Genetic Analysis of Strata of Complex Fault Block in Southern Weixinan Depression, Beibuwan Basin

The fault activities were frequent and the stratum was divided severely in Weixinan Depression of Beibuwan Basin, which makes sequential stratigraphic analysis much more difficult. In order to solve the problem of comparative study of sandbodies in tectonic fractured blocks, We found the development characteristics of the sequential stratigraphic framework and the system tracts from the different periods of fault depression in our target area by the comparison and analysis of Liushagang Formation and Weizhou Formation according to the 3 wells in the front of lower uplift in Weixinan Depression. Besides, it is confirmed that both Liushagang Formation and Weizhou Formation are third-order sequences which can be divided into four sub-sequences. However, some strata are missing due to fault cutting, meanwhile the layer loss on the top of Weizhou Formation was caused by strong denudation. The petrographic formation mechanism, sequence characteristic and development under the background of different datum depend on the tectonics, lake level changes and clastic sediment supply. The paleoclimatic changes determined the base level cycle, while the clastic supply capacity has a great effect on vertical sequence characteristics. Writers believe that large amount of debris in Weizhou Formation came from the provenance which was derived from the denudation of lower uplift of Weixinan Depression during Oligocene period.