Features Of Structures And Their Evolution In Nandaba Area

The article describes the features of combinations of structures-strata in Nandaba Structural Belt,From the old to the new,they can be divided as base combination (Pt),combination of platform-faulting depression edge (P—T₂) and combination of foreland (T3x—J).According to the studies,it is known that Nandaba Structure has mainly undertaken the generation and development of foreland,multiple structural deformations and in Himalayanian the present structural framework was formed, with obvious belt features for structural units from EN to SN. The Dabashan Thrust structural Belt can be divided into Northern Dabashan Thrust Folded Belt, Southern Dabashan Thrust Folded Thrust and Foreland Basin on planar. The structural features of Southern Daba Structure are controlled mainly by two detached surfaces (Silurian and Triassic)longitudinally.Due to the fluid change features of rocks and the features of decreased stress,longitudinally,the structures can be divided upper and lower floor with different deformational pattern.Transversely from EN-SN, due to the changes of strata engulfed from Cambrian→Silurian→Triassic,the upper, mid and lower structure floor become obviously un-harmornic and structural features are also different.The mid one is deformed seriously with developed faults,the upper one come next,dominating by folds,the lower one has belt features for various parts of it,with gentle structure and the deformed degree by engulfing being weak.According to the studies,by structural lines,Nandaba Area can be divided into two systems ofNNW,NWW and NE,NEE,on the whole,they convex toward SW while the structural line in NE,NEE has weak deformation,which mainly develop near Michang Mountain and the edges of basin,NNW ones distribute in the western area of Wanyuan of Nandaba Structural Belt and they are made of wide gentle synclines and narrow anticlines,NWW ones distribute in the eastern part of Wanyuan of Nandaba Structural Belt and they are made of wide gentle anticlines and narrow synclines.,According to the structural cross sections of field and geophysical seismic data interpreted,it is proved that the strata on surface is slip while underground ones are gentle,regional detached layers have different structures,the typical structures on ground can be divided into two types as following:overlapped faults and back-thrust structures related with structures of base engulfedâ‘¡fault-bent folds,fault propagation fold related with detachment of cover rocks and narrow folds,box-like anticlines and arc structures made of structural combinations. At the same time,underground structures can be divided into 13 structural patterns as overlapped structures and fault bent structure etc.Due to multiple reforms by several power sources, structural overlap and interrupt in various direction present in this area and lead to the differences of deep and shallow structures and the main combinations has parallel,bevel connecting and orthogonal connecting ones.According to the foundations, the features of Nandaba Structural Belt are multiple stages, multiple levels,multiple directions as well as complicated overlapped and synthesized relations on space.By statistic on terms of joints on the ground, analyses on the scraps on tensional joints formed by faults,analyses on various regional structural overlap and micro-structural analyses, geological times proves of structural deformation orders and regional structural background analyses,it is considered that Mesozoic-Cenozoic structural evolution in Nandaba Structural Belt has undertaken Indosinian initial term,Yanshanian development term and structural overlap of Hymalayanian formation term. Their dynamic sources come from the compressing stress by collision of Yangtze Block and Qingling Block in Indonisian as well as Yangtze Block thrust toward north direction into Qingling Orogenic Belt from north toward south direction.According to the relations of structural evolution and reservoiring,the article summarizes that the area has undertaken the stage of development of paleao-uplift of the edge of Daba Mountain in Caledonian～Hycenian and formation of oil & gas reservoirs,the stage of formation of traps in Indosinian～mid Yanshanian,stage of structural overlapping and stage of adjusting oil & gas as well as later reform of oil & gas in early Hymalayanian～later Hymalayanian.It is considered that the intension of structural deformation as well as stages controls the courses of oil & gas migration and reservoiring.The erosion intension and uplift of strata give impressions on oil & gas preservation, the distribution of cover rocks decide the layers that accumulate oil & gas, effective storing space also give important effects on oil & gas reservoiring.At the same time the article also points out that Tiexi-Wanyuan Area is favorable exploration area for oil & gas with good preserving conditions.