift basins. The above-mentioned fault formation and evolution law under uncoordinated extension in rift basin revealed by objective sandbox modeling can be scientifically and rationally explained with "non-coordination Criterion ", so it has universal significance in rift basins.